How to measure a window sill

Depth

1. window frame with a sill groove; the sill is slid under the frame
2. exterior sill falling outwards
3. drip nose projecting beyond the finished facade
4. bedding under the sill (wedge or insulating sub-sill, depending on the system)
5. external wall insulation with render
6. masonry below the window

Drawing no.

KV-06

Title

Exterior window sill, section and plan of end

Scale

diagram, dimensions in mm

Revision

14 October 2026

1. Measure the depth of the outer reveal – from the window frame to the finished facade – on the left and right.
2. Add about 10 mm for the part that slides under the frame.
3. Add 30–40 mm for the nose projection.
4. Round up to a depth from the manufacturer’s range.

Example (ZOFI): 160 + 10 + 30 = 200 mm → 210 mm is ordered.

If the facade has not been insulated yet, do not guess the depth. Wait for the insulation to go on, or work from the exact thickness of the insulation, adhesive and render in the build-up.

Length

When the sill is set into the render, ZOFI gives a minimum of 17.5 mm set in at each end; after deducting 2 mm for the end cap, that makes the sill 15 mm longer at each end. Sills longer than 5 m are split into two pieces with an expansion joint.

Checks before ordering

• the finished facade has been measured, not the masonry
• depth measured on the left and right, and the larger figure used
• length measured at the top and bottom by the frame, and the larger figure used
• the type of end cap and the end finishing are clear
• each window is labelled with which window it is (for example from the drawings)

The window sill calculator will do the sums for you.

Questions and answers

How do I work out the depth of an exterior window sill?

The reveal depth (from the window frame to the finished facade), plus about 10 mm for the part that slides under the frame, plus a 30–40 mm nose projection. Round the result up to the next depth the manufacturer supplies. The method follows the ZOFI fasády guide.

How do I work out the length of a sill with end caps?

According to ZOFI, measure the distance between the outer reveals (if it varies, take the larger figure), deduct 5 mm for the end caps and a further 2 mm for expansion. Example 1,800 − 5 − 2 = 1,793 mm.