Measure window width, height, and depth at three points each, then use the smallest number in each direction to get the right fit for your window replacement. On brick homes, measure the brick opening itself, not the caulk bead or the window sitting inside it.
Measuring takes about 10 to 15 minutes per window. You need a metal measuring tape, a pencil, and paper.
TL;DR
- Measure width, height, and depth at three points each and always use the smallest number.
- On brick or masonry homes, measure the brick opening, not the caulk bead or the window itself.
- Sash-only replacement windows are built to your exact frame opening, so don’t pre-subtract anything when ordering.
- Full-frame installers working from a brick or rough opening typically subtract about 3/8 inch off the tightest measurement for leveling and squaring.
- If diagonal measurements differ by more than 1/4 inch, the frame is out of square and your installer needs to know before ordering.
- Measure every window individually; frames shift and settle differently over time.
- Hidden rot, a warped sill, or a damaged jamb means you need full-frame replacement, not sash-only.
Inside or Outside: Where Do You Measure From?
Measure from inside the frame for sash-only replacement, and from outside for full-frame replacement. Most replacements in Boise and Meridian are sash-only: the existing frame stays in the wall and only the sash gets replaced, so all measurements come from inside. Full-frame replacement removes everything down to the rough opening, and on brick homes that rough opening is the brick itself, not the wood frame currently sitting in it.
| Situation | Replacement Type | Where to Measure |
| Frame is solid, no rot or damage | Sash-only | Inside the window frame |
| Frame has rot, damage, or is warped | Full-frame | Outside from the rough opening |
| Home is brick and frame is being replaced | Full-frame | Outside from the brick opening |
| Simple upgrade, frame is square | Sash-only | Inside the window frame |
Tools You Need
A metal measuring tape, a pencil, and paper. Add a flashlight for dark frame interiors and a level for checking the sill or brick row.
How to Measure Width, Height, and Depth
Width: Measure across the inside of the frame (or brick opening) near the top, middle, and bottom. Use the smallest reading.
Height: Measure from head jamb to sill (or top to bottom of the brick opening) on the left, center, and right. Use the smallest reading.
Depth: Open the window fully and measure from the outer edge of the frame to the inner edge, at three points. Most window types need at least 3 and 1/4 inches of depth; bay windows need 4 inches or more.
| Point | Example | Use This? |
| Top width | 36″ | No |
| Middle width | 35 7/8″ | No |
| Bottom width | 35 3/4″ | Yes, smallest |
Measuring Brick Openings
Look past the window frame and caulk bead to find the true brick edge, and measure to that, not to the caulk. A heavy caulk bead can make an opening look narrower than it is. If the brick line steps or angles instead of running straight, step back and find the single tightest point across the whole opening rather than trusting one reading.
How Much to Deduct
For sash-only replacement, don’t subtract anything yourself. Give the manufacturer your smallest width, height, and depth, and they build the sash to fit.
For full-frame replacement into a brick or rough opening, installers typically take about 3/8 inch off the tightest measurement, leaving roughly 3/16 inch of clearance per side to level, plumb, and square the window. Some manufacturers recommend a larger deduction, closer to 3/4 inch, to cover openings that turn out more out of square than expected; a confirmed-square opening can often use the tighter deduction instead. A window that needs a hammer to seat is a sign it was measured or ordered too large, and it will likely bind, leak, or operate poorly later.
Checking If the Frame Is Square
Measure diagonally from corner to corner in both directions and compare the two numbers.
| Difference | What It Means |
| 1/8 inch or less | Normal for most homes |
| 1/4 inch | Borderline, tell your installer |
| More than 1/4 inch | Out of square, installer must know before ordering |
On some brick homes, the original window was installed before the brick, and the brick was later built up against a sloped sill. That can make the true opening taller than what you measure with the old window still in place, so mention any signs of this when you order.
Recording Your Measurements
Write down width, height, and depth for every window separately, even ones that look identical, since frames settle at different rates. Sketch each window and label the measurements directly on the sketch.
Common Mistakes
| Mistake | Fix |
| Using the largest measurement | Always use the smallest |
| Measuring only once per direction | Measure at three points every time |
| Measuring the glass, not the frame | Measure the full frame or brick opening |
| Measuring against the caulk bead | Measure to the true brick edge |
| Forcing a tight window with a hammer | Re-measure and confirm the deduction before ordering |
Ready to Order?
Once you have your measurements, The Window Source of Western Idaho handles everything from there. We serve Boise, Meridian, Nampa, Caldwell, and the full Treasure Valley, with a 10-year labor warranty and a lifetime product warranty on every job.
FAQs
Do I measure the glass or the whole frame?
No, measure the full frame or brick opening, not the glass.
Should I subtract anything before ordering?
Depends on the type: no for sash-only (the manufacturer handles it), yes for full-frame into a brick opening, typically about 3/8 inch.
What does out of square mean?
The opening isn’t a true rectangle. Tell your installer the diagonal difference so they can shim, foam, and adjust the deduction as needed.
Can I measure my own windows?
Yes, using this guide. For bay windows, irregular brick lines, or visible frame damage, have The Window Source of Western Idaho measure on-site first.