--- title: How to Measure Windows for Replacement description: Learn how to measure windows for replacement windows accurately, with a clear step by step guide from a York installer so your new frames fit first time. --- [![SM\_Logo\_Final\_Lught](https://smwindowsanddoors.co.uk/hs-fs/hubfs/SM_Logo_Final_Lught.webp?width=140&height=80&name=SM_Logo_Final_Lught.webp "SM_Logo_Final_Lught")](https://smwindowsanddoors.co.uk) - About - [About](https://smwindowsanddoors.co.uk/about) - Doors - [uPVC Doors](https://smwindowsanddoors.co.uk/upvc-doors-york) - [Sliding & Bifold Doors](https://smwindowsanddoors.co.uk/aluminium-sliding-bi-fold-doors) - [Composite Doors](https://smwindowsanddoors.co.uk/composite-doors-york) - [Heritage Aluminium Doors](https://smwindowsanddoors.co.uk/heritage-aluminium-windows-doors) - [Door Repairs](https://smwindowsanddoors.co.uk/window-door-repairs-york) - Windows - [uPVC Windows](https://smwindowsanddoors.co.uk/upvc-windows-york) - [Casement Windows](https://smwindowsanddoors.co.uk/casement-windows-york) - [Sash Windows](https://smwindowsanddoors.co.uk/sash-windows-york) - [Timber Windows](https://smwindowsanddoors.co.uk/timber-windows-york) - [Heritage Aluminium Windows](https://smwindowsanddoors.co.uk/heritage-aluminium-windows-doors) - [Lanterns & Roof Lights](https://smwindowsanddoors.co.uk/korniche-lanterns-flat-roof-lights) - [Window Repairs](https://smwindowsanddoors.co.uk/window-door-repairs-york) - Conservatories - [Conservatories](https://smwindowsanddoors.co.uk/conservatories-orangeries-york) - [Orangeries](https://smwindowsanddoors.co.uk/conservatories-orangeries-york) - Resources - [Gallery](https://smwindowsanddoors.co.uk/gallery) - [Brochures](https://smwindowsanddoors.co.uk/brochure-downloads) - [Blog](https://smwindowsanddoors.co.uk/blog) - [Reviews](https://smwindowsanddoors.co.uk/customer-reviews) - [Contact Us](https://smwindowsanddoors.co.uk/contact) [Request A Quote](https://smwindowsanddoors.co.uk/request-a-quote) [![SM\_Logo\_Final\_Lught](https://smwindowsanddoors.co.uk/hs-fs/hubfs/SM_Logo_Final_Lught.webp?width=140&height=80&name=SM_Logo_Final_Lught.webp "SM_Logo_Final_Lught")](https://smwindowsanddoors.co.uk) [Request A Quote](https://smwindowsanddoors.co.uk/request-a-quote) - About - [About](https://smwindowsanddoors.co.uk/about) - Doors - [uPVC Doors](https://smwindowsanddoors.co.uk/upvc-doors-york) - [Sliding & Bifold Doors](https://smwindowsanddoors.co.uk/aluminium-sliding-bi-fold-doors) - [Composite Doors](https://smwindowsanddoors.co.uk/composite-doors-york) - [Heritage Aluminium Doors](https://smwindowsanddoors.co.uk/heritage-aluminium-windows-doors) - [Door Repairs](https://smwindowsanddoors.co.uk/window-door-repairs-york) - Windows - [uPVC Windows](https://smwindowsanddoors.co.uk/upvc-windows-york) - [Casement Windows](https://smwindowsanddoors.co.uk/casement-windows-york) - [Sash Windows](https://smwindowsanddoors.co.uk/sash-windows-york) - [Timber Windows](https://smwindowsanddoors.co.uk/timber-windows-york) - [Heritage Aluminium Windows](https://smwindowsanddoors.co.uk/heritage-aluminium-windows-doors) - [Lanterns & Roof Lights](https://smwindowsanddoors.co.uk/korniche-lanterns-flat-roof-lights) - [Window Repairs](https://smwindowsanddoors.co.uk/window-door-repairs-york) - Conservatories - [Conservatories](https://smwindowsanddoors.co.uk/conservatories-orangeries-york) - [Orangeries](https://smwindowsanddoors.co.uk/conservatories-orangeries-york) - Resources - [Gallery](https://smwindowsanddoors.co.uk/gallery) - [Brochures](https://smwindowsanddoors.co.uk/brochure-downloads) - [Blog](https://smwindowsanddoors.co.uk/blog) - [Reviews](https://smwindowsanddoors.co.uk/customer-reviews) - [Contact Us](https://smwindowsanddoors.co.uk/contact) [Windows](https://smwindowsanddoors.co.uk/blog/tag/windows) # How to Measure Windows for Replacement By [SM Windows & Doors](https://smwindowsanddoors.co.uk/blog/author/sm-windows-doors) Nov 10, 2026, 2:30:00 PM 3 Minutes Read If you want to know how to measure windows for replacement windows, the short version is this: measure the opening in several places, always work to the smallest figure, and give the width first then the height. Get that right and your new frames will fit snugly with room to seal. Here is how to do it properly, step by step. ## Before you start: what you will need You do not need much. A decent steel tape measure (a cloth one stretches and lies to you), a pen, a notepad and a steady pair of hands will do. If the window is upstairs or awkward to reach, get someone to hold the other end or steady the ladder. Always measure from inside the house where you can. Write everything down as you go. Trying to hold four or five figures in your head is how mistakes creep in, and a mistake here is an expensive one. ## Measure the width in three places Windows are rarely as square as they look, especially in York's older terraces where a century of settling has taken the walls out of true. So you measure the width in three spots: - Across the **top** of the opening - Across the **middle** - Across the **bottom** Measure from one side of the brickwork opening to the other, not the existing frame. You want the size of the hole the window sits in. The three figures will often differ by a few millimetres. Note all three, then use the **smallest** one as your working width. A frame built to the smallest measurement will slide in; one built to the largest will jam. ## Measure the height in three places Now do the same for the height: - Up the **left** side - Up the **middle** - Up the **right** side Measure from the underside of the top of the opening down to the sill. Again, take the smallest of the three as your working height. On older windows the sill can slope, so be sure you are measuring to the point the new frame will actually sit on. ## Always give width before height This is the convention the whole trade works to: width first, then height. A window written as 900 by 1200 means 900mm wide and 1200mm tall. Get those two the wrong way round and you can end up with a landscape window where you wanted a portrait one. When you write your notes, label them clearly so there is no doubt. ## Check the depth and the sill The opening is not just a flat rectangle. Have a look at the depth of the reveal (how thick the wall is) and the state of the sill. A rotten timber sill or crumbling render underneath the old window will need sorting before anything new goes in. It is worth noting these things now so whoever quotes you has the full picture. ## Why a survey still matters Here is the honest bit. Measuring your own windows is a genuinely useful thing to do. It helps you understand the job, spot problems early and get a rough idea of what you are dealing with before anyone visits. But no reputable installer will order frames off a figure you jotted down. The person fitting the window carries the responsibility for it fitting, so they will always come and do a proper survey with their own tools before anything is made. Think of your measurements as a helpful starting point, not the final word. If you are getting quotes on our [uPVC windows](https://smwindowsanddoors.co.uk/upvc-windows-york) or [casement windows](https://smwindowsanddoors.co.uk/casement-windows-york), your own numbers help the conversation along, and we take the binding measurements ourselves at survey. ## Common mistakes to avoid A few things trip people up every time: - **Measuring the old frame instead of the opening.** Always measure the hole, not the window sitting in it. - **Using the biggest figure.** Go with the smallest width and smallest height, every time. - **Forgetting the tape can flex.** Keep it straight and level, not sagging. - **Rounding up.** Note the exact millimetres. Rounding loses you the precision that matters. If your existing windows are simply tired rather than beyond help, it is also worth asking whether a repair would do. We handle [window repairs across York](https://smwindowsanddoors.co.uk/window-door-repairs-york), and sometimes a new sealed unit or a bit of hardware saves you a full replacement. ## FAQs ### How do I measure a window for replacement? Measure the width across the top, middle and bottom of the opening, and the height up the left, middle and right. Use the smallest width and smallest height, and always give width before height. Measure the brickwork opening, not the old frame. ### Should I measure the frame or the opening? Measure the opening, the actual hole in the wall, not the existing frame. The new frame needs to fit into that space with a little room for sealing and packing, so the opening size is what counts. ### Why measure a window in three places? Because openings are rarely perfectly square, particularly in older homes. Measuring in three spots shows you the true smallest size, and building to that ensures the new frame will fit rather than jam. ### Do I need to measure if the installer surveys anyway? You do not have to, but it helps. Your own figures give you a feel for the job and a rough idea before anyone visits. The installer always takes the final binding measurements themselves, as they carry the responsibility for the fit. ### What does 900 by 1200 mean on a window? It means 900mm wide by 1200mm tall. The trade always writes width first, then height, so keep to that order to avoid getting a window made the wrong way round. Ready to move on from measuring to choosing? [Download our latest brochure](https://smwindowsanddoors.co.uk/brochure-downloads) to see the full range, or [request a free quote](https://smwindowsanddoors.co.uk/request-a-quote) and we will come and measure up properly. #### SM Windows & Doors Author ## Phone: 07833387823 Email: [Sam@smwindowsanddoors.co.uk](mailto:Sam@smwindowsanddoors.co.uk) Opening times: Mon - Fri: 09:00 - 17:00 Sat-Sun: By Prior Appointment [Follow us on Facebook](https://www.facebook.com/people/SM-Windows-Doors/100093013267883/) [Follow us on Instagram](https://www.instagram.com/smwindowsanddoors/) ![x59dcc190ea7d320001f0083a\_logoAsset-2-1.jpg.pagespeed.ic.lk8k-280iY](https://smwindowsanddoors.co.uk/hs-fs/hubfs/x59dcc190ea7d320001f0083a_logoAsset-2-1.jpg.pagespeed.ic.lk8k-280iY.jpg?width=300&height=106&name=x59dcc190ea7d320001f0083a_logoAsset-2-1.jpg.pagespeed.ic.lk8k-280iY.jpg "x59dcc190ea7d320001f0083a_logoAsset-2-1.jpg.pagespeed.ic.lk8k-280iY") ```json { "@context" : "https://schema.org", "@type" : "BlogPosting", "author" : { "@type" : "Person", "name" : "SM Windows & Doors", "url" : "https://smwindowsanddoors.co.uk/blog/author/sm-windows-doors" }, "dateModified" : "2026-11-10T14:30:00.604Z", "datePublished" : "2026-11-10T14:30:00.000Z", "headline" : "How to Measure Windows for Replacement", "mainEntityOfPage" : { "@id" : "https://smwindowsanddoors.co.uk/blog/sm-windows-doors-blog/how-to-measure-windows-for-replacement", "@type" : "WebPage" }, "publisher" : { "@type" : "Organization", "logo" : { "@type" : "ImageObject", "url" : "https://smwindowsanddoors.co.uk/hubfs/SM_Logo_Final_Lught.webp" }, "name" : "SM Windows & Doors" } } ```