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Window Sill Calculator

Sill length is window width plus overhang at both ends; fall and mortar included.

Quick answer

With the default inputs: Window opening width 1.5 m · Overhang each end 0.06 m · Sill width 0.24 m etc.Total pieces 2 blocks

Use your own dimensions ↓

Inputs

pcs

Result

Total pieces
2blocks
Sill length per window1.620m
Pieces per window2blocks
Total sill area0.39m²
Sill volume0.012m³
Fall across depth (5%)12mm
Bedding mortar (dry)12kg
How this is worked out

Sill length = opening width + overhang at both ends. External sills normally project 60 mm each end so a drip groove can be cut and rain does not run down the wall; internal sills sit flush or project 20-30 mm. Depth follows wall thickness plus plaster, so a 240 mm wall takes a 240-260 mm sill. The fall is not optional - 5% outwards (5 mm drop per 100 mm of depth). A sill that cannot shed water drives rain back in, and that is the most common cause of window leaks. Cut pieces to the piece length when the run exceeds it, leaving 2-3 mm joints; round the piece count up. Bedding is estimated at 20 mm average with a 1:2 cement mortar, and the back of the sill must be fully bedded. Dabs at the corners only leave voids underneath, and the sill cracks underfoot or with thermal movement.

Next up: Window Glazing Area Calculator · Skirting Board Calculator

Formula

Sill lengthL = opening width + 2 × overhang
Piecesn = ⌈L / piece length⌉ × windows
Total areaA = L × depth × windows
FallΔh = depth × 5%
Bedding mortarW = A × 0.02 × 1600 (kg)
1.5 m opening, 60 mm overhang each end, 240 mm deep, 1.0 m pieces, 30 mm thick: 1.62 m sill, 2 pieces, 0.39 m² total, 0.012 m³ volume, 12 mm fall, ~12 kg bedding mortar.

Basis

FAQ

Why does a window sill need a fall?
The sill is where rainwater sits longest. With no outward fall, water stands on the sill and works into the frame-to-wall joint, or through voids in the bedding under the sill. The usual fall is 5% outwards - 5 mm per 100 mm of depth, about 12 mm over a 240 mm sill - which is invisible to the eye but enough to drain. A drip groove under the outer edge makes the water break away and drip rather than run down the wall.
Should a sill be fully bedded or spot-bedded?
Fully bedded. Spot bedding - dabs at the corners only - looks cheaper and is easier to level, but leaves voids under the sill. Three things follow: the sill cracks when someone stands on it or puts a load down, thermal movement separates it from the wall, and the voids become a path for water. Bed it solid on about 20 mm of 1:2 cement mortar and tap it down with a rubber mallet, cleaning off the squeeze-out.

Cite this page

Use this format in reports, method statements or coursework. The link is permanent.

Building Calculator. Window Sill Calculator. 2026-10-25. https://jianzhujisuan.com/en/window-sill

Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.