How to Measure Your Windows (and Why We Still Measure Them)
Measure width at three heights and height at three widths, and use the smallest of each. That gives you a rough size for budgeting. The contractor still measures before ordering, because a quarter inch error on a custom unit is not recoverable.
You can measure your own windows well enough to sanity check a quote and get a realistic budget, and doing so is genuinely useful going into an estimate.
What you should not do is order from your own measurements, and the reason is not gatekeeping. Custom units are built to size and cannot be returned.
Measuring for a rough budget
- Work from inside the house, measuring the window frame opening rather than the glass or the trim.
- Measure the width at three points: near the top, at the middle, and near the bottom. Record the smallest.
- Measure the height at three points: left side, middle, right side. Record the smallest.
- Measure the depth of the frame from the interior stop to the exterior stop, which determines whether an insert will fit.
- Measure both diagonals, corner to corner. A difference over a quarter inch means the frame has racked, which usually means full frame.
- Note the window style, whether it operates, and anything unusual such as an arched head or a mulled combination.
- Count the total openings in the house, including basement and any you were not planning to replace.
Why three measurements per dimension
Because almost no opening in a real house is a perfect rectangle. Houses settle, framing moves, and an opening that was square in 1958 rarely still is. If you measure only at the middle and the top is a quarter inch narrower, a unit built to the middle measurement will not fit. Building to the smallest measurement guarantees the unit goes in, and the small resulting gap is filled with shims and insulating foam, which is exactly what that gap is for. This is also why a quote based on standard sizes or on photographs should be treated as a budget estimate rather than a real price.
What we check that you cannot
| What we assess | Why it matters | How we check |
|---|---|---|
| Sill condition | Determines insert versus full frame | Probing with an awl at the corners |
| Jamb condition | Hidden rot behind trim | Probing and moisture reading |
| Frame square | Whether an insert will seal | Diagonal measurement |
| Header adequacy | Required for any opening change | Visual and structural assessment |
| Exterior condition | Whether capping is needed | Exterior inspection per opening |
| Wall construction | Masonry versus frame changes the install | Depth probe and exterior inspection |
| Egress requirement | Bedroom windows have a legal minimum | Net clear opening calculation |
| Safety glazing triggers | Code requires tempered in defined locations | Distance to doors, floors, tubs and stairs |
Using your measurements at the estimate
Bring your list to the estimate. It does two useful things. It tells the contractor you are engaged and paying attention, which changes the conversation. And it gives you a check: if their opening count differs from yours, or their sizes are meaningfully different from what you measured, ask why. Sometimes there is a good reason, such as counting a mulled combination as one unit rather than three. Sometimes it is an error worth catching before it is ordered. Either way you learn something.
Ranges reflect typical Capital Region projects and are for planning only. Every Sundog quote is written, itemized, and free.
Frequently asked questions
Can I order replacement windows from my own measurements?
You can, and it is a bad idea. Custom units are built to size and are not returnable, so a quarter inch error is an expensive and unrecoverable mistake. Measuring your own openings is genuinely useful for budgeting and for checking a quote, but the final measurement before ordering should be professional and should carry the contractor's responsibility.
Do I measure the glass or the frame?
The frame opening, measured from inside, not the glass and not the exterior trim. For an insert replacement you measure the existing frame opening. For a full-frame replacement the relevant measurement is the rough opening, which is only visible once the existing unit is removed.
Why does the contractor use the smallest measurement?
Because a unit built to the largest measurement will not fit through the narrowest point of the opening. Building to the smallest guarantees it goes in, and the resulting gap is filled with shims and low-expansion insulating foam, which is the intended function of that space.
How do I know if my window opening is square?
Measure both diagonals, corner to corner. In a square opening they are equal. A difference of more than about a quarter inch means the frame has racked, which usually means an insert replacement will not seal properly and full frame is the correct approach.
What is the standard window size?
There is no meaningful standard in replacement work, because nearly every unit is built to the specific opening. Stock sizes exist for new construction, where the rough opening is framed to match the window. In a replacement context, treat any quote based on stock sizes as a rough budget rather than a real price.
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A free measure and a written quote costs you nothing and gives you real numbers to work with.