Measure the length of your radiator valves and pipes. Round that number to the nearest inch, then add 2 inches.
This is the length of your cover.
HELPFUL HINT: Radiator covers look better centered under a window. It can be a good idea to order the cover a little longer than your radiator to center it below the window.
Measure from the floor to the tallest point of your radiator including valves and pipes. Round that number to the nearest inch, then add 1 inch.
Measure the front of your baseboard to the front of the radiator, including valves and pipes, round up and add one inch. This will be the depth of your radiator cover.