To measure your baseboard heater, simply measure the overall dimensions which need to be covered by your new cover. If you have a heater which ends at a wall, measure from the wall to the outermost edge of the heater. For a corner unit, measure from the wall to the outside edge, or next corner for a C shaped heater.

The heater in the picture above would be an L shaped heater with left side 3'-9 9/16" and right side 13'-11 7/16". Height would be 7" and depth 2 1/2". This heater would require an end cap at both ends ( this is a closed finished end, not a seperate piece) A heater which ends at a wall at either end does not require an end cap on theat end. Vent locations on your new cover would be at the bottom and top of the cover by default. If you need these locations changed, please make note. Since your new covers will add to the overall dimensions of the heater, please also make note of any restrictions there may be as far as clearances to trim, furniture or other items.