Go to the millwork dept. at Menard’s– they have plenty of brochures to show you how to measure and replace– easiest ones are the ones that fit inside the finished opening

Measure the width of the window in three places: top, middle, and bottom. Do the same for the height. (use the smallest measure). Also, check that the window is square.

Don’t be afraid to go window shopping. When I ordered the windows for my addition, the difference in the total price for four windows was about $300. Most expensive supplier was the home improvement center, least expensive was a lumber yard.

Also consider, for efficiency, double glazed or triple glazed. Low E glass (a coating on the glass to reduce heat transfer)? Argon filled (argon is a better insulator between the glass than oxygen)? Window features, what grid patter (if any) between the glass. Double hung (both sashes operable) or single hung (bottom sash operable)? Half screen or full screen. If you are looking for a window for a bathroom you could order the window with frosted glass.

Ask the supplier so as not to make a mistake that you will have to deal with…Some manufacturers do it different..If you do not know what they say ask again ’till you understand….

Ben gave a good answer but he left off the subtract a 1/2 an inch from your measurements. That’s the size of the window you’ll need.