What you need
A reliable tape measure, a spirit level and a way to record each reading. Photograph each opening and label it clearly. Measure only from a safe position; do not remove frames or use unsafe access to obtain a size.
BEFORE YOU ORDER · MEASURING GUIDEHOW TO MEASURE
HOW TO MEASURE
uPVC WINDOWS & DOORS.
To measure uPVC windows for replacement, take three width readings and three height readings across the structural opening, then use the smallest width and height. Deduct 10 mm from each smallest dimension for standard white windows, or 15 mm for coloured windows — once per dimension, not per side. Check internal plasterlines and your chosen cill, then ask your installer to confirm the final manufacturing sizes before ordering.
This window and door measuring guide covers replacement frame sizes, not measurements for curtains, blinds or glass alone. Follow the video and step-by-step checks below to prepare your made-to-measure window specification.
WATCH · MEASURING GUIDE
How to measure your windows and doors
Measure three widths and heights, use the smallest readings and allow for fitting. Check internal plasterlines and your cill choice, then have your installer confirm the final order sizes.
Allow for fitting. Confirm before manufacture.
Take the smallest of your three width readings and the smallest of your three height readings. For standard white windows and typical replacement doors, subtract 10 mm from the smallest width and 10 mm from the smallest height. For coloured windows, subtract 15 mm from the smallest width and 15 mm from the smallest height instead.
These are total deductions from each dimension — not off each side, and not added together. The fitting allowance is a starting point, not a substitute for a survey: your installer must confirm that it suits the frame system, opening, cill and threshold before you order.
One reference per opening
Use labels such as W01 or D01 on your notes, photos and plans. Record which room it serves and whether your sketch is viewed from inside or outside.
Millimetres, width first
Write width × height in mm and keep all readings. Do not round to a standard window size or use the visible glass size as the frame size.
01 · REPLACEMENT WINDOWS
How to measure uPVC windows
Identify the opening
Measure between the structural sides of the opening, not the glass, opening sash or decorative trims. Existing frames can conceal the true opening; internal and external reveals may differ.
Take three width readings
Record the width at the top, middle and bottom. Use the smallest width reading and subtract 10 mm for standard white windows, or 15 mm for coloured windows. This is a total deduction, not per side. Keep all three opening readings separately for your installer to confirm the order size.
Take three height readings
Record height at the left, centre and right using consistent reference points. Use the smallest height reading and subtract 10 mm for standard white windows, or 15 mm for coloured windows. Deduct it once from the total height. Explain where you measured to at the bottom and ask your installer to confirm the height with your chosen cill.
Check shape and cill details
Ask your installer to check squareness, level, reveals and frame depth. Record existing cills and any add-ons. Confirm whether the shop’s overall height includes the cill and whether extensions are included in the ordered size.
02 · ENTRANCE, FRENCH & PATIO DOORS
How to measure uPVC doors for replacement
Record the opening width at top, middle and bottom, and the height at left, centre and right. Keep all six readings. Door sizes need additional checks at floor level; a clear opening alone does not define the ordered frame height.
Agree the finished floor level
Tell your installer about planned flooring changes. Confirm the threshold type, cill, bottom reference point and drainage requirements before agreeing the manufacturing height.
Confirm the complete assembly
Specify sidelights, toplights, frame extensions and any cill as separate details. Confirm whether your quoted dimensions cover the whole assembly or just the main door frame. For typical replacement doors, subtract 10 mm from the smallest of the three width readings and 10 mm from the smallest of the three height readings — once per dimension, not per side. Your installer must confirm the allowance and final order size with the chosen threshold and cill; the 15 mm coloured-window allowance is not a blanket rule for doors.
Record opening direction
Draw hinge and handle positions, inward or outward opening, and the lead door or sliding panel. Label the viewing side and check it against the shop diagram before ordering.
Check internal plasterlines before ordering
The external opening and internal plaster opening can be different sizes. Measure the internal width at the top, middle and bottom, and the internal height at the left, centre and right. Record these separately from your structural opening measurements and photograph any stepped reveals or plaster overlapping the existing frame.
Ask your installer to compare these plasterline measurements with the proposed frame size and position. Check that the internal reveals will not cover too much of the frame, obstruct opening sashes, hinges or handles, or prevent a door from opening freely. Frame extensions or changes to the internal reveals may be needed — do not assume the usual 10 mm deduction alone guarantees a correct fit.
If the true opening is concealed or you are unsure how the plasterlines affect the fit, arrange a professional survey before placing your order.
Choose your uPVC cill before ordering
Our cill options are 85 mm, 150 mm, 180 mm. These sizes refer to the cill depth, not its length or the fitting allowance. Record which cill you want alongside each window or door specification.
Ask your installer to check the wall and reveal depth, frame position and the projection needed beyond the external wall face for drainage. Confirm the appropriate cill for your chosen system and whether the shop’s overall height includes it before finalising your measurements. For doors, also confirm how the cill works with the threshold and finished floor level.
03 · YOUR WINDOW SCHEDULEFrom opening measurements
From opening measurements
to an order-ready specification
Keep the measured opening and the installer-confirmed order size in separate columns. Never treat them as interchangeable.
| Record | What to include |
|---|---|
| Opening reference | Unique label, room, photograph and viewing side |
| Measured opening | Three widths and three heights in mm; flag the smallest readings |
| Internal plasterlines | Three internal widths and heights, reveal photographs and installer-confirmed frame clearance |
| Confirmed order size | Final width × height, agreed fitting allowance and who confirmed it |
| Layout & hardware | Fixed/opening panes, hinges, handles, door handing and sliding direction |
| Finish & glass | Inside/outside colours, glazing choice, obscure glass and ventilation |
| Cill choice | 85 mm, 150 mm, 180 mm depth; confirm projection and whether included in the overall height |
| Fitting details | Cill, extensions, threshold, finished floor and overall-size convention |
When to arrange a professional survey
Bay and bow windows, sliding sash replacements, shaped frames, concealed or stepped reveals, and structural alterations need product-specific assessment. Do not use the simple rectangular-opening diagrams to calculate these orders. Ask your installer to check the opening, support and fitting requirements.
Guidance sources
These trade guides explain measurement checks and show why fitting allowances and size conventions are not universal. They are reference material, not fitting instructions for every product we sell.
Ready to price your windows and doors?
Use your installer-confirmed sizes in the shop. If you need to discuss a specification, send your labelled photos, drawings or PDF window schedule through our contact page before placing an order.