Windows come in many different sizes. Depending upon the age of your home, they may all be the same size, or every window may be different. Newer homes have relatively standard window sizes, while older homes may not. This makes buying curtains, and even new windows a challenge. Knowing how windows are measured, will at least give you a fighting chance.
Measure the width of the window first. Measure across the window at the top; then across the middle and the bottom. These may not be the same, so you will take the smallest of these.
Measure the height next. Measure on the left, then in the middle, and then on the right. Take the largest of these.
Measure the depth of the window frame, from the window itself to the outside of the frame. Do this on the left, the right, and the top and bottom. They should all be the same, but in the event the measurements are different, use the smallest.
Measure the inside of the window frame, if you are purchasing new windows. Measure the outside of the frame if you are purchasing curtains.