How to Measure for New UPVC Windows and Doors Measuring Guide

Essential Information

Before You Start Measuring

Getting the right measurement is the single most important step in ordering a new external door. Read these golden rules before you pick up a tape measure.

⚠️

We Cannot Accept Responsibility for Incorrect Measurements

External doors are manufactured to the exact dimensions you provide — we do not make any fitting allowances on the sizes given to us. If you are unsure at any point, please consult a professional fitter, joiner, or builder before ordering. You can also call us on 01530 273365 and we will gladly help and advise.

1

Measure the Brickwork Opening

Always measure the raw brick-to-brick aperture — never the existing door, frame, or architrave. Old frames are rarely true and will give misleading readings.

2

Take Every Measurement 3 Times

Measure width at the top, middle, and bottom of the opening. Measure height at the left, centre, and right. Always use the smallest of the three readings.

3

Always Deduct 10mm

Subtract 10mm from both your final width and height measurements. This creates a 5mm fitting clearance on each side, allowing the frame to be set plumb and square.

4

Include the Frame & Cill

Your measurements must include the full door frame, the cill (sill), and any frame extenders you require. Do not measure the door leaf alone.

5

Metric Millimetres Only

All dimensions must be supplied in millimetres (mm). We do not accept imperial measurements or convert from feet and inches.

6

View Everything Externally

All design decisions — handing, opening direction, glazing position, side panel location — must be described as seen from outside the building, looking in.

Preparation

What You'll Need

Gather these items before you begin. Having the right tools makes the job quicker, safer, and more accurate.

Equipment

Tools & Materials

Steel tape measure (at least 3m) — a steel tape gives accurate, rigid readings. Avoid fabric or retractable cloth tapes for structural measurements.
Spirit level — check whether your opening is plumb (vertical) and level (horizontal) before measuring. A twisted opening needs to be noted at ordering stage.
Notepad and pencil — record every reading as you go. Write down all three width readings and all three height readings before selecting the smallest.
Step ladder or platform — for tall openings, especially when measuring the height at the lintel. Never stretch or estimate — get close to the measurement point.
Helper (recommended) — one person to hold the tape at the top while the other reads the measurement at floor level ensures accuracy and safety.

💡 If your opening is in poor condition — crumbling mortar, damaged brickwork, or a bowed lintel — repair it before measuring. The opening should be structurally sound before you order.

Gather all tools before starting — accuracy depends on using the right equipment

Section 1

Measuring a Standard External Door

Follow these two steps for any standard external door — whether composite, upvc, french, patio or bifold doors. Both width and height must be measured separately and independently.

Step 1

Measure the Opening Width (Brick to Brick)

Always measure the raw brick-to-brick aperture — never the existing door, frame, or architrave. Old frames are rarely true and will give misleading readings.
Position your tape at the inner face of the brickwork on one side of the opening. The inner face is the surface that faces into the opening itself — not the outer wall face.
Extend the tape horizontally across to the inner face of the brickwork on the opposite side. Keep the tape perfectly horizontal — use your spirit level to check.
Take this reading at three heights:
- Near the top of the opening (25–50mm below the lintel)
- At the middle of the opening
- Near the bottom of the opening (25–50mm above the floor)
Record all three readings and identify the smallest value. This is your critical width measurement.
Subtract 10mm from the smallest reading. This is the width you order.

Smallest width reading − 10mm = Your Order Width

💡 Example: You measure 908mm, 902mm, and 910mm. The smallest is 902mm. Subtract 10mm → your order width is 892mm.

⚠️ If your three width readings vary by more than 15–20mm, your opening may be significantly out of plumb or the lintel may be bowing. Contact us before ordering — do not simply take the smallest and proceed.

Measure brick-to-brick width at three heights. Always use the smallest reading, then deduct 10mm.

Top width

Middle width

Bottom width

Step 2

Measure the Opening Height (Brick to Brick)

Identify the lintel — the structural beam or brick course that spans the top of your opening. Your height measurement starts from the underside of this lintel.
Position the end of your tape at the underside of the lintel. Hold it firmly against the brickwork — do not allow a gap between the tape end and the lintel face.
Extend the tape vertically downward to the floor or existing threshold. Keep the tape plumb — use your spirit level or let it hang naturally.
Take this reading at three positions across the opening:
- On the left side (near the left brickwork)
- At the centre of the opening
- On the right side (near the right brickwork)
Record all three readings and identify the smallest value.
Subtract 10mm from the smallest reading. This is the height you order.

Smallest height reading − 10mm = Your Order Height

💡 If you are ordering a door with a new external cill, your height measurement must include the cill depth. Check with us if you are unsure — the cill is part of the overall frame height.

📐 Most standard UK external door openings are around 2050mm–2150mm tall and 840mm–920mm wide. If your measurements fall well outside these ranges, double-check your readings before proceeding.

Measure from underside of lintel to floor at left, centre, and right. Use the smallest, deduct 10mm.

Left

Centre

Right

Order Width

Smallest of 3 width readings, minus 10mm

Order Height

Smallest of 3 height readings, minus 10mm

Viewed From

Always describe the door as seen from outside the building

Units

Millimetres only — no inches, no conversion

Section 2

Door with a Side Panel

A side panel (also called a sidelight) is a fixed glazed panel that sits beside the main door within the same frame. You will need to measure the total opening width plus the individual side panel width — these are two separate measurements.

Step 1

Measure the Total Opening Width (Full Brick to Brick)

Measure the full brick-to-brick width of the entire opening, exactly as you would for a standard door (top, middle, bottom — take the smallest).
Deduct 10mm from the smallest reading. This gives your total frame order width — the combined width of the door and the side panel together.
Note that the total frame order width includes: the door frame, the door leaf, the mullion (the vertical dividing profile between the door and the panel), and the side panel frame.

Smallest total width − 10mm = Total Frame Order Width

💡 The total order width covers the entire aperture. The door width and side panel width are calculated from this total — you do not order them separately unless requested.

The total order width spans the full aperture — door, mullion, and side panel combined

Step 2

Measure the Side Panel Width (Mullion Centre to Outer Frame)

Locate the mullion — this is the vertical profile that runs between the door and the side panel. It is part of the frame system and its centre point divides the door width from the side panel width.
Measure from the centre of the mullion across to the outer edge of the side panel frame (i.e., the edge that meets the brickwork). This measurement gives the side panel width.
Provide this side panel width in addition to your total frame order width when placing your order.
The door leaf width is then calculated as: total frame order width minus the side panel width minus the full mullion width (typically 70mm).
The height measurement is the same as a standard door — brick to brick at left, centre, and right; use the smallest; deduct 10mm.

💡 Side panel position: When viewing externally, specify whether the side panel is to the left or right of the door. This affects handing and frame construction.

⚠️ Do not guess the mullion centre — measure to it. If the frame has been removed, the mullion centre will be where the door frame ends and the panel frame begins. Typically the mullion is 70–100mm wide; its centre is at half this measurement.

Side panel width = mullion centre to outer edge of side panel frame

📋

What to Provide When Ordering a Door with Side Panel

1. Total frame order width (full brick-to-brick width minus 10mm) · 2. Side panel width (mullion centre to outer frame edge) · 3. Frame order height (brick-to-brick height minus 10mm) · 4. Side panel position — left or right as viewed externally

Section 3

Door with a Top Light

A top light (also called a fanlight or transom light) is a fixed glazed panel that sits above the main door, separated by a horizontal transom bar. You need the total opening height and the transom drop — the depth of the top light.

Step 1

Measure the Total Opening Height (Full Brick to Brick)

Measure the full brick-to-brick height of the entire opening — from the underside of the lintel down to the floor. Take three readings (left, centre, right) and use the smallest.
Deduct 10mm from the smallest reading. This is your total frame order height — the combined height of the door and the top light together.
This single height measurement covers the full frame assembly: top light, transom bar, and main door panel.

Smallest total height − 10mm = Total Frame Order Height

📐 The width measurement for a door with top light is taken exactly the same as for a standard door — brick to brick at top, middle, bottom, smallest reading minus 10mm.

Total height = lintel to floor, smallest of three readings minus 10mm

Step 2

Measure the Transom Drop (Top Light Depth)

Locate the transom bar — this is the horizontal frame member that separates the top light from the main door. If measuring an existing installation, find its centre point.
Measure from the top of the overall door frame (or the underside of the lintel if the frame has been removed) down to the centre of the transom bar. This measurement is the transom drop.
The transom drop tells us where to position the horizontal bar within the frame — it defines how deep the top light panel will be.
Door height = Total frame order height minus the transom drop (approximately).

Top of frame → centre of transom bar = Transom Drop

💡 A standard transom drop is typically 300–400mm from the top of the frame, giving a proportional top light. However, you can specify a custom transom drop position to suit your design preferences or existing opening proportions.

⚠️ Do not confuse the top light height with the transom drop. The transom drop is measured from the top of the frame downward to the bar centre — not the height of the glazed top light panel itself.

Transom drop = top of frame to centre of transom bar. This defines the top light depth.

📋

What to Provide When Ordering a Door with Top Light

1. Frame order width (brick-to-brick width minus 10mm) · 2. Total frame order height (brick-to-brick height minus 10mm) · 3. Transom drop (top of frame to centre of transom bar)

Section 4

Door Handing & Opening Direction

Once you have your measurements, you need to specify how the door opens. Getting this wrong means the door will be manufactured the wrong way around and cannot be corrected after manufacture.

Handing Guide

How to Determine Handing — Always View Externally

Stand outside the building, facing the door from the outside. All handing descriptions are based on this external view.
Identify the hinge side — the side the door is fixed to and pivots from.
- Left-hand hung — the hinges are on the left as you face the door from outside.
- Right-hand hung — the hinges are on the right as you face the door from outside.
Identify the opening direction:
- Inward opening — the door swings toward you when you open it from inside (i.e., into the building). This is the most common for UK external doors.
- Outward opening — the door swings away from you when opening from inside (i.e., out from the building). Often used for back doors or outbuildings.
Provide both the hinge side and opening direction when placing your order.

⚠️ Handing is one of the most common ordering errors. If you are unsure, describe the door to us on the phone and we will confirm the correct handing together before your order is placed.

Always determine handing by standing outside, looking at the door from the external face

Section 5

Measuring a UPVC Window

The same brick-to-brick principle applies to windows as to doors. Always measure the raw aperture in the brickwork — never the old frame, glazing bead, or existing window unit.

Step 1

Measure the Window Opening Width (Brick to Brick)

Locate the inner face of the brickwork on both sides of the window reveal. Measure from one face directly across to the other — do not follow the reveal angle if the brickwork is splayed.
Take the width measurement at three heights:
- Near the top of the opening (just below the lintel)
- At the middle of the opening
- Near the bottom of the opening (just above the cill)
Record all three readings and identify the smallest value.
Subtract 10mm from the smallest reading. This is your order width.

Smallest width reading − 10mm = Order Width

💡 On older properties, window openings are often noticeably out of square. Taking all three measurements protects you from ordering a frame too wide to fit at its narrowest point.

Measure brick-to-brick at top, middle, and bottom. Use the smallest reading, then deduct 10mm.

Top width

Middle width

Bottom width

Step 2

Measure the Window Opening Height (Brick to Brick)

Measure from the underside of the lintel (or the top inner edge of the brickwork) down to the underside of the existing cill. Always measure to the underside of the cill — the cill itself must be included in your height measurement.
Take this reading at three positions: left side, centre, and right side of the opening.
Record all three and identify the smallest value.
Subtract 10mm — this is your order height.
The measurement must always include the full depth of the existing cill. Measure from the underside of the lintel right down to the underside of the cill — not the top of it. If the existing cill is being replaced, measure to the underside of where the new cill will sit.

Smallest height reading − 10mm = Order Height

💡 Always measure to the underside of the existing cill — the cill is part of the frame assembly and its full depth must be included. If no cill exists, measure to the floor of the reveal.

⚠️ Do not measure to the top of the cill — always measure to the underside. The cill depth is part of your total height measurement.

Measure from underside of lintel to underside of cill at left, centre, and right. Use the smallest, deduct 10mm.

Left height

Right height

Step 3 — Optional

Measure the Mullion Width (Multi-Pane Windows)

Identify the mullion — this is the vertical profile bar that divides the window into two or more panes side by side. If your window has only a single pane, there is no mullion and this step does not apply.
Measure the full mullion width — from the outer edge of the mullion profile on one side to the outer edge on the other side. This is a face-on measurement taken horizontally across the bar itself.
For ordering purposes, provide the position of the mullion as a distance from one side of the frame — measured from the inner edge of the outer frame to the centre of the mullion. This tells us where to position it within the overall frame width.
If the window has multiple mullions, provide a separate centre-line position for each one.

Inner frame edge → centre of mullion = Mullion Position

💡 A typical UPVC mullion is 60–90mm wide. If you are replacing an existing window and want the new mullion in exactly the same position, measure from the inner edge of the existing outer frame to the centre of the existing mullion before removing the frame.

📐 The mullion position determines how each individual pane width is calculated. If the panes need to be equal, simply specify this — we will position the mullion at the halfway point of the total frame width automatically.

Mullion width = face-to-face across the bar. Mullion position = inner frame edge to centre of mullion.

Mullion width

Mullion position

Total frame width

Step 4 — Optional

Measure the Transom Drop (Horizontal Dividing Bar)

Identify the transom bar — this is a horizontal profile bar that divides the window into upper and lower sections (e.g. a fixed upper pane with an opening lower casement, or a top-hung fanlight above a fixed pane). If your window is a single-height unit with no horizontal bar, this step does not apply.
Measure the transom drop — from the top inner edge of the outer frame (or the underside of the lintel if measuring in the opening before the frame is fitted) down to the centre of the transom bar. This is a vertical measurement.
The transom drop tells us how far down from the top of the frame the horizontal bar is positioned. It defines the depth of the upper and lower sections of the window.
If the window has multiple transoms, provide a separate drop measurement for each, always measured from the top of the frame.

Top of frame → centre of transom bar = Transom Drop

💡 A typical transom drop for a window with a top-hung opening vent is around 300–450mm from the top of the frame, giving a narrow upper ventilation pane. For an equal split, the transom drop is half the total frame height.

⚠️ Do not confuse the transom drop with the height of the upper pane. The transom drop is always measured from the top of the frame downward to the bar centre — not from the bar upward to the lintel.

Transom drop = top of frame to centre of transom bar. Always measured downward from the top.

Transom drop

Total frame height

Transom bar

📋

What to Provide When Ordering a Window with Mullion or Transom

Standard window: order width + order height · + Mullion: also provide mullion position (inner frame edge to mullion centre) · + Transom: also provide transom drop (top of frame to centre of transom bar) · Equal panes: simply state this and we will calculate the positions automatically

Order Width

Smallest of 3 width readings (top / middle / bottom), minus 10mm

Order Height

Smallest of 3 height readings (left / centre / right), minus 10mm. Always measure to underside of cill.

Mullion Position

If applicable: inner frame edge to centre of mullion bar (mm)

Transom Drop

If applicable: top of frame down to centre of transom bar (mm)

Section 6

Measuring a Square Bay Window

Square bay windows consist of three window panels and two 90° corner posts. They require a combination of internal width measurements and an external height measurement — different from standard windows.

Bay Window

Internal Back Span, Side Panel Widths & External Height

Measure the internal back span — from the inner edge of the existing frame on the left side to the inner edge on the right side. This is measured internally, across the full width of the bay.
Measure each individual side window internally — from the inner edge of one frame to the inner edge of the adjacent frame. Both side windows should be equal; if they are not, note the difference and contact us.
We add 140mm to the external measurement to account for the two 90° corner posts, which are 70mm square each. You do not need to measure the posts yourself.
- Example: internal back span = 1,800mm → external order width = 1,800 + 140 = 1,940mm
Measure the height externally — from the underside of the lintel down to the underside of the existing cill. The cill must be included in the measurement. Take three readings (left, centre, right) and use the smallest.
Deduct 10mm from the smallest height reading only. The width calculation already accounts for fitting with the 140mm post addition.

Internal back span + 140mm = External Order Width

💡 The centre window panel of a square bay is typically the widest. Measure it separately from the two side panels — provide all three individual panel widths when ordering.

⚠️ Height must be measured externally brick to brick — unlike the width measurements, which are taken internally from the existing frame edges.

Add 140mm (two 70mm corner posts) to the internal back span to get the external order width

Internal back span

Side panel widths

70mm corner posts

📋

What to Provide When Ordering a Bay Window

1. Internal back span (we add 140mm for corner posts) · 2. Left side panel width (internal frame-to-frame) · 3. Centre panel width (internal frame-to-frame) · 4. Right side panel width (internal frame-to-frame) · 5. External height (brick-to-brick, smallest of 3, minus 10mm)

Section 7 (Advanced)

Measuring a Bow Window

Bow windows are more complex than bays. They consist of multiple panels at shallower angles, creating a curved projection. Three key measurements are required: the internal back span, individual facet widths, and the projection depth.

Bow Window

Back Span, Individual Facet Widths & Projection

Measure the internal back span — from the inner edge of the frame on the left, straight across to the inner edge of the frame on the right. This is a straight horizontal measurement taken internally.
Measure each individual facet internally — each glazed panel has its own width. Measure from the inner face of the frame on one side of that panel to the inner face on the other side. Provide the width for every facet individually.
Measure the projection — from the inside face of the back span (the wall face) out to the inside face of the furthest-forward window frame. This tells us how far the bow projects from the wall.
Measure the height externally — from the underside of the lintel down to the underside of the existing cill. The cill must always be included. Take three readings (left, centre, right) and use the smallest, minus 10mm.

Back span + Each facet + Projection = Full specification

💡 All bow window width measurements are taken internally. Only the height is measured externally. When in doubt, call us on 01530 273365 — bow windows are the most complex to specify and we are happy to guide you through exactly which measurements we need.

⚠️ Bow windows require a precise projection measurement. An inaccurate projection means the window will not fit flush against the wall or the cill will not sit correctly. Take extra care with this measurement.

Plan view from above — back span, each facet width, and projection are all measured internally

Internal back span

Projection depth

Individual facet widths

📋

What to Provide When Ordering a Bow Window

1. Internal back span (straight across, edge to edge) · 2. Width of each individual facet (measured internally) · 3. Projection depth (from back span face to front frame face, internally) · 4. External height (brick-to-brick, smallest of 3, minus 10mm) · 5. Number of facets (typically 3 or 5)

Final Checklist

Pre-Order Checklist

Before you submit your order, confirm every item in this checklist. A missed detail is the most common cause of a door not fitting or being manufactured incorrectly.

✓	Check Item	Details
✓	Measured brick to brick	Raw brickwork aperture only — never the existing door, frame, window unit, or architrave
✓	Three width readings taken	Top, middle, and bottom of the opening — all three recorded
✓	Three height readings taken	Left side, centre, and right side of the opening — all three recorded
✓	Smallest reading used	The smallest of the three width readings and the smallest of the three height readings
✓	10mm deducted from both	10mm subtracted from both the width and height smallest readings to get order dimensions
✓	All dimensions in millimetres	No inches, no feet — metric millimetres only
✓	Frame, cill & extenders included	Measurements must account for the full frame assembly including the cill and any add-ons
✓	Handing confirmed (doors)	Hinge side (left or right) and opening direction (inward or outward) specified as viewed externally
✓	Side panel position specified	If applicable: side panel width measured and left/right position confirmed as viewed externally
✓	Transom drop specified	If applicable: transom drop measured from top of frame to centre of transom bar
✓	Window cill confirmed	Windows, bay & bow: always measure to the underside of the existing cill — cill depth is always included in the frame height
✓	Bay: internal back span & side widths	If ordering a bay window: all three panel widths (left side, centre, right side) measured internally; height measured externally
✓	Bow: all facet widths & projection	If ordering a bow window: every facet measured individually (internally) plus projection depth; height measured externally
✓	Opening is structurally sound	Lintel is sound, brickwork is stable, and any damaged masonry has been repaired before measuring

📞

Need Help? We're Here Monday to Friday

If you have any doubt about your measurements, please call us on 01530 273365 (Mon–Fri 9:00–17:30) or email sales@justdoorsuk.com before placing your order. Our team will talk you through the measurement process and confirm your dimensions before manufacture begins. We cannot accept responsibility for incorrect measurements supplied to us, but we will do everything we can to help you measure correctly.