Start by measuring the depth of your window in front of any obstructions. For a double hung window, the depth needs to be measured in front of the outer most window pane. Similarly, measure the window’s depth in front of any latches or handles if the window you are measuring has such parts.
Determine the distance from one inside edge of the window all the way to the other inner edge of the window. This will be the inner-width of the window you are measuring. If the sides of you window seem to be slanting at an angle, measure the inner-width at three different spots along the vertical length of the window and use the smallest of the three measurements.
Determine the inside height of the window. It is the vertical distance from the bottom of the window all the way up to the top of the window. As before, measure the inside height of the window at three different spots along the horizontal length of the window. However, you will have to use the largest measurement in this case.
Keeping the frame of the window in your measurements, determine the distance between the outer edges of the window at three different spots; top, middle, and bottom. This will be the outside width of the window and make sure to use largest of the three measurements.
Measure the outside height of the window in a similar way in which you measure the outside width.
If you are planning to mount an object above the frame of the window, you will need to compensate the additional length by adding however many inches to the outside height of the window.