Use a steel tape measure
Measure the width of the window frame
Measure the height from the top of the frame to the desired curtain end point
Decide where the curtain rod will be placed
Measure the rod width, not just the window width
Add extra width beyond the window on both sides for full coverage
Measure from the rod position down to the floor, sill, or desired length
For sill-length curtains, measure to just below the sill
For apron-length curtains, measure to a few inches below the sill
For floor-length curtains, measure to just above the floor
For puddle-length curtains, add extra length below the floor
Measure each window separately
Record all measurements in inches or centimeters consistently
Check measurements twice before buying curtains
Consider curtain fullness when choosing width
Include space for curtain rod brackets and finials
Use the largest width and height if the window is uneven
Round up if measurements fall between standard curtain sizes