This is the measurement you need to take if you want your blind to fit inside the window recess.

Measure the full width and height of the recess (the size of the hole, in three places. We need the minimum measurement. We will make the appropriate deductions to ensure a perfect fit.

 Use a metal tape measure.

 Measure the exact size of the area you would like the blind to overlap. To minimize light leakage we recommend that the blind overlaps the window recess by at least 70mm at the top and 50mm each side.

 The overlap at the bottom of the blind is a matter of personal choice but please take into account obstacles such as radiators.