Using a metal tape measure, find the width, accurate to the 1/16th of an inch, at the point where the brackets are to be mounted. (Keep in mind that you will need to place your brackets low enough from the top of the frame in order to drop your shade into the bracket when you install it.) Take measurements at the top of each window for which you are installing a shade
To the 1/8 th of an inch
To the whole inch
#4 OUTSIDE MOUNT Measure where the brackets are to be mounted. Keep in mind the actual shade material will be 1/2 inch less on each side.
For inside measuring, the entire ruler case should be inside the frame. The backside of the ruler should rest against the inside of the frame. Different brands of rulers have different add-ons for inside measures. It will say somewhere on the ruler (usually the back) how much to add for an inside measure. This particular ruler has an add-on of 2 inches. For example, if you read 36 3/16 when the case and hook are inside the frame, the real measurement with a 2-inch add-on is 38 3/16.
The hook can make a different of 1/16th of an inch on measuring! The hook should be pushed in for an inside measure against the frame. For an outside measure, the hook should be extended.