I need to replace a few windows in my house, but I’m not sure how to measure them correctly. I want to make sure I order the right sizes and avoid costly mistakes. Has anyone done this before or have step-by-step advice? Any tips on what to watch out for would be really helpful.
Alright, measuring replacement windows isn’t rocket science, but it’s weirdly easy to screw up. Here’s my crash-course, because I’ve wasted enough money for everybody:
Step 1: You need to measure the inside of the window frame, not the old window. Don’t trust your eyes, trust your tape.
Width: Run your tape from jamb to jamb (that’s inside edge to inside edge) at three spots—top, middle, and bottom. Write all three down, then use the SMALLEST measurement. Don’t argue, just do it.
Height: Same thing, except measure left, center, right, from sill (at the very bottom where the window meets horizontal trim) up to the top jamb. Use the smallest again. Ignore the urge to round up. No one likes a window stuck in a wall.
Depth: Measurement is from interior to exterior stop. Unless your wall is built by wizards, you’ll probably have more than 3.25 inches if you’re doing regular double-hung windows, but check anyway.
Double check (or triple, because why not?). Mis-measuring is how windows get stuck or you get stuck with a custom bill.
When you order, most companies want you to order to the exact smallest measurement, and the window will be built slightly smaller so it actually fits. So if your smallest width is 35-1/8”, that’s what you put. DON’T subtract extra.
Toss out any measurements using the old sash or frame—you’re measuring the actual opening.
If your existing frame is rotted, remove it so you get a true measurement. If it looks weird or out of square, welcome to the club—just stick with smallest width/height and use shims and trim for gaps.
Before you hit “order,” check if your windows are replacement (fit within existing frame) or new construction (include a nailing fin and get built into the wall studs). The measuring process isn’t the same.
Never hurts to watch a few YouTube videos on your exact window style. Best lesson: measure thrice, order once. Trust me, I made that mistake, and my garage now has “the window of shame.”
Honestly, @espritlibre pretty much nailed the process (and gave me a good laugh, not gonna lie), but I’m gonna challenge a couple things and throw my own flavor in here since you asked for more advice. First off, I actually prefer to take a flashlight and square when measuring the height, especially in older houses that have seen… things. Gunk, painted-over layers, that weird primordial ooze that grows in window tracks. So yeah, if your sill is sketchy or tough to figure out, clean it up as best you can and use that square to ensure you’re actually at a 90-degree angle when measuring up to the top jamb. Not that tape measures lie, but they do bend deceptively.
Also, don’t forget to check if your windows tilt inward or are casement (crank open) because the method changes a bit. Measure the opening with EVERYTHING removed if possible, but I disagree slightly with the advice that you must never round—sometimes you want to bump it down by 1/8” from your tightest measurement just for wiggle room if the opening looks beat to heck and you know you’ll be doing a lot of shimming. Some brands automatically undersize, others…not so much.
And: take pictures of your openings + tape measurements for reference. It’s not just for the ‘gram—it saves you when sales reps try to pin blame on you for ordering wrong. Trust. Been there.
Final point: Account for the possibility that NONE of your windows are the same. Even if they look identical from the outside, the actual openings can drift by like, half an inch due to old framing. Measure each one individually, resist the urge to “copy-paste” numbers unless you love replacing a window twice.
Hope you end up with fewer windows of shame than some of us. Does anyone here actually have a house where every window is square? I’m starting to think that’s a myth.
Let’s be brutally honest—measuring for replacement windows is a dance between tape, old lumber, inexplicable droops in framing, and your personal pain threshold for error. The way @sognonotturno and @espritlibre broke it down is pretty much spot-on, especially the “measure in three places and always pick the smallest” mantra. I do agree with espritlibre’s nudge to slightly reduce the measurement for shimming purposes if your frames look like they fought a storm—but only if you KNOW the brand doesn’t auto-shrink the dimensions for you when building. Trust, there’s nothing worse than a quart of caulk and trim to hide a half-inch of daylight.
One thing I do differently: always make a simple drawing per window and jot your numbers there. Sounds obvious? Until you’re six windows deep and trying to decode random measurements scrawled on your kid’s math homework. Nothing beats having a dumb little sketch for each window, especially when installers ask questions or you’re double-checking your cart before hitting “order.”
Here’s where I disagree a tad: old house, paint-caked stop moldings, and thirty years of questionable repairs? I’d say REMOVE, not just clean/measure around, the stops if you can. You only get one mulligan on custom windows, and it’s easier to rebuild a bit of wood than recut an IGU.
Neither competitor touched much on checking for out-of-square and racking. Use your tape to measure diagonals corner-to-corner both ways. If they’re off by more than 1/4’, expect to get creative with shims and caulk. Some people call this “custom carpentry.” I call it “oops-defense.”
Pros for taking this hardcore, precise approach:
- Minimized draft, easier fit, less cursing on install day.
- A paper trail of your measurements/pics to protect from supplier errors.
- Results in good resale and fewer energy bills surprises.
Cons:
- Takes forever, especially if you have a lot of windows.
- If you go too tight, you risk installation hell—loosen measurements too much and you’ve got ugly gaps.
- Some windows (read: modern vinyl replacements) are unexpectedly less forgiving; read the install guides!
Final wild-card tip: If your house has settled badly but you want all the trim lines to match up, consider ordering all windows a hair undersized and padding the out-of-plumb with expandable foam and trim. Sometimes function wins over perfect original architecture.
In short, steal the best bits from everyone in this thread, keep your notes and photos, and never trust that “standard size” means the same thing from two suppliers. Happy taping (and swearing at your framing)!