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Metric vs. Imperial: Why We Have Two Systems

If you've ever been confused by the fact that the US measures in inches while the rest of the world uses centimeters, you're not alone. The existence of two measurement systems is one of the most persistent sources of confusion in everyday life.

A Brief History

The Imperial System

The imperial system (technically "US customary units") traces back to Roman and medieval English measurements. An inch was originally based on the width of a man's thumb. A foot was literally the length of a king's foot. A mile comes from the Roman "mille passus" — 1,000 double paces of a marching soldier.

These units were standardized in the British Empire and carried to America by colonists. By the time the metric system was invented, the US had already built its entire infrastructure around imperial units.

The Metric System

The metric system was created during the French Revolution (1790s) as a rational, science-based alternative. Instead of arbitrary body measurements, it's based on universal constants:

• The meter was originally defined as one ten-millionth of the distance from the equator to the North Pole
• The kilogram was the mass of one liter of water
• All other units are powers of 10 — making conversions as simple as moving a decimal point

Why the US Hasn't Switched

The US has actually tried to go metric several times:

• 1866: Congress legalized the metric system
• 1875: The US signed the Treaty of the Meter
• 1975: The Metric Conversion Act declared metric the "preferred system" — but made conversion voluntary
• 1988: Congress required federal agencies to use metric for procurement

The problem? Converting every road sign, manufacturing specification, recipe, and product label would cost an estimated $300+ billion. And Americans are stubbornly comfortable with their inches and pounds.

The US Hybrid System

Here's what many people don't realize: the US already uses metric extensively in professional contexts.

• Science: All scientific research uses metric (grams, meters, liters)
• Medicine: Drug dosages are in milligrams and milliliters
• Military: The US military uses metric for distances and ammunition
• Nutrition: Food labels show both metric (grams) and imperial (ounces)
• Automotive: Engine sizes are in liters; tools are metric and imperial

Quick Conversion Cheat Sheet

Length

Weight

Volume

The One Thing to Remember

If you remember nothing else from this page, remember these four numbers:

• 2.54 — inches to centimeters
• 1.6 — miles to kilometers
• 2.2 — pounds to kilograms
• 1.8×+32 — Celsius to Fahrenheit

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