If you are in this section, it is likely that you are not a professional, so if you are doing a renovation or if you are building your house from scratch, the first piece of advice is to ask the builder or contractor who is doing the work to give you the measurements of the window sills, which you can then buy directly on this site.

If, on the other hand, you want to proceed on your own or even if you just want to check for accuracy that the measurements you have received are correct, then in order to calculate the measurements of your windowsill you need some basic information:

• Finished size of wall (with coat and eventual final shaving).
• Positioning of window frames relative to the shoulder of the wall.

If you have this information available, you can easily calculate the measurements of your windowsill.

Depending on the position in relation to the window frame, we can have

• External sill
• Internal sill 
• Sill both inside and outside. (full)

To determine the width (or depth, the short side in short) of the window sill, we must measure from a few centimetres below the positioning of the window frame, until we consider an outward projection, beyond the finished coat of about 4 centimetres. 4 centimetres of overhang is recommended to allow for the possibility of a drip edge at the bottom of the window sill.

The length (the long side) of the sill, on the other hand, should be calculated by adding approximately 4 cm on each side from the edge of the plastered wall.

So ultimately

Short side measurement: Width

Measuring from below the window frame (min. 1 cm below) to the edge of the shaved external wall + 3 cm

Long side measurement: Length

Measurement from the edge of the window shoulder shaved to the other + 4 cm+4 cm (i.e. 4 cm per side)

For the internal sill, the same criteria are used as for the outer sill, but since the drip edge is not necessary, the inner projection can be reduced to 3 cm (instead of 4 cm).

So ultimately

Short side measurement: Width

Measurement starting from below the window frame (min. 1 cm below)

In rare cases, it may happen that the window sill covers the entire section of the wall and faces both the outside and inside in one unbroken piece near the window frame.

In this case, the indications given for internal and external window sill measurements are added together.

So ultimately:

Short side measurement: Width

Measurement from the flush of the plastered inner wall to the flush of the finished outer coat + 3 cm + 4 cm (total 7 cm)

Long side measurement: Length

From the edge of the plastered window shoulder to the other + 4 cm+ 4 cm (i.e. 4 cm per side)