{"id":3599,"date":"2026-10-29T16:15:34","date_gmt":"2026-10-29T08:15:34","guid":{"rendered":"http:\/\/www.amaroneonline.com\/blog\/?p=3599"},"modified":"2026-10-29T16:15:34","modified_gmt":"2026-10-29T08:15:34","slug":"how-do-i-choose-the-right-size-of-upvc-windows-44c0-6d1bd5","status":"publish","type":"post","link":"http:\/\/www.amaroneonline.com\/blog\/2026\/10\/29\/how-do-i-choose-the-right-size-of-upvc-windows-44c0-6d1bd5\/","title":{"rendered":"How do I choose the right size of UPVC windows?"},"content":{"rendered":"

Hey there! If you\u2019ve ever stared at a set of blueprints for your home, squinted at the window openings you measured, and wondered, \u201cWait\u2014did I get this right?\u201d you\u2019re not alone. As someone who\u2019s been in the UPVC window game for years (no stuffy office, just a guy who\u2019s helped homeowners, builders, and even small contractors nail the right size more times than I can count), I get it. Picking the wrong UPVC window size isn\u2019t just a \u201clittle oops\u201d\u2014it means ill-fitting seals, drafts that turn your heating bill into a horror show, or worst of all, having to dump a set of windows that cost you a chunk of cash. Today, let\u2019s break this down like we\u2019re chatting over a coffee\u2014no jargon, no confusing spreadsheets, just the real, actionable stuff you need. UPVC Windows<\/a><\/p>\n

<\/p>\n

First off, let\u2019s get one thing straight: UPVC window sizes aren\u2019t arbitrary. They\u2019re tied directly to the opening in your wall, right? That\u2019s the starting point, but here\u2019s where most folks mess up\u2014they measure the finished opening, not leaving room for the frame, the sill, and those little gaps the installers need to make sure it sits level and doesn\u2019t leak. I\u2019ve seen a guy measure his bathroom window as 1200mm wide x 900mm tall, skip the gap, and end up with a window that jammed 5mm too big on all sides. The installers had to whittle at the frame, which messed up the insulation. Not cool.<\/p>\n

So how do you actually measure correctly? Let\u2019s start with the basics\u2014you need a decent tape measure, not the flimsy 1m one you keep in your junk drawer. A 5m or 8m steel tape works best, it doesn\u2019t bend all over the place. Now, for each window opening, you need three measurements: width, height, and depth. Let\u2019s go one by one.<\/p>\n

Width: Don\u2019t just measure the top, or the middle, or the bottom of the wall opening. Walls aren\u2019t perfectly straight\u2014your house has been settling for decades, maybe a bit of damp shifted a stud here and there. Measure the width at the TOP, MIDDLE, and BOTTOM of the rough opening (that\u2019s the actual hole in the wall before you add any plaster or trim). Write down the smallest of those three numbers. That\u2019s your width to work with. Why smallest? Because if you go for a window that fits the top, it\u2019s gonna be too big at the bottom, and we all know what happens then\u2014you force it in, warp the frame, and goodbye to airtight seals.<\/p>\n

Height is the same vibe\u2014measure the left side, right side, and middle of the opening. Grab the smallest number here too. Again, walls aren\u2019t level, so that middle spot might be a centimeter shorter on one side. Use the smallest to avoid headaches later.<\/p>\n

Depth is the one people sleep on, and it\u2019s super important for UPVC windows because UPVC frames have a standard thickness, but some are deeper for insulation. The depth is how thick the wall is at that window spot\u2014you measure from the outside of the wall to the inside. If your wall\u2019s a standard brick wall, that\u2019s usually around 100mm, but if it\u2019s a cavity wall (most modern homes), that\u2019s 200mm or more. If you get this wrong, the window frame will stick out too far inside or outside, making the whole place look off, or not sit flush with the trim. I had a customer once who bought a window with a 70mm depth for a 150mm cavity wall\u2014half the frame was hanging inside, like a shelf, and he had to buy extensions. Wasted time, wasted money.<\/p>\n

Now, here\u2019s the rule most installers and I swear by: once you have your three smallest numbers (width, height, depth), you need to subtract about 10-15mm total from the width and 10-15mm total from the height. Wait, why both? That little gap\u2014called the \u201cworking tolerance\u201d\u2014is for the foam insulation, the shims the installers use to level the window, and a tiny bit of wiggle room if the frame\u2019s got a little manufacturing quirk. If you skip this gap, the window is gonna be too tight to fit, and you risk cracking the frame when you hammer it in, or not being able to adjust it to be plumb.<\/p>\n

Wait, but what if you\u2019re replacing an old window? That\u2019s a whole other scenario, right? A lot of folks are swapping out their aluminum or wooden windows for UPVC because they last longer, don\u2019t rot, and are way better at keeping heat in. When you\u2019re replacing, don\u2019t just measure the old window\u2019s size\u2014measure the opening that the old window is sitting in, same as we talked about for new builds. The old window might have been forcing itself into the wall, so the frame is bigger than the actual hole. I\u2019ve seen people grab the old window\u2019s label (which says, say, 1200mm x 900mm) and buy that exact size, only to realize the old label is for the window itself, not the wall opening. That\u2019s a rookie move. Always measure the opening, not the old window.<\/p>\n

Now, let\u2019s talk about standard vs. custom sizes, because that\u2019s a big one. Most of the time, you\u2019ll be looking at standard UPVC window sizes\u2014like 600x600mm for a bathroom, 1200x1200mm for a living room double window, 1500x900mm for a kitchen. Standard sizes are cheaper, faster to get, and the installers know exactly how to handle them. Custom sizes are for if you have a super unique opening\u2014like a curved window in a bay, or a tiny window in a attic that\u2019s not a standard square. But custom comes with a catch: it\u2019s more expensive, takes longer to make, and you have to measure perfectly because there\u2019s no wiggle room for standard sizing. I always tell customers: if your opening is even 5mm off a standard size, that\u2019s worth custom, not forcing a standard window that doesn\u2019t fit.<\/p>\n

Wait, another thing: window type affects size too. Are you getting a casement window, a sliding sash, a fixed pane? For casement windows, the ones that swing open, you need to leave room for the window to open. If you put a casement window right next to a wall or another window, it can\u2019t swing out, which defeats the whole purpose of a casement. So when you\u2019re measuring, if it\u2019s a casement, leave an extra 50-100mm clearance on the side it\u2019s going to swing open. Sliding sash windows are different\u2014they slide horizontally, so clearance isn\u2019t as big an issue, but you need to make sure the tracks fit the opening. Fixed windows are just panes that don\u2019t open, so they\u2019re the most flexible, but you still need to get the size right for insulation.<\/p>\n

Let\u2019s also talk about a few common mistakes I see all the time, just so you can avoid them. First, measuring in the wrong units. A lot of countries use mm, some use inches\u2014if you mix them up, you\u2019re toast. I once had a guy from the US order a window that was 36 inches wide, and he told me it was 36mm. No, that\u2019s not a typo, he actually said mm. The window was like 1.4 inches wide, useless for a house. Double check your units, always. Second, forgetting the sill. The bottom of the window needs to sit on a solid sill, right? So when you measure height, don\u2019t go all the way to the floor, leave a little room for the sill\u2014usually 10-15mm. Third, not checking with the installer first. Installers are the ones putting these in every day\u2014they know what gaps work for their tools, what sizes cause the least headaches. If you\u2019re not sure, shoot them a pic of your measurements, or even send me a pic (full transparency here, if you reach out) and I\u2019ll help double check.<\/p>\n

Wait, let\u2019s get practical. Let\u2019s say you have a new build, the wall opening for a kitchen window is 1210mm wide at all three spots, 910mm tall at all three, and the wall depth is 150mm. So your smallest numbers are 1210 and 910. Subtract 15mm from each, so 1195mm wide, 895mm tall. That\u2019s the size you order. Easy. If that\u2019s a standard size (say, 1200×900 is common, so this is just a tiny bit under), that\u2019s perfect for a standard window, and you\u2019ve got the gap for foam and shims. If your opening was 1250mm wide, that\u2019s a 1250 size, which is custom, no big deal, but you pay a bit more.<\/p>\n

Also, let\u2019s not forget about the finish. If you\u2019re planning to add plaster, tile, or trim around the window, make sure you leave enough room for that too. For example, if you\u2019re tiling around a bathroom window, the window shouldn\u2019t stick out past the tile line. So when you measure depth, account for the tile thickness, or the plaster on the interior and exterior. That\u2019s another small thing that trips people up.<\/p>\n

What about if you\u2019re on a budget? Standard sizes are definitely cheaper, so I always recommend fitting your opening to a standard size if you can, instead of making a custom opening. A good builder can adjust the wall a little to make the opening fit a standard window, which is way cheaper than paying for custom UPVC. But if the opening is already there (like when replacing), then measure exactly and go with what fits, even if it\u2019s a custom.<\/p>\n

Another point: building codes. Wait, right, you can\u2019t just pick any size. Depending on where you live, there are rules about window size for fire escape, ventilation, energy efficiency. For example, bedrooms usually need a window that\u2019s at least 5% of the floor area for ventilation, so if your bedroom is 20 sqm, your window needs to be at least 1 sqm. That\u2019s not something you can ignore\u2014if you pick a window too small, you\u2019ll have to replace it later to meet code. So before you order, check your local building regs, or I can help with that too, just shoot me a message.<\/p>\n

Wait, let\u2019s address a question I get all the time: \u201cIs there a way to order too big?\u201d No, not really\u2014if it\u2019s only a little too big, installers can adjust, but if it\u2019s way too big, you\u2019ll have to cut the wall, which is a huge mess. But way too small is worse, because then you\u2019ve got gaps around the window that let in drafts, water, bugs. That\u2019s a way bigger problem than a little too big. So err on the side of measuring the opening correctly, using the smallest numbers, and leaving the working gap.<\/p>\n

Let me share a quick story, because stories stick. A few months back, a guy named Jake reached out to me, super stressed. He was replacing his living room window, had measured the old window\u2019s size (which was 1500x1000mm), ordered that exact size, and when he tried to fit it, it was 20mm too big on all sides. The old window had been forced into the wall, so the opening was actually 1460x960mm. He\u2019d ordered 1500×1000, which was way off. By the time he reached me, he\u2019d already tried to hammer it in, warped the frame, so he had to order a custom size, pay extra, and wait 2 weeks longer. He told me he lost almost $500 in extra costs, just because he didn\u2019t measure the opening, just the old window. That\u2019s the kind of mistake I want you to avoid.<\/p>\n

So to wrap this up, let\u2019s make it a step-by-step, no-fluff checklist you can pull out right now:<\/p>\n

    \n
  1. Grab a steel tape measure (not the flimsy one), pen and paper.<\/li>\n
  2. For your window opening, measure width at top, middle, bottom\u2014write down the SMALLEST number.<\/li>\n
  3. Measure height at left, right, middle\u2014write down the SMALLEST number.<\/li>\n
  4. Measure wall depth (outside to inside of the opening) and note that down.<\/li>\n
  5. Subtract 10-15mm from width and 10-15mm from height to get your working size.<\/li>\n
  6. Check if that working size is a standard UPVC window size\u2014if yes, go standard. If not, custom is fine.<\/li>\n
  7. If it\u2019s a casement window, add 50-100mm clearance on the swing side.<\/li>\n
  8. Double check building codes for ventilation\/fire escape, don\u2019t skip that.<\/li>\n<\/ol>\n

    And a final tip: if you\u2019re at all unsure about your measurements, don\u2019t guess. Send me a pic of the opening, list out your measurements, and I\u2019ll help you work through it. I\u2019ve seen way too many people get stuck because they tried to do it alone, and it\u2019s not worth the hassle.<\/p>\n

    Wait, I should also mention that UPVC is solid, but it\u2019s not unbreakable. When you get your window, don\u2019t open it right away once you take it out of the packaging\u2014let it acclimatize to the room temperature for a few hours, especially if it\u2019s cold outside. The frame might contract a little, so opening it too early can warp it. That\u2019s another small thing that affects fit, but it\u2019s easy to remember.<\/p>\n

    At the end of the day, choosing the right UPVC window size is all about being precise, not perfect. You don\u2019t need to get every measurement to the exact millimeter, but you do need to account for gaps, walls not being straight, and what the window actually needs to do (open, insulate, fit the space). If you follow those steps, you\u2019ll avoid the headaches Jake had, and end up with windows that fit like a glove, last for decades, and save you money on heating.<\/p>\n

    <\/p>\n

    If you\u2019re ready to get your windows ordered, or just want to run your measurements by someone who\u2019s done this a hundred times, hit me up. No pushy sales stuff, just honest help to get you the right window for your home.<\/p>\n

    Aluminum Windows<\/a> References:<\/p>\n

      \n
    1. British Standards Institution. (2015). BS EN 14351-1: Windows and doors. Product standard, performance characteristics. External pedestrian doorsets.<\/li>\n
    2. National Fenestration Rating Council. (2022). Fenestration Installation Best Practices Guide.<\/li>\n
    3. U.S. Department of Energy. (2021). Energy Efficiency Guidelines for Residential Windows.<\/li>\n<\/ol>\n
      \n

      Wuhu Xupin Architectural Decoration Engineering Co., Ltd.<\/a>
      As one of the most professional UPVC windows manufacturers and suppliers in China, our products have good reputation in the market. Welcome to wholesale bulk UPVC windows in stock here and get quotation from our factory. Customized orders are welcome.
      Address: No. 11 Weiba Road, Fangcun Industrial Concentration Zone, Jinghu District, Wuhu City, Anhui Province
      E-mail: 13470752088@163.com
      WebSite:       https:\/\/www.xupinwindows.com\/<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

      Hey there! If you\u2019ve ever stared at a set of blueprints for your home, squinted at … How do I choose the right size of UPVC windows?<\/span>Read more<\/a><\/p>\n","protected":false},"author":4,"featured_media":3599,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[3562],"class_list":["post-3599","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-industry","tag-upvc-windows-47ac-6d55ab"],"_links":{"self":[{"href":"http:\/\/www.amaroneonline.com\/blog\/wp-json\/wp\/v2\/posts\/3599","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/www.amaroneonline.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.amaroneonline.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.amaroneonline.com\/blog\/wp-json\/wp\/v2\/users\/4"}],"replies":[{"embeddable":true,"href":"http:\/\/www.amaroneonline.com\/blog\/wp-json\/wp\/v2\/comments?post=3599"}],"version-history":[{"count":0,"href":"http:\/\/www.amaroneonline.com\/blog\/wp-json\/wp\/v2\/posts\/3599\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"http:\/\/www.amaroneonline.com\/blog\/wp-json\/wp\/v2\/posts\/3599"}],"wp:attachment":[{"href":"http:\/\/www.amaroneonline.com\/blog\/wp-json\/wp\/v2\/media?parent=3599"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.amaroneonline.com\/blog\/wp-json\/wp\/v2\/categories?post=3599"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.amaroneonline.com\/blog\/wp-json\/wp\/v2\/tags?post=3599"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}