How to Replace Windows in your House: How to Measure for New Windows. Standard window depth is 3 1/4 . This is how to measure window for opening size. When measuring opening size you will have to make deductions you would want a window a little smaller to fit in the opening.Standard deduction is 1/4 off of the height and width. to the windows for example if your opening size is 32x50 then an ideal window would be 31 3/4 x 49 3/4.Please post any questions and I'll do my best to answer them.his is how to measure window for opening size. When measuring opening size you will have to make deductions you would want a window a little smaller to fit in the opening.