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Window sill calculator

The calculator works out what sill depth to order and what lengths to cut. You can enter the depth measured on the finished facade, or calculate it from the wall build-up if the insulation is not on yet.

Sheet
Sill calculator
Revision
Sources
1 item
Type
calculation aid

Sill depth

How will you enter the depth?

from the window frame to the finished facade

to match the depth steps the manufacturer supplies

Window lengths (optional, for the cutting list)

WindowLength between reveals (mm)Quantity

The result will appear when you press Calculate.

What the calculation is based on

  • depth = reveal depth to the finished facade + slide under the frame (~10 mm) + nose projection of 30–40 mm, rounded up (ZOFI fasády),
  • length with end caps = length between reveals − 5 mm for the caps − 2 mm for expansion,
  • length when set into the render = length between reveals + 2 × 15 mm + 2 mm,
  • over 5 m the sill is split into two pieces with an expansion joint.

For the wall build-up, we add the distance of the frame from the masonry face, the insulation and the render layers – that gives the depth of the future reveal. Always measure it again once the insulation is on.

Section through an exterior window sill The sill is slid under the window frame, falls outwards, and its nose projects 30 to 40 mm beyond the finished facade. Bottom right: plan of the sill end with its end cap. 30 – 40 ≈ 10 under frame falls outwards plan of sill end end cap 1 2 3 4 5 6
  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

Next: How to measure a window sill · Build an enquiry

Technical sources

The sources this sheet draws on. We checked prices and values from standards on the date shown; for prices, whatever the source currently states always applies. Sources marked “in Slovak” or “in Czech” are only available in that language.

ItemSourcePublished byVersion / dateChecked
01 Measuring a window sill correctly before fitting it to the facade (projection 30–40 mm, +10 mm under the frame, end caps, expansion) (in Czech) ZOFI fasády s.r.o. undated 14 October 2026
Prepared by
KlampiarskeVýrobky.sk
Revision
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