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The foot, pound, second (fps) system of units is widely-used, particularly in the UK and the USA, in many industries and in most public transactions. When SI units are used, fps units are often offered as an alternative. In this section, fps units are listed and conversion factors relevant to heat and mass transfer are tabulated.

The foot, ft, is one third of the Imperial Standard yard, yd, which is defined as 0.9144 meter exactly. The foot is therefore 0.3048 meter.

The pound, 1b, is defined as 0.453 592 37 kilogram.

The gallon, gal, is a unit of volume. The British gallon is defined as the volume of 10 Ibs of water of density 988.859 kg/ m3 weighed in air with a density of 1.217 kg/m3 against weight of density 8 136 kg/m3, i.e., 1 gal = 4.546 09 × 10−3 m3. The US gallon is smaller than the British; 1 USA gallon = 3.785 412 × 10−3 m3 = 0.8327 gal.

Table 1 gives a list of the most common secondary units in the British System, and Table 2 lists factors for converting fps units to SI units and vice-versa. (See Table 2 on pages 244 and 245.)

Table 1. Secondary units in the British FPS system

Units of Length 
12 inches= 1 foot (ft)
3 feet= 1 yard (yd)
22 yards= 1 chain
10 chains= 1 furlong
8 furlongs or 1760 yards= 1 mile (mi)
6080 feet= 1 UK nautical mile*
6 feet= 1 fathom
Units of Mass 
16 ounces (oz)= 1 pound (lb)
14 pounds (lb)= 1 stone
28 pounds= 1 quarter
4 quarters or 112 pounds= 1 hundredweight
20 hundredweight (cwt) or 2240 lb= 1 ton
Units of Area 
4840 square yards= 1 acre
640 acres= 1 square mile
Units of Volume 
20 fluid ounces (fl. oz)= 1 pint
2 pints (pt)= 1 quart
4 quarts (qt)= 1 gallon

Table 2. Conversion factors, FPS/SI


Taborek, J. (1984) Heat Exchange Design Handbook, Hemisphere Publishing Corp.


  1. Taborek, J. (1984) Heat Exchange Design Handbook, Hemisphere Publishing Corp.
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