What is the difference between SI and metric?

The metric system is the informal name for the decimal measurement system developed in France in the 1790s. SI (International System of Units) is the modern, internationally standardized version of the metric system, defined since 1960. All SI units are metric, but SI includes more precise definitions and additional derived units not in the original metric system.

Why doesn't the US use the metric system?

The US has made multiple attempts to metricate, including the Metric Conversion Act of 1975. Adoption remained voluntary and industries with large installed bases (construction, consumer goods, road infrastructure) did not convert. US science, medicine, military, and international trade all use SI — the gap is mainly in consumer and construction contexts.

Is the imperial gallon the same as the US gallon?

No. The US gallon is 3.785 L; the imperial (UK) gallon is 4.546 L — about 20% larger. This matters for fuel economy comparisons: a car rated at 40 mpg (UK) is about 33 mpg (US) for the same fuel consumption.

SI vs Imperial Units: A Complete Guide

Two major unit systems divide the world: SI (the metric system) and the imperial system. Engineers, scientists, and tradespeople encounter both daily. This guide explains where each system came from, what makes them different, when each is appropriate, and which conversions matter most in practice.

Published March 19, 2026

Key takeaways

SI (the metric system) is the international standard used by every country except the US, Liberia, and Myanmar for everyday measurement.
Imperial units survive in the US for construction, road distances, body weight, and cooking, but US science and medicine use SI.
The two systems are legally defined by exact conversion factors — 1 inch = 25.4 mm exactly.
Understanding both systems prevents costly errors in engineering, manufacturing, and international trade.

What is the SI system?

The International System of Units (SI, from French Système International d'Unités) is the modern form of the metric system, adopted by the General Conference on Weights and Measures (CGPM) in 1960. It defines seven base units: the meter (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), mole (amount of substance), and candela (luminous intensity). All other SI units are derived from these seven.

SI is a decimal system — every unit scales by powers of 10 using standard prefixes: kilo (×1,000), centi (×0.01), milli (×0.001), and so on. This makes mental arithmetic and unit conversion straightforward: 1 kilometer = 1,000 meters, 1 centimeter = 0.01 meters.

What are imperial units?

Imperial units descend from a collection of English and British customary measures standardized by the British Weights and Measures Act 1824. The system includes familiar units like the inch, foot, yard, mile (length), ounce, pound, stone, hundredweight, ton (mass), and fluid ounce, pint, quart, gallon (volume).

Unlike SI, imperial units do not have a consistent scaling factor. There are 12 inches in a foot, 3 feet in a yard, 1,760 yards in a mile. There are 16 ounces in a pound, 14 pounds in a stone, and 8 pints in a gallon. This complexity is why most of the world moved to SI for technical work.

Note: US customary units and imperial units are closely related but not identical. The most important difference is the gallon: the US gallon is 3.785 L while the imperial gallon is 4.546 L.

Which countries use which system?

The United States is the only large economy that has not officially metricated. The US uses a mix:

Everyday life: miles, feet, pounds, Fahrenheit, gallons (US)
Science, medicine, military, aviation: SI units
Manufacturing: a hybrid — torque specs in lb·ft, tolerances in thousandths of an inch

The United Kingdom officially uses SI but retains imperial for road distances (miles), draught beer (pints), and body weight (stones and pounds). Australia, Canada, and most of Europe completed metrication in the 1970s–1980s.

In engineering contexts worldwide, SI is dominant. The 1999 Mars Climate Orbiter was lost because one engineering team used pound-force·seconds and another used newton·seconds — a $327M lesson in the cost of mixing unit systems.

Key differences in practice

Note

The most consequential everyday differences:

Length: 1 inch = 25.4 mm (exact). 1 foot = 30.48 cm. 1 mile = 1.60934 km. In construction, US drawings use feet and inches; international drawings use millimeters.

Mass/Weight: 1 pound = 0.453592 kg (exact by US legal definition). 1 kilogram ≈ 2.20462 lb. Body weight: a 75 kg person weighs 165.3 lb or 11 st 11 lb (UK).

Temperature: °C and °F require a non-linear conversion. Water freezes at 0°C / 32°F and boils at 100°C / 212°F at sea level. Formula: °F = (°C × 9/5) + 32.

Volume: 1 US gallon = 3.785 L. 1 imperial gallon = 4.546 L. 1 US fluid ounce = 29.57 mL; 1 imperial fluid ounce = 28.41 mL. Fuel economy: US uses miles per gallon (US); UK uses miles per gallon (imperial), about 20% higher for the same vehicle; most of the world uses liters per 100 km.

How conversion factors are defined

Formula

Since 1959, the inch has been exactly 25.4 mm by international agreement. The pound has been exactly 0.45359237 kg since the International Yard and Pound Agreement. These exact definitions mean conversions between SI and imperial are lossless — no rounding is needed at the definition level, only when expressing the result in finite digits.

When to use each system

Use SI when: communicating with international colleagues, writing scientific or engineering documentation, working in medicine, chemistry, or physics, or specifying anything that crosses national borders.

You will encounter imperial when: reading US construction documents, understanding US product specifications, working with road distances in the US or UK, or dealing with consumer products sold in the US market.

Many industries use both simultaneously. Aviation uses nautical miles and feet for altitude alongside SI for fuel mass. Automotive engineering in the US specifies torque in lb·ft but may use mm for dimensions.