Short answer
Measure the existing window frame (not the drywall) at three points for width and height, use the smallest numbers, and subtract about 1/4 inch for fitting. Also check the diagonals for squareness, measure the jamb depth, and identify the sill angle. Order the window using width x height to the nearest 1/8 inch.
Why accurate measuring matters
Replacement (insert/pocket) windows are designed to slide into your existing frame. If you measure the drywall opening or trim instead of the true frame, your new window can be too big to fit or too small to seal properly. Correct measurements help you avoid drafts, leaks, and costly returns.
Tools and materials
- Tape measure (25 ft, with 1/8-in graduations)
- 2-ft or 4-ft level
- Framing square
- Notepad or phone for recording sizes
- Digital angle finder or bevel gauge (for sill angle)
- Straightedge or ruler
- Flashlight
- PPE: safety glasses and gloves (if removing any trim or checking for rot)
Time: 10–30 minutes per window. Tool cost: $20–$80 depending on what you already own.
Step-by-step: measuring for insert (pocket) replacement windows
These steps assume you’re keeping the existing frame and trim.
1) Identify the true frame surfaces
- Open the sashes and find the inside faces of the side jambs, head jamb (top), and the high point of the sloped sill (bottom). Do not measure drywall, casing, or the stool (interior shelf).
- If there are vinyl/metal jamb liners, measure the clear opening they create—the replacement must fit through that space.
2) Measure width (jamb to jamb)
- Take three measurements: top, middle, bottom of the opening.
- Record to the nearest 1/8 inch.
- Use the smallest of the three as your “frame width.”
3) Measure height (sill to head)
- Take three measurements: left, center, right from the highest point of the sloped sill to the head jamb.
- Use the smallest of the three as your “frame height.”
4) Check for square (diagonals)
- Measure both diagonals inside the frame (upper-left to lower-right, and upper-right to lower-left).
- A difference greater than about 1/4 inch suggests the opening is out of square; your installer may need additional shimming, or you may consider a full-frame replacement if it’s severe.
5) Measure jamb depth
- Measure from the interior stop face to the exterior stop face (or from interior trim plane to the outermost inside frame plane). Typical insert windows need 3-1/4 in jamb depth; verify your chosen product.
6) Determine sill angle
- Place a digital angle finder on the sill to read the slope, or use a bevel gauge and transfer to a protractor. Many sills are 8–15 degrees. Some replacement units ship with adjustable or cut-to-fit sill adapters—knowing the angle improves fit and weather seal.
7) Apply fitting allowance and format your order
- Manufacturers generally recommend subtracting 1/4 in from the smallest width and height to allow for shimming and squaring.
Ordered width = smallest measured width − 1/4 in
Ordered height = smallest measured height − 1/4 in
Round/record to nearest 1/8 in.
Example order specs:
Size: 35-3/4" W x 59-1/2" H (order as 35.75 x 59.50)
Jamb depth: 3-1/4"
Sill angle: 8°
Operation: Double-hung (specify handing for casements/awnings)
Color/finishes and grid options as desired
Measuring for full-frame replacement or new-construction windows
If you’re removing the entire window (including jambs), measure the rough opening after exposing the framing:
- Width: stud to stud
- Height: header to sill plate
- Confirm plumb/level and check diagonals
- Most manufacturers size the unit to the rough opening by specifying the RO or subtracting their standard clearance. Follow the product’s guide.
Safety considerations
- Use ladder safety if measuring upper floors; keep three points of contact.
- Wear gloves and glasses if you probe for rot or gently pry any stop to verify true jamb edges.
- Homes built before 1978 may have lead paint. Avoid sanding or aggressive scraping. If paint is chipping and you need to disturb it, follow EPA RRP guidelines or hire a certified pro.
Tips for best results
- Measure each window individually—even identical-looking openings often differ by 1/8–3/8 in.
- Photograph each opening with a label (room + window number) and note measurements on the photo for ordering.
- Use a rigid steel tape for accuracy; avoid a floppy fabric tape.
- If the sill has a pronounced bow, place a straightedge on it and measure to the straightedge, then note the low spots.
- For casements/awnings, also check the interior clearance for hardware and returns on deep jambs.
Common mistakes to avoid
- Measuring drywall-to-drywall or trim-to-trim instead of jamb-to-jamb.
- Forgetting to subtract clearance, resulting in a too-tight unit that won’t fit.
- Measuring to the window stool (shelf) instead of the sill’s high point.
- Ignoring out-of-square conditions—this can cause racking, poor operation, and gaps.
- Mixing up width and height on the order; most suppliers use W x H.
- Overlooking rot or soft spots in the sill/jambs. Insert windows need a sound frame.
When to call a pro
- You find rot, insect damage, or a severely out-of-square frame (>1/2 in diagonal difference).
- Masonry openings needing exterior trim/fins or flashing integration.
- Historic homes with weight-and-pulley pockets where energy upgrades and lead-safe practices are needed.
- You need egress-compliant sizing for bedrooms or code-specific installations.
A careful hour spent measuring and documenting each opening pays off with smoother installation, tighter weather sealing, and fewer surprises on install day. If you’re unsure about your numbers, many window suppliers will verify measurements on-site for a modest fee—well worth it before placing a custom order.