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How to take the Measurements of a Window Sill

Window sills can visually upgrade the house facade and be an eye-catcher inside of any home. If anyone is thinking about installing window sills they must take precise measurements of the old ones. Only exactly measured components will later fit perfectly in the wall opening. This manual will help you with the measurements of the old sill – and you do not need previous experience. The following manual will give you detailed information about the different steps and the necessary tools.


  • Bubble level
  • Tape measure

1. Preparation

If you want to determine the measurements of a window sill, you will need the appropriate tools and materials at hand and ready to use. This will ensure a smooth procedure. A blueprint of the house can be very helpful too. But you will have to re-measure the dimensions in order to identify deviations anyway. The new measurements can then be written in the blueprint and marked with the current date (Fig. 1).

2. Measurements of the outer Construction Element

The dimensions of aluminium, wooden or uPVC windows sills outside are determined in two different ways. This will ensure that length and depth are perfectly defined.

a) The Length of the outer Window Sill

  • First you have to open the window. This ensures that the sill is easily accessible and the procedure can be performed smoothly.
  • Then you will need the tape measure. Determine the length of the sill by measuring the distance of the reveal from left to right. The reveal is defined as the space between the left and right walls and the distance from top to bottom.
  • Now you have to deduct 10 mm from the determined dimension. The result is the necessary measurement of the profile. These 10 mm have to be deducted to ensure that the new construction element will fit into the reveal.
  • If the component is embedded at the sides, you will have to measure the additional distance and add it to the previously determined dimension. This value must be included in the required measurements.

Especially old sills often have a warped profile. A bubble level can help to work precisely.

b) The Depth of the outer Window Sill

  • The depth is defined by the distance between the lower edge of the frame and the outer edge of the wall opening.
  • For the overhang you have to add 30 - 40 mm to the determined distance. This overhang protrudes from the opening of the wall and protects the facade from water. Anyone thinking about placing flowers on the sill should definitely consider the installation of an overhang.

An overall appealing appearance can be achieved with side finishes. The selection of appropriate side finishes is of significant importance for the aesthetic of the house facade. A plastered finish matches well with a plastered house facade. Boarded facades look better with clinker finishing elements.

3. Measurements of the inner Construction Element

Wooden or uPVC window sills also need to be measured in two different ways. The correct length and depth are significantly important for later installation of new components.

a) The Length of an inner Window Sill

  • Take the measurements for the length inside the window by determining the distance from left to right.
  • To ensure that the new sill can be installed smoothly later, you have to deduct 5 mm.
  • Do you want to embed the your sills into the masonry? Then do not forget to add the respective distance to the measurements.

b) The Depth of an inner Window Sill

  • Take the measurements between the front edge of the wall opening and the lower edge of the frame.
  • Then add 10 - 20 mm to this value. The added value defines the part of the component that is later embedded under the profile of the frame.
  • If you want an overhang for visual or practical reasons, you also have to add the respective value to the measurements - usually 30 -40 mm.

It is also possible to order side finishes for outer and inner window sills. These are not part of the accessory items, but are already installed in the sill and should therefore be part of the required measurements.

Taking precise measurements of the old components is obligatory for the installation of new construction elements. This can help you achieve a smoother installation process. If you follow the steps of this manual closely, you can not only learn a new procedure but the job can be performed by yourself.

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