How to Measure a Window Sill

Window sill depth, thickness and dimensions

There is no standard window sill size — the piece is cut to the opening — but these are the ranges most jobs fall in.

• Depth — most interior sills run 5" to 12" deep. Depth is the wall face to the window frame, plus however far the sill projects into the room.
• Overhang — 1" to 1½" past the wall face is typical. Enough to read as a shelf and to shed condensation clear of the wall, without becoming something people knock.
• Thickness — the slab is 3/4". A laminated build-up edge makes the front read as 1½", 2½", 3" or 4" while keeping the weight down.
• Height — set by the window, not by the sill. The sill sits on the rough opening, so measure from the frame rather than from the floor.
• Length — jamb to jamb, from 18" up to 144" in one piece.

Window sill angle, slope and pitch

A common question, and the answer differs sharply between interior and exterior work.

Interior sills are level. The sills on this site are interior stools, and they are cut flat — a sloped interior sill would not hold anything you set on it, and there is no water to shed. If your opening itself is out of level, the sill is still cut square and shimmed to sit true.

Exterior sills are pitched to drain, typically between 5 and 15 degrees away from the building, and are usually a different material and detail entirely. If you are looking for an exterior sill slope, that is masonry or cladding work rather than an interior stool.

One angle does matter indoors: whether the opening is square. Most are not, which is why every piece is cut 1/16" short per side as a reveal, to be caulked after fitting.

Straight sill vs. return sill

A straight sill is a single rectangular piece that sits between the two jambs — you only need length and depth. A return sill (also called a horned sill) has ears that wrap back to the wall on each side; in addition to length and depth you'll measure how far each horn returns past the opening, left and right. A single continuous sill can span one, two or three windows, with the openings separated by interior jambs.

Tips for an accurate measurement

• Measure the length in a few places. Check jamb-to-jamb at the front, middle and back of the opening. If the numbers differ, the opening is out of square — use the smallest, and note it so we can build in fit clearance.
• Decide your overhang first. Depth runs from the window frame to the front edge; add however far you want the sill to project past the wall face before you write the number down.
• Left and right returns can differ. Measure each horn separately — they don't have to match.
• Work to the nearest 1/16". The configurator accepts lengths from 8 to 160 inches to half-inch precision, and shows a live drawing so you can confirm the geometry.
• Measure to the finished surface. For tiled or stone jambs, measure to the finished face, not the rough opening.
• When in doubt, send a photo with your rough numbers and we'll confirm the drawing before anything is cut.

How precise do the measurements need to be?

Every sill is made to your exact dimensions, accurate to ±0.5mm. You don't have to be a professional to get a good result — enter your best measurements in the online configurator and it draws the sill live as you go, so you can sanity-check the shape. We also review every order and confirm drawings within one business day before fabrication begins.