1. Clearance & Rubber Bead Check:

Ensure your window has a rubber beading between the window frame and the glass. 
The Perfect fit framework needs a minimum of 28mm clearance between the edge of visible glass and any obstruction, such as handles, hinges or vents.
Ensure the rubber beading is less than 6mm wide, from the edge of the frame to the edge of the glazing. 
If your rubber beading is wider than 6mm Perfect Fit blinds will not be suitable.

2. Find Depth:

Measure the depth of the window. To do this, simply place a credit card over the corner of the frame and use a metal tape to take the measurement. This will usually be between 18 and 30mm.

3. Find Width:

Measure from the inside edges of the frame where the glass meets the rubber bead. You shouldn't include any of the beading or rubber seals in your measurements. Measure the top, the middle & the bottom of the glass to the nearest mm, use the smallest measurement as your width.
Only measure the visible glass area of each window or door.

4. Find Drop:

Measure the drop of the visible glass in three places. Use the smallest measurement as your drop.