How to Measure a Window for Replacement

The three-measurement rule

Window openings are almost never perfectly square, especially in a home more than a couple decades old — settling, framing tolerances, and old installs all add slight variation. That's why the standard measuring method takes three readings for width and three for height, then uses the smallest number from each set. Ordering to the smallest measurement means the new window will fit even where the opening has narrowed slightly; ordering to the largest measurement risks a window that doesn't fit at all.

Step by step

• Remove the interior sash stops or trim if needed so you can measure the true opening rather than the visible trim.
• Measure width at three points: top, middle, and bottom of the opening, jamb to jamb (the inside vertical frame pieces, not the exterior trim or brick mold). Record the smallest number.
• Measure height at three points: left, middle, and right side, from the sill to the top of the opening. Record the smallest number.
• Measure depth (front to back) of the opening, which matters for choosing the right window and confirming it will clear interior trim.
• Check for square by measuring both diagonals corner to corner — if they don't match within about 1/4 inch, the opening is out of square and your installer needs to know before ordering.
• Note the opening style (single-hung, double-hung, casement, slider, etc.) since replacement windows are typically ordered to match the existing operation type unless you're changing it intentionally.

Pro tip from our network contractors

DIY measurements are useful for getting a rough budget number before you call anyone — but have a professional confirm the exact measurements before an order is placed. A window built to a mismeasured opening either doesn't fit or needs expensive on-site modification, and most reputable installers include a professional measure as part of the quote process anyway.

Full-frame vs. insert (pocket) replacement changes what you measure

If you're doing a full-frame replacement (removing the entire old frame down to the rough opening), the measurements above apply directly. If you're doing an insert (pocket) replacement — the more common approach, where the new window fits inside the existing frame — your installer measures the inside of the existing frame instead, which is a different, usually simpler process. Ask which approach your project needs; it affects both the measurement and the cost. See window replacement process for how the full installation typically goes.