Replacement windows are measured from inside the existing opening, taking three widths and three heights and using the smallest of each. The smallest number is what the new unit has to clear, because almost no opening in a real house is perfectly square.
- 3widths, 3 heightsper opening, not one of each
- Smallestmeasurement winsthe unit must clear the tightest point
- 1/8"typical precisionto the nearest eighth of an inch
Measuring is the step that decides whether a window fits, and it is the one step you should not do as the final word yourself - but understanding it tells you whether the person quoting you is doing it properly.
Why three measurements, not one
An opening in a real house is rarely square. Frames settle, houses move, and an opening can be a quarter-inch wider at the bottom than at the top. If you measure once, in the middle, and the top is tighter, the window does not go in.
So the method is:
- Three widths — at the top, the middle and the bottom of the opening.
- Three heights — at the left, the middle and the right.
- Use the smallest of each. The new unit has to pass the tightest point, not the average one.
What the measurement is taken from
For an insert replacement, measurements are taken from inside the existing frame, jamb to jamb - the old frame is staying, so it defines the space available.
For a full-frame replacement, the old frame is coming out, so the relevant measurement is the rough opening in the wall behind it. This is why the two methods cannot share a quote: they are not measuring the same thing.
Taking a rough measurement yourself
Useful for getting a ballpark before anyone visits, not for ordering:
- Work from inside the house.
- Measure the width at top, middle and bottom, jamb to jamb. Write down the smallest.
- Measure the height at left, middle and right, sill to header. Write down the smallest.
- Round down to the nearest eighth of an inch, never up.
- Check the opening is square by measuring both diagonals - if they differ noticeably, say so when you get quotes, because it affects the install.
Do this for every opening. Counting and sizing your openings is the single most useful thing you can have ready before a quote appointment, and it is covered further in what it costs.
Common questions
Can I just measure the old window itself?
No - the sash you can see is smaller than the opening it sits in, so measuring the glass or the sash gives a number that is too small. The measurement is of the opening, not the window.
Why round down instead of up?
Because a window slightly too small can be shimmed and sealed, and a window slightly too large cannot go in at all. Rounding down is the safe direction of error.
Does the installer measure again before ordering?
A competent one does, and the responsibility for that final measurement should be stated in the contract. Treat any quote that skips a measuring visit entirely as incomplete.