Ever notice how old houses have weirdly sized windows?

Also, I read somewhere that you’re supposed to measure from “jamb to jamb” and not include the trim, but honestly, sometimes it’s hard to tell what counts as trim or jamb in these old houses.

That’s a super common headache. Older homes are all over the place with what’s considered “trim” vs “jamb”—sometimes you’ve got layers of paint or weird molding that hides the real edges. Here’s how I usually tackle it:

- Look for where the window actually stops and the wall begins. The jamb is basically the side of the window frame that touches the window sash (the part that moves), not the decorative wood on top.
- If you’re unsure, pop off a piece of trim carefully (pry bar + patience). You’ll see the actual frame underneath. That’s your true measuring point.
- If removing trim isn’t an option, shine a flashlight along the edge—sometimes you can spot where things transition.

A few other things I’ve run into:

- Like you mentioned, always measure top/middle/bottom (and left/right if it’s a tall window). I’ve seen some windows bow out so much that there’s nearly half an inch difference from top to bottom.
- Don’t trust old blueprints or “standard” sizes. I once ordered a whole set of replacements based on what was supposed to be a standard 32" opening… turned out every single one was different by at least 3/8". Ended up trimming and shimming every last one.
- Watch for settled sills—sometimes they slope down toward the outside if there’s been water damage. Measure both inside and outside if possible.

One trick: If you’re ordering custom windows and your measurements are weirdly close to a standard size, go just a hair smaller. It’s way easier to shim/fill gaps than try to force a too-tight fit. And don’t forget—old houses move with the seasons, so leave yourself some wiggle room.

Funny enough, I’ve found more oddball sizes in 1940s bungalows than in Victorians. Maybe they just eyeballed it back then… or maybe my house is just extra quirky.

Hope that helps someone avoid a costly mistake.

I totally get the confusion with old houses—mine’s from the 1920s and every window seems like it was custom built for that exact spot, no rhyme or reason. I once tried to measure for a new sash and ended up discovering three different “jambs” under layers of caulk and trim. What finally worked for me was running my finger along the inside edge until I hit bare wood, then using a small mirror to double-check what I was actually measuring. Still had to shave down the new sash a bit, but at least it fit. It’s wild how nothing is ever square or standard in these places... makes you wonder how anyone managed before laser levels.

Had the same headache with my 1915 place. The window openings are all over the map—nothing is plumb or level. Measuring for replacement sashes turned into a full-on forensic investigation.

"I once tried to measure for a new sash and ended up discovering three different “jambs” under layers of caulk and trim."

I ended up using a digital caliper for the final measurements, but still had to custom plane the frame. Even then, it was a tight fit on one side and a gap on the other—had to use backer rod and flexible sealant to compensate. Laser levels help, but honestly, nothing beats just taking your time and expecting a bit of trial and error.

I kind of appreciate the quirks now, but it definitely makes any “standard” window part basically useless. Custom seems like the only way unless you want to completely reframe, which... no thanks.

I ended up using a digital caliper for the final measurements, but still had to custom plane the frame.

Standard parts rarely fit these old frames, and even custom orders can be a gamble if the opening isn’t square. I usually end up scribing the new jambs to match the existing opening. It’s tedious, but it avoids big gaps or binding sashes.

Standard parts rarely fit these old frames, and even custom orders can be a gamble if the opening isn’t square.

That’s exactly what I’m running into with my place. It’s kind of frustrating, but honestly, seeing it all come together after all the scribing and planing feels worth it. Nice to know I’m not the only one dealing with these weird angles.

Totally get what you mean about the satisfaction after all that fitting and trimming. Sometimes I wonder if these frames shifted over time or if builders just eyeballed it back then. Have you run into any spots where the old plaster or trim made things even trickier? That’s tripped me up more than once.

Definitely ran into that—old plaster is a wild card. Sometimes you pull off the trim and there’s a half-inch gap or weird buildup from past repairs. Makes it tough to get anything square. I’ve had to shim behind new casings just to get things to line up. It’s like every window tells its own story... and not always a good one.

Haha, yeah, I thought it was just me being clumsy with a pry bar, but every time I mess with the trim in my place (1920s), it’s like opening a weird time capsule. Sometimes there’s a random chunk of wood stuffed behind the plaster, or the window’s off level by half an inch. I tried to square up one frame and ended up shimming so much it looked like a game of Jenga. Guess “standard size” just wasn’t a thing back then...

- Had the same thing in my 1915 place—nothing lines up, and every window is a different height off the floor.
- Once pulled off some baseboard and found a brick just sitting behind it. No idea why.
- “Standard size” is a myth with these old houses. You end up custom ordering or hacking things to fit.
- Honestly, sometimes I wonder if the builders were just using whatever was lying around... but I guess it’s held up this long, so maybe there’s something to it.