Davies Publishing SPI Practice Test

In unit conversion, you use conversion factors as fractions that equal 1, and you carry them along through the calculation. A conversion factor expresses the same quantity in two different unit terms, so its value is 1 when you look at the right ratio. By multiplying by this “1,” you change the units without changing the numeric amount, and the unwanted unit cancels out while the desired unit remains. For example, converting 12 inches to feet: 12 inches × (1 ft / 12 in) equals 1 ft. The inches cancel and the number stays the same. Using a zero-valued fraction would erase the result, and treating the factor as an addition or ignoring units would break the dimensional check that keeps conversions valid.