1.1.1 SI Base and Derived Units

While the Americans are still clinging on to the imperial system (e.g. 1 foot = 12 inches, 1 pound = 16 ounces), the rest of the world have switched to the metric system (e.g. 1 m = 1000 mm, 1 kg = 1000 g). More precisely, we have adopted the International System of Units (Le Système International d’Unités), aka SI Units, which is built upon 7 base units of 7 base quantities.

What about the units for other quantities? Well, because the other quantities can be defined in terms of these 7 base quantities, their units can always be expressed in terms of base units. They are thus called derived units of derived quantities.