SI vs. Customary Units: A Complete Guide

Introduction

If you grew up in almost any country other than the United States, you were taught metric. If you grew up in the US, you learned feet, miles, pounds, and gallons, with metric as a secondary system mentioned in science class. Both are used to describe the same physical world, and both can be converted to each other with exact or near-exact precision. But they are not designed the same way, and understanding the difference helps you use both confidently.

The SI System

The International System of Units (SI) is the modern form of the metric system, maintained by the Bureau International des Poids et Mesures (BIPM) in France. It is the world's most widely used measurement system, adopted officially by nearly every country.

The seven base units

SI is built on seven base units from which all others are derived:

Derived units

Every other unit is a combination of these base units. The newton is kg·m/s². The joule is kg·m²/s². The watt is kg·m²/s³. This coherence is the source of SI's elegance: physical equations work without any unit conversion factors as long as all quantities are expressed in base units.

Decimal prefixes

SI multiplies units by powers of ten using standardized prefixes. Kilo means 1000, mega means 10⁶, giga means 10⁹. Milli means 0.001, micro means 10⁻⁶, nano means 10⁻⁹. Since 2022, the system also includes ronna (10²⁷), quetta (10³⁰), ronto (10⁻²⁷), and quecto (10⁻³⁰).

The US Customary and Imperial Systems

The US customary system and the British imperial system are closely related but not identical. They share a common origin in English units but diverged after American independence. Some units have the same name but different values — the gallon is the most famous example.

US customary units

Used in the United States for everyday commerce and daily life. Key units include inch, foot, yard, mile, pound (avoirdupois), ounce, US gallon, and US fluid ounce. Many of these are now defined as exact fractions of SI units:

• 1 inch = 2.54 cm exactly
• 1 foot = 0.3048 m exactly
• 1 yard = 0.9144 m exactly
• 1 international mile = 1,609.344 m exactly
• 1 pound = 0.45359237 kg exactly
• 1 US liquid gallon = 3.785411784 L exactly

Imperial units

The UK officially adopted the metric system for most purposes in the 1960s and 1970s, but imperial units remain in use for road distances (miles), beer and cider (pints), and personal height and weight in some contexts. The imperial gallon (4.54609 L) differs from the US gallon (3.785411784 L) by about 20%.

Structural Differences

The two systems differ in how they relate units to one another:

Coherence and decimal structure

SI is coherent: every derived unit is a product or ratio of base units without any numeric factor. Length × length gives area in m². Force × length gives energy in J = N·m. In US customary units, the same operation requires care: foot × pound gives a "foot-pound" that is not the same as the pound-force applied over one foot — there is a gravitational constant gc involved.

Prefixes vs. named units

SI uses prefixes for magnitude: kilometer, millimeter, microsecond. US customary units use distinct names for different scales: inch, foot, yard, mile. This makes US customary units more intuitive at first glance but harder to compute with, because the ratios between them are not decimal (12 inches per foot, 3 feet per yard, 5,280 feet per mile).

Where each is used

SI is used almost everywhere for science, medicine, engineering, aviation (altitude in feet is an exception), and commerce. US customary units dominate in the US for consumer goods, weather reporting, and everyday life. The UK uses a hybrid: metric for most things, imperial for road distances and personal weight.

Exact Conversions

Many US customary units are now defined as exact fractions or multiples of SI units, which means US→SI conversions are exact in principle:

Everyday Implications

When traveling, working internationally, or reading foreign documents, you will constantly encounter both systems. Knowing a few rough conversions mentally is useful:

• 1 meter ≈ 3.3 feet
• 1 kilometer ≈ 0.62 miles
• 1 kilogram ≈ 2.2 pounds
• 1 liter ≈ 0.26 US gallons (or 0.22 imperial gallons)
• 1 inch ≈ 2.5 cm
• 20 °C ≈ 68 °F (room temperature)
• 0 °C = 32 °F (freezing)
• 100 °C = 212 °F (boiling at sea level)

For anything requiring precision — engineering, medicine, cooking by weight, navigation — use M-Convert or another exact-factor tool rather than mental math.

A Brief History

The metric system was created during the French Revolution (1790s) as a rational alternative to the chaotic collection of local units that plagued pre-revolutionary France. It spread throughout Europe during the 19th century and became truly international in the 20th century with the creation of the SI in 1960.

US customary units trace their roots to English units, which themselves derived from Roman, Anglo-Saxon, and Norman sources. The US adopted its own federal standards in the 19th century and officially defined them in terms of metric units in 1893 (the Mendenhall Order) and again in 1959 (the international yard and pound agreement).

Conclusion

SI and customary units are not rivals — they are two parallel descriptions of the same physical world, and modern definitions make them exactly interconvertible. The key to using both well is to know which system a document or conversation is using, and to apply the exact factor when precision matters.

For conversions, use M-Convert's Length, Mass, Volume, or Temperature converters. For deeper questions about significant figures, see Significant Figures & Rounding.