This drawing shows the three measurements needed. The length includes the valve shown on the left and the pipe on the right. The height should be measured at both ends since some radiators are noticeably higher at one end. It may help to place a place a book on top to simplify measuring the height. The depth is measured from the baseboard molding or quarter round to the front of the radiator. The measurements do not have to be extremely precise. Measure to the nearest 1/4 inch or a little larger. When we build the enclosure, we add 1/2 inch to each end, to the height and to the depth. The enclosure fits as shown in the drawing below.

This drawing shows the size of the enclosure relative to the radiator. As noted above, we add 1/2 inch clearance at each face. Thus, the length of the base is 2 1/2 inches longer than the radiator. The depth is 1 3/4 inches greater and the height is 2 1/4 inches taller. The top of the enclosure overhangs 1/2 inch at the front and on each end. You must allow 1 3/4 inches between the ends of the radiator valves and pipes and any walls, doors or other obstructions. There must be 2 1/4 inches between the top of the radiator and any window sills or other top obstacles. There must also be room between the wall and the radiator for the back. Usually about 1 inch is sufficient.