Systems of Measurement Units
Throughout human history, distinct regional measurement systems developed to quantify length, mass, area, volume, and time. Today, two major international systems predominate:
1. International System of Units (SI / Metric)
The modern metric system is the universal standard for science, medicine, and global trade. It relies on decimal base-10 prefixes (kilo-, centi-, milli-) scaling systematically from base units such as the meter (m), gram (g), and liter (L).
2. US Customary & Imperial Systems
Derived from historic English units established before the American Revolution, US Customary units use historical non-decimal conversion ratios (e.g. 12 inches per foot, 16 ounces per pound, 4 quarts per gallon).
Why the United States Still Uses Customary Units
When European nations metricated during the 19th century, the United States had already built massive industrial manufacturing plants, rail networks, building construction standards, and land survey grids based on feet, inches, and acres.
While the US officially recognized the metric system in 1866 and defined Customary units in terms of exact metric standards (such as defining 1 inch as exactly 2.54 cm in 1959), the immense cost of replacing machine tools, building codes, highway signs, and consumer packaging led the US to retain Customary units for domestic commerce while utilizing metric standards for scientific, medical, and military operations.