How carefully you measure depends on who will be wrong if the window does not fit. If an installer is supplying and fitting, their surveyor measures every opening before anything is made, and a frame that does not fit is their problem. If you are buying supply-only, your figures go straight to the factory, the window is made to them, and four of the supply-only retailers we read say a wrong measurement is yours to pay for. Most of this guide is for the second case. It follows the Glass and Glazing Federation's good practice guide on installing replacement windows (April 2016, reflecting BS 8213-4:2016), the retailers' own measuring instructions and the consumer law that decides whether you can send a window back.
First: are you measuring for a price or for an order?
For a price, you need a count, not measurements. Our window price calculators ask for the number of windows, the style, the frame material, the glazing, fitted or supply-only, your property type and your region. There is no width or height field. Count your windows by style, enter the count in the uPVC window calculator, and keep your millimetres for the supplier or surveyor. Size moves where you land within the per-window range rather than being typed in, and it moves the price less than people expect: the Federation of Master Builders' July 2026 figures put a fitted uPVC casement at £525 for 600 x 900 mm and £670 for 1,200 x 1,200 mm, which our cost per window guide sets out by material.
For a fitted quote,rough measurements are enough. Everest, an installer, says measurements taken for quotes should be taken again by the installer. Eurocell's home route asks you to measure from inside, frame included, then sends a surveyor to check, and says it honours prices within a tolerance of its own measurements without stating how big that tolerance is. The GGF guide expects the surveyor to show a valid Minimum Technical Competence card on request or to be certified to NVQ level 3, and says any defects found at survey should be agreed with the customer before the windows go in. Our installation process guide covers that survey.
For a supply-only order, the measurements below are the contract. Supply-only saves £200 to £600 a window on our figures, and part of what you give up for that saving is the surveyor, as supply-only vs fully fitted explains.
How to measure a window opening, step by step
1. Find the brick edge, outside
Every supply-only retailer we found asks for the brickwork opening, measured from outside, not the old frame. The reasons they give are about the old frame: it may have warped, it may fit the opening badly, and in House of Windows' words "Old frames are rarely true and will give misleading readings." Scrape back render or old foam until you reach the brick edge that will form the opening. On a rendered house, Modern uPVC Windows notes that you may also need frame extenders, depending on the thickness of the render. Where outside measuring is impossible, the same retailer accepts the existing window as a fallback.
2. Three widths, three heights, in millimetres
Measure the width brick to brick across the top, middle and bottom, and the height at the left, centre and right, from the underside of the lintel to the underside of a removable cill. Work in millimetres: House of Windows will not accept imperial measurements or convert them, and Travis Perkins notes that suppliers usually want the width first. One retailer asks you to leave out a metal lintel.
3. Use the smallest, and do not average
The GGF guide, paragraph 2.16: "The smallest measurement of width and height is used for manufacturing sizes". Six retailers and suppliers say the same. The reason is that the frame has to fit the tightest point of the opening; a frame made to the average will not go in where the opening is narrowest. Glasstec puts it bluntly: "Do not simply average the three measurements."
4. Check the diagonals
Measure corner to corner both ways. If the two are the same, the opening is square; if they differ, it is out of square, or racked. How much difference is acceptable depends on who you ask, and the sources disagree. The GGF surveyor's checklist asks whether the aperture is "square and even to within 5mm height and width and 10mm diagonals". Two trade suppliers, Trade Window Store and Trade Windows Scotland, treat a difference of more than 5 mm between the diagonals as out of square. The GGF figure is the trade body's; the 5 mm is the stricter supplier view. Either way, a badly racked opening is worth showing to the supplier before you order.
5. Compare the inside with the outside
The GGF guide asks for the difference between the internal and external reveal sizes to be found, and for a check that plaster, render or tiles will not stop the opening light moving. If the outside opening is noticeably smaller than the inside, you may have a check reveal, and the next section applies before you go any further.
6. Settle the cill
Three retailers agree on measuring the height to the underside of an existing removable cill, because the cill depth is part of the height. Where a stone cill is staying, measure to the bottom of the window frame instead. What they then want on the order differs: Modern uPVC Windows asks for overall sizes including the cill, while Aluminium Trade Windows' pages say both that you should enter overall sizes including the cill and that it includes the cill for you. Ask each supplier. A new cill should overhang the face of the wall by at least 25 mm (GGF and Sidey).
7. Apply the deduction the supplier asks for
The frame is made smaller than the opening to leave room for packers, fixings and sealant. The GGF guide's table is the reference, and its first note matters most: "These deductions are from the total width or height and are not 'per side'".
| Frame material | Up to 1.5 m | 1.5 to 3.0 m | 3.0 to 4.5 m | Over 4.5 m |
|---|---|---|---|---|
| uPVC, white | 10 mm | 10 mm | 15 mm | 20 mm |
| uPVC, coloured or foiled | 15 mm | 15 mm | 22 mm | 28 mm |
| Timber | 10 mm | 10 mm | 10 mm | 15 mm |
| Aluminium | 10 mm | 10 mm | 15 mm | 20 mm |
| Steel | 8 mm | 10 mm | 12 mm | 15 mm |
| GRP | 5 mm | 10 mm | 15 mm | 15 mm |
Source: GGF, A Guide to Good Practice: Installation of replacement windows and doors (April 2016), Table 1. Totals, not per side. Deduct 4 mm when fitting aluminium or steel frames into existing timber sub-frames; allow for any sealant or mortar bed at the sill.
Retailers mostly use a flat 10 mm, about 5 mm each side, which matches the GGF for white uPVC, timber and aluminium up to 3 m. No retailer page we found varies it by colour, so on a coloured or foiled uPVC frame the GGF asks for 5 mm more than the usual retailer advice, and Everest's 5 to 10 mm is below it. The bigger risk is a double deduction. House of Windows wants you to deduct 10 mm and says "we do not make any fitting allowances on the sizes given to us"; no other supplier page we read said whether it deducts for you. So do what Everest advises: ask your manufacturer how much to deduct, and say clearly what you have measured. The full fitting sequence, from sill to sealant, is in how to fit a double glazed window. The GGF applies the same totals to an external doorset, which has its own fixing and threshold rules, covered in how to fit a uPVC door.
Write down what each figure is: brick to brick or frame, before or after the deduction, cill in or out. A supplier who reads your opening as a frame size will make exactly what you sent, and the terms say that is yours to pay for.
Double Glazing Cost Calculator Team
Check reveals: when you add instead of deduct
A check reveal is an opening "with an external reveal size less than that of the internal reveal size" (Sidey), so the window frame sits partly behind the outer brickwork. NHBC's standards say new homes in Scotland, Northern Ireland and areas of very severe exposure should use them, with the frame set back behind the outer leaf and overlapping it by at least 12 mm. illbruck, a sealing tape maker, calls them "particularly common in Scotland", and the GGF guide points to properties with box sash windows, where the side boxes are hidden behind the face brickwork. We found no source saying they are common in older English houses generally.
In a check reveal the outside opening is the smallest part, so measuring it and deducting 10 mm under-sizes the frame. The trade guides size the window so the frame overlaps the outer brick by at least 12 mm on both sides, found by drilling through the old frame jamb or taking off an internal architrave. Two sash window suppliers, Colin's Sash Windows and Sliding Sash Windows Online, tell buyers with a check reveal to measure the external opening and add 40 mm to the width and 20 mm to the height, "providing there is sufficient clearance", and to check that the internal width and height are larger than the external. Travis Perkins' guide tells sash buyers to measure the box frame opening and deduct 10 mm, with no mention of check reveals. In a check reveal, following that would leave a frame about 50 mm narrower than the suppliers' figure.
The two instructions cannot both be right for the same opening. Ask the supplier which it wants before you send any figures. Every source that covers check reveals in detail is written for the trade, so this is a good point to have the supplier's surveyor or a joiner size it.
Bay windows and sash windows
Bays.Three supply-only retailers agree on the method: widths and angles from the inside, height from the outside. Measure each section separately and note the angle between sections, typically 90 or 135 degrees; Modern uPVC Windows suggests an angle finder. Corner posts are where retailers differ. House of Windows measures a square bay's internal back span and adds 140 mm for two 70 mm corner posts, so an 1,800 mm back span is ordered at 1,940 mm, while the height, measured outside, has 10 mm taken off the smallest reading only. Modern uPVC Windows makes 135 degree bays slightly smaller than ordered to fit its posts. Travis Perkins says many homeowners have bay measurements confirmed by a professional installer. And the GGF guide warns that the loads in an existing bay must be checked before removal so it can be supported, with bay poles specified for those loads. Our bay windows guide covers the rest.
Sashes.Sidey's survey guide says replacement box sash windows are usually installed into an existing check reveal, and flush reveals are unusual, so the section above applies to most sash jobs. The GGF guide's tip is to take off an internal architrave to see how deep the brick return really is, and to drill through the head of the old box to find the maximum height the new window can go behind the brick, which may differ from the width allowance. Where a new sash goes into an old timber box, Trade Windows Scotland measures the inside of the box at three points. Our sash windows guide covers the choice of window.
Escape windows. If a window is a fire escape route, measure its clear opening with the sash fully open before it comes out, as well as the brick opening. The new window must give at least the same escape potential, and our guide to fire escape windows sets out the three measurements it has to meet.
Window sizes in the UK: is there a standard?
No published one. Search for UK window sizes and you will find the same list on several installer blogs: widths of 488, 630, 915, 1,200 and 1,770 mm, with Everest adding heights from 450 to 1,500 mm. None of the pages we fetched says where it comes from, and a search for a British Standard or trade body list behind it found nothing. Everest, one of the pages carrying it, also says of casements, the commonest UK style, that "the industry doesn't specify a standard size".
The list does not describe stock either. B&Q's stock uPVC casements come in widths of 440, 610, 820, 905, 1,190 and 1,770 mm, which share only 1,770 mm with the list, and heights of 610 to 1,190 mm, which match none of its heights. Selco says it stocks widths from 610 to 1,770 mm and heights from 1,040 to 1,190 mm.
| Stock size, width x height | B&Q price, supply only |
|---|---|
| 440 x 610 mm (top hung) | £150 |
| 610 x 820 mm | £151 to £187 |
| 610 x 1,040 mm | £168 to £197 |
| 610 x 1,190 mm | £213 |
| 820 x 820 mm | £186 to £234 |
| 905 x 965 mm | £213 to £281 |
| 905 x 1,040 mm | £249 |
| 1,190 x 965 mm | £234 |
| 1,190 x 1,040 mm | £253 to £307 |
| 1,190 x 1,115 mm | £301 |
| 1,770 x 965 mm | £342 |
| 1,770 x 1,040 mm | £365 |
| 1,770 x 1,190 mm | £541 |
Source: B&Q windows category, prices as listed on 19 September 2026. Ranges reflect colour and handing. Supply only, before fitting, removal and certification.
The replacement suppliers we read make to measure: Everest, Trade Window Store and Aluminium Trade Windows all make every window to order, and Travis Perkins notes that replacement windows are made to order. A stock size only helps if your opening happens to suit it after the deduction. It does carry one advantage in law, below. The sizes on our cost pages, such as 600 x 900 mm, are pricing reference sizes from the cost guides, not standard sizes.
Who pays when a measurement is wrong
On a fitted job, the installer measures and the installer answers for it. Which?, in guidance dated 16 September 2026, says the Consumer Rights Act 2015 requires installation work to be done with reasonable care and skill and to be as described, and that a company using subcontractors is responsible for their work. Everest, an installer, tells buyers to insist the installer takes responsibility for measuring, and says it puts right any problem with its own measurements.
On a supply-only order, the four retailers whose terms we read put the risk on you in almost the same words.
| Supply-only retailer | What its terms say about wrong measurements |
|---|---|
| House of Windows | "We Cannot Accept Responsibility for Incorrect Measurements" |
| Modern uPVC Windows | "We are unable to accept responsibility for incorrect measurements." |
| Aluminium Trade Windows | "you will still have to pay for a window that doesn't fit"; no returns for ordering "the incorrect size" |
| Window Supply Direct | Goods wrong because of "misinformation being given to us" cannot be returned; its paid site visit, £65 plus VAT, is "for advice and guidance only" |
Source: each retailer's terms, returns policy or measuring guide, fetched 19 September 2026. Research in lib/research/measure-windows-2026.md.
The law does not rescue you. Regulation 28(1)(b) of the Consumer Contracts Regulations 2013 excludes "goods that are made to the consumer's specifications or are clearly personalised" from the cancellation right. Trading Standards' Business Companion guidance names "double glazed windows that have been made specifically to the measurements for the customer's home" as an example, and adds that the exemption does not apply to "a standard sized window unit bought from a trade supplier". Which? is blunter: made-to-measure windows carry "no right to cancel". What survives is your right to goods that are not faulty and are made as ordered; Aluminium Trade Windows notes that its policy does not affect statutory rights where goods are faulty, damaged or not as described. The same exemption is why a fitted contract can come without a cooling-off period, which our double glazing finance guide covers.
Three practical points from the retailers' own terms. Aluminium Trade Windows strongly recommends not removing the old windows or booking a fitter until the new ones have been delivered and checked, and asks for delivery to be inspected within 72 hours. Changes are time-limited: Window Supply Direct allows cancellations or amendments within 48 hours of ordering, and Aluminium Trade Windows offers a goodwill refund within 24 hours of payment, before manufacture starts. Those are one retailer each, not the law, so read your supplier's terms before you pay. If the numbers still favour doing it yourself, the should you fit your own windows tool weighs them.
If you would rather someone else measured
On a fitted job the surveyor measures every opening, the installer answers for a frame that does not fit, and the certificate comes with it. If that is the route you want, tell us what you have and Leads 2 Trade will arrange quotes from installers covering your postcode. Our other double glazing guides cover the survey, the fitting and the paperwork.
Frequently asked questions
Measure the brickwork opening from outside, not the old frame, in millimetres. Take the width at the top, middle and bottom and the height at the left, centre and right, and use the smallest of each; do not average them. Check the two diagonals, compare the inside and outside reveals to spot a check reveal, and note whether a removable cill is included. Then take off the fitting allowance your supplier asks for, and tell them exactly what you measured.
From outside, brick to brick, for an order: every supply-only retailer we found asks for the external brickwork opening. Inside measurements, frame included, are fine for a rough price where a surveyor will measure again, which is how Eurocell's home route works. The trade guide asks for both to be compared, because an outside opening noticeably smaller than the inside can mean a check reveal.
The GGF good practice guide takes 10 mm in total off the width and off the height for white uPVC, timber and aluminium frames up to 3 m, and 15 mm for coloured or foiled uPVC, with more above 3 m. The figure is a total, not per side, which leaves about 5 mm each side. Most retailers we read use a flat 10 mm. At least one asks you to deduct and adds nothing itself, so ask each supplier which figure it wants and who does the deducting.
There is no published standard. The list several sites repeat, widths of 488, 630, 915, 1,200 and 1,770 mm, cites no source, and we found no British Standard or trade body list behind it. It does not match stock either: B&Q's stock uPVC casements come in widths of 440, 610, 820, 905, 1,190 and 1,770 mm, sharing only 1,770 with the list. The replacement suppliers we read make every window to order, so measure your own openings.
Usually not. Regulation 28(1)(b) of the Consumer Contracts Regulations 2013 excludes goods made to the consumer's specifications from the cancellation right, and Trading Standards' guidance gives double glazed windows made to the measurements of your home as an example. Supply-only retailers' terms say the same about wrong measurements: the loss is yours. A standard sized stock window is not covered by that exemption, and goods that are faulty or not made as ordered keep your statutory rights.
An opening where the outer brickwork is smaller than the inside, so the window frame sits behind the outer brick. NHBC says new homes in Scotland, Northern Ireland and areas of very severe exposure should use them, and box sash windows usually sit in one. Measuring the outside and deducting under-sizes the frame: two sash suppliers instead add 40 mm to the width and 20 mm to the height, and the trade guides want at least 12 mm of frame behind the brick on each side. Ask the supplier which it wants.
No. The calculator takes the number of windows, the style, frame material, glazing, fitted or supply-only, property type and region; it has no width or height field. Count your windows by style and enter the count. Size moves where you land within the per-window range, but the millimetres are for the supplier or surveyor, not the calculator.
Widths and angles from the inside, height from the outside, according to three supply-only retailers. Measure each section separately, note the angle between sections (usually 90 or 135 degrees), and measure the height brick to brick outside. Retailers treat the corner posts differently: one adds 140 mm for two 70 mm posts on a square bay. Travis Perkins says many homeowners have bay measurements confirmed by a professional installer, and the bay's structural load needs checking before removal.