The number of measurement units that have been devised and utilized throughout history is, quite frankly, enormous. The purpose of this page is to attempt to bring a bit of order to chaos, by providing basic information and techniques for comparing and inter-converting physical quantities from unit to unit, and from system to system.

Before the development of theoretical physics, virtually all measurement units concerned only the quantities of length, mass (given as "weight"), time, area, and volume (a.k.a. capacity, liquid or dry). For each of these quantities (which were usually considered basic), a sort of hierarchy congealed, in which units were conceived as "chunk-ed"... that is, built out of multiples of smaller units, and with each size of unit often specialized for some specific use. And, of course, many countries had developed their own unit systems independently.

The introduction of the Metric System in the late 18th century was a step in the right direction: a uniform system of clearly defined units of measurement that were not a priori tailored to any specific culture or application, and would thus be universally applicable. However, in the following century, what with the rise of theoretical physics (thermodynamics, electromagnetism, etc.), new units were continually invented to express other such quantities as the need arose, regardless of whether or not those units were reasonably compatible with the older (traditional) units or the newer (metric) units, or even with each other.

Well, to make an exceedingly long story short, the concept of "intra-system unit coherency" eventually caught on. Thus arose the MKS system (meter, kilogram, second), the CGS system (centimeter, gram, second), and the USCS system (foot, pound_force, second). The MKS system (as modified by additional base units) was officially adopted by SI (System International) and is currently the international standard in scientific work. CGS, although popular with astronomers of the past century or so, appears to be on the decline. As for USCS (not to be confused with the hodge-podge of English units that is still used by us stubborn Americans in our everyday lives, for some reason), it is sort of a contrivancy, and often introduced in physics text as an example of a coherent system with an alternate set of base quantities (length, force, time).

(to be continued...)

Note: Each physical quantity is accompanied by a vector (x,y,z), which is a representation of its dimensional breakdown in terms of length, mass, and time. In SI, this corresponds to m^xkg^ys^z. Click an entry in the following tables for a list of units for that quantity, sorted by size.