Unit Converters

Almost every conversion is a single multiplication, and almost every error comes from somewhere else. The three recurring causes are using a rounded factor where an exact one exists, applying a length factor to an area, and assuming that two units with the same name are the same size.

The first is easy to avoid. Since 1959 the imperial units have been defined exactly against the metric ones, so an inch is 25.4 millimetres and a pound is 0.45359237 kilograms with no approximation involved. These tools use the full definitions and round once, at the end.

The second is a matter of dimension. A factor applied to a length is applied twice for an area and three times for a volume. One metre is 3.28 feet, but one square metre is 10.76 square feet and one cubic metre is 35.31 cubic feet. Rooms, flooring and paint quantities are where this goes wrong most often.

The third is a naming problem. A US gallon and an Imperial gallon share a name and differ by about 20%. A metric tonne, a US short ton and an Imperial ton are three different weights. Every unit here is labelled with its system, so there is no unit that is simply called a gallon.