# Window Sill Calculator

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

- Page: https://jianzhujisuan.com/en/window-sill
- Category: Finishes (https://jianzhujisuan.com/en/c/finishes)
- Chinese version: https://jianzhujisuan.com/zh/window-sill.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Window opening width 1.5 m, Overhang each end 0.06 m, Sill width (depth) 0.24 m, Piece length 1.0 m, Number of windows 1 pcs, Sill thickness 0.03 m): **Total pieces 2 blocks**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Window opening width | number | 1.5 | m |  |
| Overhang each end | number | 0.06 | m |  |
| Sill width (depth) | number | 0.24 | m |  |
| Piece length | number | 1.0 | m |  |
| Number of windows | number | 1 | pcs |  |
| Sill thickness | number | 0.03 | m |  |

## Formula

- **Sill length**: L = opening width + 2 × overhang
- **Pieces**: n = ⌈L / piece length⌉ × windows
- **Total area**: A = L × depth × windows
- **Fall**: Δh = depth × 5%
- **Bedding mortar**: W = A × 0.02 × 1600 (kg)

## Worked example

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.

## Questions

### 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.

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