How to Measure for Replacement Windows nine measurements per opening, and why the smallest one wins
Nine numbers per opening and one rule: the window is ordered to the smallest of them, because glass does not compress.
Nine window measurements, smallest wins
Measuring one opening properly takes 10 minutes per window, and the whole procedure is nine measurements: width in three places, height in three places, depth in two, and both diagonals.
The reason for nine rather than two is that houses move. A frame that was square when it was built settles, and an opening can be a quarter of an inch narrower at the middle than at the top without anything looking wrong. Order to the widest measurement and the window does not go in. Order to the smallest and it does, with a gap you fill and flash.
Do this before you take a single quote. It is the fastest way to tell whether a bidder actually measured or eyeballed the job, and it lets you read a quote in the same units the installer wrote it in. Whoever supplies the window is responsible for the final measurement, and you should still have your own.
What you need before you measure a window
This is ground-level work, and the one real risk is the ladder. Measure upper-floor windows from inside the room, never leaning out of one, and never from a ladder standing on a soft or sloping surface.
Nothing specialist. The steel tape does almost all of it, and the level is what turns nine numbers into a diagnosis rather than a list.
- Steel tape measure
- A rigid steel tape, not a cloth or fiberglass one. Those stretch, and an eighth of an inch of stretch is the whole tolerance you have.
- A short level
- Two feet or less, so it fits inside a sill. It tells you whether the opening is out of true, and which way.
- Pencil and paper
- One sheet per window, sketched. Typing nine numbers into a phone loses which number belonged to which place, and that is the only thing that matters.
- Your phone
- For a photograph of each opening from inside and outside, with the sketch beside it.
- A stiff putty knife
- Optional. Useful for finding the edge of the frame under decades of paint on an older window.
Decide which replacement window you are measuring for
Before any tape comes out, settle one question: is this an insert or a full-frame replacement? The two are measured from different surfaces and the numbers are not interchangeable.
An insert fits a new window inside the existing frame, so you measure the frame. A full-frame replacement takes everything back to the rough opening in the wall, so you measure the structural opening once the frame is out, or you estimate it and confirm at removal. Almost every homeowner measuring their own windows is measuring for an insert.
- If the existing frame is sound, square and dry, an insert is measured from the inside faces of that frame.
- If the frame is rotten, out of square or has ever leaked, it is a full-frame job and your measurements are indicative only.
- Write which one you measured at the top of every sheet. A number with no reference surface is worthless.
Measure the window width in three places
Measure horizontally between the inside faces of the frame, from left to right, at three heights: near the top of the opening, at the middle, and near the sill.
Record all three. Do not average them and do not round. The number you will order to is the smallest of the three, because a window has to pass through the narrowest point of the opening on its way in.
- Measure to the frame, not to the stop, the trim or the sash. On an older window, scrape the paint at one point if you cannot see where the frame ends.
- Read to the nearest sixteenth of an inch. Eighths are usually enough and sixteenths cost nothing.
- A spread of more than a quarter inch across the three means the opening has moved, and it is worth noting on the sheet for the installer.
Measure the window height in three places
Measure vertically from the highest point of the sill to the underside of the head jamb, at three positions: left, center and right.
The sill on most windows slopes outward to shed water, which is exactly why the measurement is taken from its high point. Measuring from the low point gives you a window that will not fit under the head.
- Again, record all three and order to the smallest.
- On a sloped sill, put the tape at the highest part of the slope, closest to the interior.
- If the head jamb sags in the middle, the center measurement will be the smallest. That is normal in an older house and is what the three readings are for.
Measure the depth of the frame
Measure the depth of the existing frame from the inside face of the exterior stop to the inside face of the interior stop, at two points on opposite sides.
This is the measurement most homeowners skip and the one that most often stops an order. A replacement window needs a minimum frame depth to sit in, typically around three and a quarter inches, and a shallow frame rules out whole product lines.
- Take out the interior stop if you cannot see the pocket clearly. It comes off with a putty knife and goes back on.
- If the depth is under about three and a quarter inches, say so when you request quotes rather than discovering it at the order stage.
- Note anything intruding into the pocket: old weights, pulleys, security hardware or a storm window track.
Why the smallest window measurement wins
A window is ordered to the tightest point in the opening, because a gap can be filled and flashed and glass cannot be made narrower on the day
Measure both window diagonals
Measure corner to corner, both ways. On a square opening the two figures match. On a settled one they will not, and the difference tells you how far out of true it is.
This is the check that separates a measurement from a diagnosis. It costs thirty seconds and it is what tells an installer whether the window can be shimmed square inside the opening or whether the frame has to come out.
- A difference under a quarter of an inch is normal and shims will absorb it.
- Half an inch or more means the opening has racked. Note it, photograph it, and expect a full-frame conversation.
- Check the sill with the level while you are there, and write down which end is low.
Write the window measurements down so they survive
Sketch the opening on its own sheet, mark the nine numbers where they were taken, and write the room, the elevation and whether it was measured for an insert or full-frame.
Then photograph the opening from inside and from outside with the sheet in the frame. When three companies quote on the same house, this is what lets you tell whether they are quoting on the same windows.
- Number the openings and use the same numbers in every quote conversation.
- Note anything that changes the job: a bay or bow, a fixed unit beside an operating one, obscured glass in a bathroom, a tempered-glass requirement near a door or a tub.
- Keep the sheets until the windows are installed and accepted, not until they are ordered.
When a window measurement is not enough
Your numbers are for reading quotes and catching mistakes. These situations need a professional measurement before anything is ordered, and in every one of them the supplier owns the final figure.
- The diagonals differ by half an inch or more, or the sill is visibly out of level.
- The frame is soft anywhere, or paint is lifting off bare grey timber at the sill corners.
- The opening is a bay, a bow, an arch, or anything that is not a rectangle.
- The window is within eighteen inches of a floor, beside a door, or in a bathroom, where code may require tempered glass.
- The house is old enough that the frame may contain sash weights, or old enough for lead paint, which changes how the trim can legally be disturbed.
- The frame depth is marginal, because that decides which products can be used at all.
What to do with your window measurements
Nine numbers per opening and a photograph turn a window quote from a sales conversation into a comparison. Use them in that order.
- Compare like with like
- Give every bidder the same numbered sheet. Quotes that describe different openings cannot be compared, and that is usually not an accident.
- Check the quote against your sheet
- If a quote lists sizes that do not match your smallest measurements, ask which surface they measured from before you ask about the price.
- Let the supplier take the final measure
- Whoever manufactures the window owns the measurement that goes on the order. Yours is the check, not the instruction.
- Keep the depth figure to hand
- It is the one number that eliminates products rather than adjusting the price, and the earliest one worth raising.
What homeowners ask when measuring windows
Why measure in three places instead of one?
Because openings are rarely square after a few decades of settlement. Three readings across the width and three up the height show you the narrowest point, and the narrowest point is the one the window has to fit through.
Do I measure the glass or the frame?
The frame, always, and specifically the inside faces of it. The glass area is a consequence of the frame you order, not an input, and measuring the visible glass will give you a window several inches too small.
How much smaller than the opening should the window be?
That is the supplier’s decision, not yours. Manufacturers subtract their own clearance from your measurements, usually a quarter to a half inch, and the gap is filled with low-expansion foam and flashed. Never subtract it yourself, or it gets taken off twice.
What if the diagonals do not match?
Under a quarter inch, shims absorb it and nobody mentions it again. Half an inch or more means the opening has racked, and an insert fitted into it will bind. That is a full-frame conversation, and better to have it before the order than after.
Should I measure from inside or outside?
Inside, for an insert, which is what most people are measuring for. Outside measurements pick up brick mould, siding and trim rather than the frame, and none of those are what the window sits in.
Does my measurement go on the order?
It should not. Every reputable supplier sends someone to take the final measurement and takes responsibility for it, because a window ordered to the wrong size is a custom item nobody can return. Your numbers exist to check theirs.