The rule most usually adopted for determining the number of square feet of heating surface of different sized steam pipes is to calculate that a foot length of 4-in. pipe has a superficial, i.e. square, foot of surface. Then the areas of other sizes can be readily estimated. A 1-in. pipe, for instance, has one-fourth of a square foot of surface per foot run, or a square foot to 4 ft. run. This would also apply to bends, fittings, and other hot parts of the installation. These calculations are based on the interior diameters of pipes. Often the exterior is taken, by which a 1 1/2 in. pipe, 1 ft. long, would be said to have half a square foot of surface, because it is of 2 in. exterior diameter (nearly). This, however, is not a correct way, for it gives a certain size of pipe a variable super surface according to the thickness of the material of which it is made, whereas the thicker material would decrease heating efficacy rather than increase it.