What Size Replacement Window Fits My Opening?
You measured the window and the numbers do not match anything on a shelf.
Reply within one business day. Or call / text 763-250-3621.
What size replacement window fits my opening? One built a little smaller than the opening it goes into: an insert is ordered to the inside of the old frame less a small gap for shims, a full-frame window to the rough opening less a bit more. JMB Builders measures every opening before ordering, Twin Cities metro. Send photos and we reply within one business day, or book a free in-home estimate.
- Window built to your measured opening, to the eighth inch
- Insert: to the old frame; full frame: to the rough opening
- A small gap for shims and foam, covered by trim
Replacement windows are made to order, to the eighth of an inch, in almost any width and height within the line's range. There is no standard size to fit; the window is built to fit your opening. The question is what you measure and how much you take off, and that depends on how the window goes in.
An insert window sits inside the old window frame. The size is the inside of that frame, jamb to jamb and sill to head, measured at three places each way, with the smallest numbers used, less a small gap, usually around a quarter inch each way, so the new frame slides in and can be shimmed plumb. A full-frame window replaces the whole old window, so its size is the rough opening, stud to stud and rough sill to header, less a bit more, often a half inch each way, for shims, the sill pan and foam. Either way the gap is foamed and covered by the trim; you never see it.
That is why the measure is in person. A quarter inch too big and the window does not go in; a half inch too small and the gap is more than the trim covers. We measure every opening ourselves before the order, write the deductions, and the window arrives the size the hole needs. A stock window in a standard size is used only when the opening happens to fit it, which is rare in an older house and common in a newer one.
Get a written quote for this job
Three photos and a sentence is enough: a reply within one business day, or ask for a free in-home estimate. Or text the photos to 763-250-3621.
What you supply
- Photos of each window from inside and outside, labeled by room
- Your own rough measurements, width and height, if you want a quote before the measure
- Whether you expect insert or full-frame, or want us to recommend
- Your window style, color and grille choices
What JMB supplies
- A measure of every opening in person before anything is ordered: width and height at three places each, plus the depth
- The call on insert vs. full frame, written per window on the quote
- The deductions worked out and the windows ordered through ABC Supply to the eighth inch
- Install with shims, sill pan, flashing, foam and sealant, trimmed inside and out
- Photos of the finished windows in your job folder
How it is priced: per window
Quoted in writing from your photos, with a reply within one business day. What moves the number:
- Custom size is the normal order and sets no extra charge on most lines; very large or odd shapes do
- Insert vs. full frame, which changes what is measured and how much trim is replaced
- Window style, glass and frame color
- A stock size that happens to fit, which can be quicker and cheaper
- Openings out of square or with rot that change the size once the old window is out
Send your measurements with the photos and we quote from them; the order goes in only after we measure in person, so a small error on your side costs nothing.
How long it takes
Measure: a few minutes per window. Custom-sized windows run with the line's lead time; install is about half a day per window.
How your project works
- 1
Send photos, get a written quote
Send three photos of the space and a sentence about what you want. You get a clear, itemized reply within one business day. Prefer a look in person? A free in-home estimate is always an option.
- 2
Pick materials, or bring your own
We help you choose profiles and materials that fit the house and the budget, or match your existing millwork exactly — or you buy the door or trim yourself and we install it. Everything is confirmed before the crew is scheduled.
- 3
Clean, precise installation
The crew shows up on time, protects floors and furniture, and installs with tight joints and consistent reveals. Photos of the finished work go in your job folder.
- 4
Walkthrough and invoice
We walk the finished work with you. If anything is not right, we fix it before we call the job done. Then one invoice, paid by check or direct deposit — no card fees, no payment links.
What Size Replacement Window Fits My Opening?: questions, answered
Is a replacement window the same size as the opening?
No. It is built a little smaller than the opening it goes into, so it can be slid in, shimmed plumb and level, and foamed. The gap is small, a fraction of an inch on each side, and it is covered by the interior stop or casing and the exterior trim. A window the exact size of the opening cannot be set straight.
How much smaller is the window than the opening?
For an insert into the old frame, around a quarter inch less than the inside of the frame in width and height. For a full-frame window into the rough opening, closer to a half inch less each way, to leave room for shims, the sill pan and foam. The exact deduction depends on the line and the install, and we work it out at the measure.
Can I order a standard size and fill the gap?
Only when the gap is small. A window more than about a half inch short of the opening in either direction leaves a gap the trim does not cover and the foam should not fill, and a filler strip to make up the difference is a weak point for water. Replacement windows are custom sized for the same price on most lines, so there is no reason to force a stock size.
Can a replacement window be bigger than the old one?
Yes, with framing. Making the opening wider or taller means cutting the wall, resizing the header, and patching siding and drywall; it is a framing job with the window, and often a permit. Within the existing opening, a full-frame replacement gains some glass over an insert, because the old frame is gone and the new one fills the rough opening.
Do you measure before ordering, or use my numbers?
We measure. Your numbers get you a quote; our measure of every opening in person, with the deductions written down, is what the order is placed from. A window is a custom-built part and it cannot be returned for being the wrong size, so the measure is ours to get right.
What is the difference between insert size and full-frame size?
An insert is sized to the inside of the old window frame, because the old frame stays and the new window sits inside it. A full-frame window is sized to the rough opening, the framed hole in the wall, because the old frame comes out. The full-frame window is larger, with more glass, and the insert is a quicker install that keeps the old trim.
Recent JMB work, from our job log
59 installs recorded across 30 Twin Cities communities since Dec 2025. Window and door installs for homeowners and the builders we work with. Counts come straight from our job log; no customer names.
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Read morePage reviewed October 27, 2026 · JMB Builders LLC, MN license BC809005 · All windows pages
Where we work
Window Replacement & Installation near you
Based in Zimmerman, scheduling across the Twin Cities metro. Closest first: