We measure windows every day for our own installs across Westchester, Putnam and Fairfield County, and we now check measurements for contractors and homeowners ordering Harvey windows through us. This is the method we use. Take your time with it, and if you're inside our service area and would rather not, we'll measure for you.
One note first: this guide covers replacement (insert) windows going into an existing frame. Full-frame and new construction windows are measured to the rough opening and need a different approach. If you're not sure which you have, send us a few photos and ask.
What you need
A steel tape measure, a pencil and a sheet with a line for every window. Label each window by room and position (for example, Kitchen, left of sink) so the numbers end up on the right opening. Measure in inches to the nearest eighth.
Step 1: Measure the width in three places
Measure the opening between the side jambs, inside the frame where the new window will sit, at the top, the middle and the bottom. Old frames are rarely perfectly straight. Write down all three and use the smallest.
Step 2: Measure the height in three places
Measure from the head jamb at the top down to the point where the sill meets the frame, on the left, in the center and on the right. Again, record all three and use the smallest.
Step 3: Check that the opening is square
Measure both diagonals, corner to corner. If the two numbers are within about a quarter inch of each other, the opening is reasonably square. A bigger difference means the frame has moved, and you'll want a professional to look before ordering.
Step 4: Measure the depth
Measure the depth of the jamb, from the inside edge of the interior stop to the outside edge of the exterior stop. The replacement window needs enough depth to sit securely in the frame. Note it on your sheet.
Step 5: Note everything else
For each window, write down the style you want, the Harvey line if you've chosen one, interior and exterior color, grid pattern, and anything unusual: rot on the sill, a frame that's out of square, an opening wider than the others. Photos of the inside and outside of each window help a lot.
Common mistakes that cause reorders
Using the biggest measurement instead of the smallest. Measuring the old sash instead of the frame opening. Mixing up width and height (width is always first). Rounding to the nearest inch. Measuring one window and assuming the identical-looking one next to it is the same size. They often aren't.
And the expensive one: ordering replacement inserts for frames that are rotted or badly out of square. An insert is only as good as the frame it goes into. If the frame is shot, the job needs full-frame windows, and that's a different order.
Replacement insert or full frame: which do you need?
Before you measure, take a hard look at the existing frames. Press a screwdriver into the sill and the bottom of the side jambs. If the wood is soft, crumbles or is stained dark from water, the frame is likely rotted, and putting a new insert into it just hides the problem. Check the outside too: failing caulk, peeling trim and gaps around the frame are all signs worth noting.
If the frames are solid and reasonably square, replacement inserts are usually the efficient choice. If they aren't, plan for full-frame replacement, which is measured to the rough opening after the trim is removed. When in doubt, send us photos of the sills and corners before you order.
Bays, bows and wide openings
Bay and bow windows are measured differently from a single window. You'll need the overall width across the opening, the height, the projection (how far the unit sticks out from the wall), and the angle of the side units. These are worth having measured professionally, because small errors compound across several units joined together. The same goes for wide openings with two or three windows mulled together: measure the whole opening and note how the units are divided.
Measuring for a patio or entry door
Doors follow the same logic: measure the width in three places and the height in three places, and use the smallest. Also measure the jamb depth, and note which side the door should hinge or slide from. For a patio door, stand outside and say which panel operates. For an entry door, note whether it swings in or out and which side the hinges are on, viewed from outside. Write it down at the opening rather than from memory.
A simple measuring sheet
For every opening, write: the location, the style, width top / middle / bottom, height left / center / right, the two diagonals, the depth, and any notes. Then circle the smallest width and the smallest height. Those are the numbers that go on the order. Keep the sheet with your photos and send both with your quote request so we can check them together.
Before you send the numbers
Measure every window twice, ideally on different days or with a second person reading the tape. Compare the two sets. If any number is off by more than an eighth of an inch, measure that opening a third time. It sounds like overkill, but a custom window built to the wrong size costs far more than an extra hour with a tape measure, and it pushes your whole project back while a new one is built.
Let us check it, or let us measure
When you order Harvey windows through Window Inserts, we look over every measurement before the order goes in and call you if anything looks off. Inside our service area in Westchester, Putnam and Fairfield County, you can check the measuring box on the quote form and a Window Inserts tech will measure every opening for you.
By The Window Inserts team. Published November 6, 2026.