How to Measure for Replacement Windows

To measure for a replacement window, installers take the width at three points (top, middle and bottom) and the height at three points (left, center and right), and use the smallest of each. They also check the depth of the existing frame and whether the opening is square by comparing its two diagonals. That is the method in Andersen's and JELD-WEN's measuring guides ¹².

The smallest number wins because a window that is a little small can be shimmed, sealed and trimmed, while a window that is too big will not go into the opening at all. An old frame is rarely a perfect rectangle, so the narrowest point is the size that is certain to fit. Your own measurements are good for comparing quotes; the order is usually placed from the installer's.

Why measuring matters more than it looks

A replacement window is usually made to order for the opening. JELD-WEN's guide for its pocket (replacement) windows says windows made to user-specified dimensions cannot be returned ². A wrong measurement can mean a window that will not fit and a second order. And a window that is badly fitted performs badly: the U.S. Department of Energy says even the most energy-efficient windows can result in a drafty house and moisture condensation if they are not properly installed ³.

Which opening is measured depends on the kind of replacement. An insert goes inside the existing frame, so the frame's inside dimensions matter. A full-frame replacement removes the old frame and goes into the rough opening. Full-frame vs. insert window replacement explains the difference, and standard window sizes explains how size codes and rough openings relate.

Step 1: Width at three points

Andersen's guide says to measure from inside, from jamb to jamb (or drywall to drywall if there is no trim), near the top, at the center and near the bottom, round down to the nearest 1/8 inch, and use the smallest measurement as the width ¹.

JELD-WEN's guide for its pocket windows is more specific about where the tape goes: wood surface to wood surface between the side jambs, temporarily removing jamb liners if needed and avoiding stops, weatherstripping, balance cords and pulleys ². Measuring to a stop or a liner instead of the jamb gives a number that is too small, and the gap shows up later as extra trim work.

Step 2: Height at three points

Height runs from the head (the top of the frame) to the sill. Andersen says to measure at the left, center and right, to the nearest 1/8 inch, and use the smallest ¹. JELD-WEN says to measure at the head directly from the wood, avoiding stops, weatherstrip and liners, and at the sill on the exterior side next to the sill stop, at the highest point of the sloped sill ².

The sloped sill is the detail most likely to go wrong. Sills are usually sloped so water runs off, and measuring to the low point or to the inside stool gives a height that may be too tall for the opening.

Step 3: Depth

Depth is the space between the inside and outside stops, sometimes called jamb depth or the pocket. An insert window needs enough of it to sit in. Andersen gives a minimum jamb depth of 3 1/4 inches for an insert replacement ¹. JELD-WEN's guide requires 3 1/4 inches or more for its pocket windows, except casements, which need at least 4 inches ². Other products have their own minimums, which is one reason installers measure with a specific product in mind.

Step 4: Squareness

A square opening has four corners at 90 degrees. Installers check it by measuring both diagonals, corner to opposite corner.

• Andersen says if the two diagonals are within 1/8 inch of each other, the window is square; if not, sizing adjustments are needed when ordering and the rough opening may need repair ¹.
• JELD-WEN says if the diagonals differ by more than 1/4 inch, an out-of-square adjustment must be subtracted from the smallest width, measured with a plumb line from the corners ².

The two guides use different tolerances for different products, so neither number is a universal rule. Andersen also says an insert needs an opening that is plumb, level and square ¹. A badly out-of-square frame is one of the signs a full-frame job may be needed rather than an insert.

Where the numbers disagree

JELD-WEN says that if there is more than 1/8 inch of deviation within a set of measurements, the cause should be investigated, with professional help to correct problems ². A large difference between top and bottom widths, or between left and right heights, may point to a frame that has moved, a sill that has sagged or rot in the frame. The guide also says a pocket window will not fix decay or water leaks around the existing frame, and those must be investigated and fixed before it goes in ². Drafty windows: what to try first covers the signs of a frame problem, and window frame replacement cost covers the repair.

JELD-WEN adds two habits worth copying: measure every window separately, because similar-looking windows are not always the same size, and record each window's location so the right unit goes into the right opening ².

What a homeowner can check

You do not need to place the order to get value from measuring. A tape measure and a notebook are enough to:

• Count and list the windows, room by room, with the style of each (double-hung, slider, casement). The types of windows guide helps name them.
• Take rough widths and heights at three points and note the smallest. That lets you compare quotes on the same sizes.
• Compare the diagonals. A big difference is worth mentioning to every installer you talk to ¹².
• Look and press. Soft wood at the sill or the bottom corners, water stains or peeling paint around the frame are things to point out.
• Flag bedroom windows. A bedroom window may have to work as an escape opening; see egress window requirements.

Then let the installer's measurement decide the order. Ask each one:

• Who takes the final measurements, and when, before or after the contract is signed?
• Are you measuring for an insert or a full-frame window, and why?
• What happens if a window arrives the wrong size?
• How will gaps between the new window and the old frame be sealed and trimmed?

How replacement windows are installed walks through installation day, how to choose a window installer covers vetting, and how to compare window quotes covers what a written quote should list. When you have your list, you can compare quotes from window installers.

We are not affiliated with the manufacturers named.

About these figures

• ¹ Reported: Andersen measuring guide: three-point width and height, rounding down to 1/8 inch, smallest measurement, diagonals within 1/8 inch, and the 3 1/4 inch minimum jamb depth for an insert, checked October 26, 2026.
• ² Reported: JELD-WEN measurement guide for its pocket windows (JMI001, copyright 2011): no returns on custom sizes, where to measure width and height, 3 1/4 inch and 4 inch pocket depths, 1/4 inch diagonal adjustment, 1/8 inch deviation, and measuring each window separately, checked October 26, 2026.
• ³ Official record: DOE consumer guide stating that improperly installed windows can result in a drafty house and condensation, checked October 26, 2026.