Interior window sills are prepared for a specific opening. Even a small measurement error can leave the sill too short or require further cutting. Record the length and depth separately, rather than measuring only the visible width of the opening.
When should you measure?
The most reliable time is after the window has been installed and the final wall and reveal positions are known. If decoration is unfinished, allow for plaster, plasterboard or other finishes. Measurements taken from an old sill before replacing a window are suitable for an initial estimate, but the new frame position may change the final dimensions.
1. Measure the sill length
Measure between the left and right reveals at the sill position. Check near the window and at the outer edge of the opening: walls are not always parallel. If the ends will be recessed into the reveals, add the planned recess on each side.
Do not rely only on the old sill length. Check the opening and planned final finish.
2. Measure the sill depth
Measure from the position beneath the window frame to the finished wall edge, then allow for the chosen projection into the room. The sill normally extends beneath the frame, so the visible distance from the frame front to the wall edge is not the complete product depth.
3. Consider the radiator and airflow
A deep sill above a radiator can obstruct warm air rising along the window. Choose a projection that is practical without unnecessarily covering the heat source. Check the installation arrangement if the sill needs to be unusually deep.
Common measurement mistakes
- Ignoring the recess into the reveals.
- Forgetting the section beneath the window frame.
- Measuring in one place when the opening is not square.
- Ignoring the thickness of the final wall finish.
- Choosing an excessive projection above a radiator.
- Confusing sill width (depth) with length.
What should you include in your enquiry?
For each sill, provide the quantity, material, finish, length and depth in millimetres. Specify narrowing, end lamination, joining, end caps and installation if required. In our window sill calculator, you can select PVC or MDP and enter the dimensions of every sill.
When should you ask for help?
Ask for professional measurement advice when reveals are uneven, sills meet at a corner, non-standard finishing is needed or windows are being replaced at the same time. This allows the installation height, recesses and finishing details to be agreed together.
Measurements, prices and order confirmation
The calculator provides an estimate, but final cutting dimensions and configuration must be confirmed before production. Width means the sill depth; length runs along the window opening. Prepared sills can be collected at Verkių g. 44, Vilnius, LT-09117, at an agreed time. For window sill enquiries, call +370 641 76065.