Measure the width from the inside edge of the left jamb to the inside edge of the right jamb
Measure the height from the inside edge of the top jamb to the inside edge of the sill
Measure the width at the top, middle, and bottom
Measure the height at the left, center, and right
Record the smallest width measurement
Record the smallest height measurement
Measure the window depth if required
Use a steel tape measure
Round measurements to the nearest 1/8 inch
Measure each window separately
Double-check all measurements before ordering or installing