Windows

Measure the window frame's outer width, then add 4-6 inches on each side for the standard look — an 8-12 inch total addition — to get the rod length to buy or cut to.

For an inside mount, measure the exact width and height inside the window frame and let the deduction happen at manufacturing — don't subtract anything yourself unless the product instructions say to. For an outside mount, add a few inches of overlap on every side.

Mount the rod 4-6 inches above the window frame and 4-6 inches past each side, then measure from the rod down to where you want the curtains to end — the floor, sill, or somewhere between.

Measure the width at the top, middle, and bottom of the frame and use the narrowest number; measure the height at the left, middle, and right and use the shortest. Also check the two diagonals to confirm the frame is square.