There have two installation styles: Inside mount and Outside mount 

Inside Mount Measuring: Inside Window Width, Height, Depth

Custom width=Inside Window Width;

Custom Height=Inside Window Height

An inside mount simply means that the window covering fits neatly inside the window, outlining crisply the inside of the window frame and snugly fitting all around the inside. An inside mount offers the cleanest and most minimal look and can be installed as deep or as shallow as you desire.

For inside mount custom blinds, you can just enter your exact window size. 

And we will help you take a 0.635 cm(1/4 inch) deduction off the width for better installation.

Outside Mount Measuring: Inside Window Width, Height, Depth

Custom width=Outside Window Width + at least 10cm(4 inch)

An outside mount is when blinds or shades are installed on the wall above the window or directly on the window trim. Blinds or shades are made larger than the window opening for complete coverage.

For outside mount custom blinds, we recommend adding at least 10cm to the original size of the window.