Short answer: measure the width from jamb to jamb at the top, middle, and bottom of the opening, measure the height from the sill to the head at the left, center, and right, and keep the smallest number of each. Round down to the nearest 1/8 inch, check that the two diagonals are within 1/4 inch of each other, and confirm the frame is at least 3 1/4 inches deep. The window maker takes 1/4 inch off your numbers so the new unit slides in with room to shim and seal.
That is the whole method, and anyone can do it in ten minutes per window. At Gorilla Works we measure every opening ourselves before we order, because the tape measure only tells half the story. This guide gives you the full method first, then the half a tape cannot see, and it links to our window installation service for when you want the numbers taken by the crew that will stand behind the fit.
The 5 steps at a glance
| Step | What to measure | The rule |
|---|---|---|
| 1. Width | Jamb to jamb at the top, middle, and bottom | Keep the smallest of the three |
| 2. Height | Sill to head at the left, center, and right | Keep the smallest of the three |
| 3. Square | Both diagonals, corner to corner | Within 1/4 inch of each other, or the frame is out of square |
| 4. Depth | Front of the frame to the back, at the sill | At least 3 1/4 inches for a standard insert window |
| 5. Record | Width first, then height, rounded down to 1/8 inch, one line per window | Never assume two windows match. Measure every one |
What do you measure when replacing a window?
You measure the inside of the existing frame, not the trim and not the glass. The two vertical sides of the frame are the jambs. The flat piece at the bottom that the sash rests on is the sill. The piece across the top is the head. The decorative shelf that sticks into the room is the stool, and it is not part of any measurement. An insert replacement window, sometimes called a pocket or retrofit window, is built to fit inside that jamb-to-jamb, sill-to-head opening, so those are the four surfaces your tape touches.
How do you measure window width for replacement?
Hold the tape flat against the left jamb and extend it to the right jamb. Take that measurement at three heights: the top with the window closed, the middle with the sash out of the way, and the bottom with the window open so you can see the jambs. Write down all three and circle the smallest. Frames settle and bow over the years, and the narrowest point is the one the new window has to pass.
How do you measure window height for replacement?
Same idea turned sideways. Put the tape on the sill, where the bottom sash sits when closed, and run it up to the head. Do it at the far left, the center, and the far right. Circle the smallest. If your sill is sloped, which most exterior sills are so water runs off, measure from the highest point of the sill at the inside edge, since that is where the new window will sit.
Do you subtract 1/4 inch when measuring for replacement windows?
No, you do not. The window manufacturer does. You give them the smallest width and smallest height, rounded down to the nearest 1/8 inch, and they build the unit 1/4 inch smaller than your numbers, accurate to about 1/8 inch. That 1/4 inch is the room the installer needs to set the window plumb, shim it, and fill the gap with low-expansion foam and sealant. If you subtract it yourself and the maker subtracts it again, the window arrives 1/2 inch small and the gap is too wide to seal well. Order from the raw measurement and say so on the order.
How do you know if a window opening is square?
Measure corner to corner both ways: top left to bottom right, then top right to bottom left. If the two numbers are within 1/4 inch of each other, the opening is square enough for an insert window. If the difference is bigger than 1/4 inch, the frame has racked, and a factory-square insert will bind, leak, or leave a wedge-shaped gap. That opening needs a full-frame replacement, which takes the old frame out and starts from the rough opening in the wall.
How deep does the frame need to be?
Open the window and measure from the front edge of the sill to the back, in front of the stool. A standard insert window needs at least 3 1/4 inches of depth. Less than that is a specialty order or a full-frame job. Old aluminum sliders from the 1970s and 1980s, common across Portland and Vancouver, are the ones that come up short most often.
Insert or full-frame: which one are you measuring for?
Everything above is for an insert. An insert sets the new window inside your existing, sound frame, which is why the frame has to be square, deep enough, and solid. A full-frame replacement removes everything down to the rough opening in the framing, then installs a new window with a nail fin and fresh flashing. It is the right call when the old frame is rotted, water damaged, badly out of square, or when you are changing the window size. Full-frame openings are measured differently, from stud to stud and header to rough sill, and they are not a homeowner measurement, because you cannot see those surfaces until the old unit is out.
| What you find | Insert or full-frame | Why |
|---|---|---|
| Diagonals within 1/4 inch, frame solid, depth 3 1/4 inches or more | Insert | The existing frame can carry the new window |
| Diagonals off by more than 1/4 inch | Full-frame | An insert cannot be shimmed square in a racked opening |
| Soft or dark wood at the sill or lower jambs | Full-frame | Rot spreads behind an insert. It has to come out |
| Frame depth under 3 1/4 inches | Full-frame or specialty | A standard insert will not seat |
| You want a bigger window or a different shape | Full-frame | The opening itself changes |
What a pro measures that a tape measure cannot
The numbers get you a window that fits the hole. They do not tell you whether the hole is worth putting a window in. Here is what our crew checks on every opening before we order, and why each one matters in Portland's wet climate.
- Rot at the sill and lower jambs. We press a screwdriver into the wood at the bottom corners. Soft wood means water has been getting behind the frame, usually for years, and an insert would seal that damage in.
- Sill slope and sill condition. A sloped or cupped sill changes where the new window sits and how the sill adapter has to be cut.
- What the exterior is. Lap siding, stucco, and brick each take a different install detail on the outside, and the measurement for the exterior trim or flush fin is separate from the opening measurement.
- Flashing and the weather barrier. On a full-frame job we check whether there is a house wrap behind the siding and how the old window was flashed, because the new flashing has to tie into it or water finds the gap.
- Interior finishes. Tile sills, drywall returns, and plaster all change the interior trim plan and sometimes the window depth we order.
- Egress. Bedroom windows have to open wide enough for a person to get out. If the old window was undersized, the replacement cannot just copy it.
None of that changes the width and height you measured. It changes whether an insert is the right window, and it is the reason the crew that installs the window should be the crew that measures it. When we measure, the fit is on us.
Want the crew that installs it to measure it?
We measure every opening in person, check for rot and square, and order the window against our own numbers, so the fit is our responsibility, not yours.
Licensed OR CCB #198186 and WA L&I #EXTERDC884PP · Milgard-certified · 20+ years in the Portland area
Frequently Asked Questions
Do you measure a window from the inside or the outside?
Inside, for an insert replacement. The opening the new window slides into is the inside of the existing frame, jamb to jamb and sill to head. Exterior measurements only come into play for the outside trim or a flush fin, and the installer takes those.
Do you write width or height first?
Width first, then height, every time. A window written as 36 x 48 is 36 inches wide and 48 inches tall. Reversing them is the most common ordering mistake in the business.
Should you measure the glass or the frame?
The frame. Glass size tells you nothing about the opening. Measure the inside of the frame the sash sits in.
Can you measure for replacement windows yourself?
Yes, and the method above is the same one manufacturers publish. Where homeowners get caught is not the numbers, it is rot, out-of-square frames, and shallow aluminum frames that the numbers do not reveal. Measuring yourself is fine for a quote. Ordering against your own measurement puts the fit on you.
What if my measurements are all slightly different?
That is normal. Wood frames move over decades. Use the smallest width and the smallest height. If any two readings differ by more than 1/4 inch, measure again, and if they still do, the opening is likely out of square and needs a full-frame replacement.
What tools do you need to measure for replacement windows?
A steel tape measure, a notebook and pencil, a small level, and a second person for anything wider than about four feet. Take the measurements in a notebook, one window per line, labeled by room and direction it faces.
How long does it take to measure a window?
About ten minutes per window done carefully: three widths, three heights, two diagonals, one depth, and a look at the sill. A whole house is usually under two hours.
The bottom line
Three widths, three heights, smallest wins, round down to 1/8 inch, diagonals within 1/4 inch, depth of at least 3 1/4 inches, and let the manufacturer take off the 1/4 inch. That gets you a number good enough for any quote. Whether the opening deserves an insert or needs a full-frame replacement is a different question, and it is the one the crew answers at your house.
If you are in Portland or Vancouver and want the measuring, the rot check, and the fit all on one set of shoulders, our window installation team does the measurement visit free.
Across the river? Our Vancouver WA window replacement page covers the same service on the Washington side.
Wondering what you should be paying? Our guide to window replacement cost in Portland walks through what sets the price, including the insert versus full-frame difference.
Sources
Gorilla Works proudly serves Portland, Gresham, Beaverton, Hillsboro, Tigard, Lake Oswego, Happy Valley, Milwaukie, Troutdale, Vancouver WA, and Camas WA. Call (971) 343-4964 or request a free window measurement.
