Measure the smallest width and height of the opening, then subtract 1/8 inch for channel-mounted screens. Measure to the nearest 1/16 inch.
A screen that’s a quarter inch too wide won’t slide, and one that’s too narrow won’t stay seated. The measurement that decides it is the smallest opening dimension, not the number you read off the top of the frame. This walkthrough covers the three install types, the deductions each one needs, and the hardware details that sink a fit after the width and height look right.
Which Method Applies to Your Window?
The measuring method changes with how the screen sits in the window, so identify the install type before you pick up a tape.
Milgard’s screen measuring guide covers the basics: width first, then height, to the nearest 1/16 inch. From there the type of installation decides the rest.
- Channel-mounted (insect screen): the screen slides into a track built into the window frame. RiteScreen’s guidance is to measure inside one side channel to the opposite edge and subtract 1/8 inch for width; repeat for height. Depth matters too — measure from the front inner edge to the opposing channel wall to confirm the screen will sit in the track.
- Existing screen replacement: pull the screen and measure outside edge to outside edge of its frame. That measurement is the target, not the opening.
- Inside mount (roller or pull-down): the screen mounts inside the opening rather than in a channel. Roll Away’s instructions call for width at the top, middle, and bottom, and height at the left and right, then use the smallest of each. A 1/16 inch deduction applies for proper fit in this style.
How Do You Handle an Opening That Isn’t Square?
Measure in several places and trust the smallest number. An opening that’s out of square by more than 1/4 inch is a real problem, and Andersen says custom insect screens can’t be made to fit it.
Windows bow at the middle, so a single centerline reading often overstates the width. Andersen’s method is to measure width at the sill, 6 inches up from the sill, and at the midpoint, then record the smallest figure. For height, measure at the center and at one edge, and keep the smaller number.
Run a diagonal check corner-to-corner as well. If the two diagonals differ by more than 1/4 inch, the unit is out of square — and that’s the point where an adjustable or custom screen stops being an option until the opening itself is addressed.
| Install Type | Where to Measure | Deduction |
|---|---|---|
| Channel-mounted insect screen | Inside one channel to the opposite edge; width and height | Subtract 1/8 inch from each |
| Existing screen replacement | Outside edge to outside edge of the old frame | None — the frame is the target |
| Inside mount (roller/pull-down) | Width at top, middle, bottom; height at left and right | Use smallest of each, subtract 1/16 inch |
| Spring hardware present | Deepest channel inside edge to opposite channel outside edge | Per manufacturer’s tension-spring spec |
| Out-of-square opening | Diagonals, plus width at three points and height at two | Over 1/4 inch off — custom screen won’t fit |
Once you have the numbers, the next step is matching them to a screen whose published range covers both. Our roundup of adjustable screens for windows lists model ranges side by side so you can check the width and height against your reading before ordering.
What Hardware Details Affect the Fit?
Hardware determines whether the screen locks in place, so note it before you buy anything. Metro Screenworks’ guidelines call for recording spring hardware and its exact location, plus the frame thickness.
Also capture pull tabs, clips, the crossbar position, and any handle, lock, or obstruction that crowds the opening. Adjustable screens are sized by opening range, so confirm the exact model’s minimum and maximum width and height — those ranges differ by brand, and a range that clears your width may still miss your height.
Frequently Asked Questions
Do I measure the window or the old screen?
For a replacement, pull the existing screen and measure outside edge to outside edge of its frame. For a new channel-mounted screen with no old frame to reference, measure the opening itself and deduct 1/8 inch from both width and height.
Why subtract 1/8 inch instead of ordering the exact opening size?
A screen cut to the full opening won’t slide or seat into the channel. RiteScreen’s measuring guidance builds in the 1/8 inch reduction so the screen drops into the track with room to move, which is what lets it sit flush once installed.
What if my width and height land in two different size ranges?
Match both dimensions to the same model’s published range. Adjustable screens state a minimum and maximum width and a separate height figure, and a model that clears your width can still fall short on height.
References & Sources
- Milgard. “How to Measure Window and Patio Door Screens” Covers width-then-height order and 1/16 inch measuring precision.
- Andersen Windows. “Custom-Sized Insect Screens” Source for diagonal checks, the 1/4 inch out-of-square limit, and the smallest-measurement method.
- Metro Screenworks. “Measuring Guidelines: Window Screens” Source for frame thickness and spring hardware documentation.