Use a steel tape measure
Measure each window separately
Decide inside mount or outside mount before measuring
For inside mount, measure the exact width at the top, middle, and bottom of the window opening
Record the narrowest width for inside mount
For inside mount, measure the exact height at the left, center, and right of the window opening
Record the tallest height for inside mount
Measure the window depth for inside mount clearance
For outside mount, measure the full width you want the blinds to cover
Add extra width beyond the window frame for better light control and coverage
For outside mount, measure the full height you want the blinds to cover
Add extra height above and below the window as needed
Measure in inches and write down all measurements clearly
Round down only if the manufacturer requires it
Double-check every measurement before ordering
Do not assume all windows in the same room are the same size