Hello Ladies (and you too Gentlemen).  Have you looked at a measuring tape with fear and anxiety in getting the measurement wrong. This post gives you all you need to measure with accuracy, and it’s easy.1) Decide if your blind is an inside or outside mount.  Two points here:

a)  The depth of the head rail on the blind and the depth of the window sill will determine if an inside mount is possible.
b) Once your location of installation is determined…
2) Use a rigid measuring instrument to measure.
3) Always round down to the nearest 1\8” for width.
4) Always round up to the nearest 1\8” for length.

This image will help guide you:

ruler-for-blinds

5) If using inside measurements, they are referred to as tight measurements and the manufacturer will make appropriate deductions.measuring-for-blinds
6) If using outside measurements, they are referred to as finished measurements and no deductions are required.
7) Measure in 3 locations for width and length as seen above, then order the smallest width and the largest length.
8) If outside mount is determined then a selection of casing or wall mount must be determined before measuring.
9) Good luck and enjoy your blinds