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Metric to imperial unit conversion formulas and reference

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The metric system (SI) is based on decimal powers of ten, making conversions between centimeters, meters, and kilometers simple multiplication. The imperial and US customary systems, however, rely on historical fractional definitions (12 inches to a foot, 16 ounces to a pound, 4 quarts to a gallon).

Converting accurately between these systems is crucial in international engineering, cooking, global trade, science, and travel. By international treaty since 1959, all imperial units are defined by exact metric equivalents.

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Step by step

  1. Length & Distance Conversions

    1 inch is defined as exactly 2.54 cm. 1 foot = 12 inches = 0.3048 m. 1 yard = 3 feet = 0.9144 m. 1 mile = 1.609344 km. To convert km to miles: divide by 1.609344 (or multiply by 0.621371).

  2. Mass & Weight Conversions

    1 pound (lb) is legally defined as exactly 0.45359237 kg. 1 kilogram ≈ 2.20462 lbs. 1 ounce (oz) = 28.3495 grams. 1 metric ton = 1,000 kg ≈ 2,204.62 lbs.

  3. Temperature Conversions: The Offset Difference

    Because Fahrenheit and Celsius have different zero points and degree scaling: °F = (°C × 9/5) + 32. °C = (°F - 32) × 5/9. Water freezes at 0°C (32°F) and boils at 100°C (212°F). At -40°, both scales intersect.

  4. Liquid Volume Conversions

    1 US Fluid Gallon = 3.78541 Liters. 1 Imperial Gallon (UK) = 4.54609 Liters. 1 Liter = 33.814 US fl oz. When following international recipes, always verify whether liquid ounces refer to US or UK customary measurements.

Things worth knowing

  • Never mix US customary gallons (3.785 L) with UK imperial gallons (4.546 L); the UK gallon is roughly 20% larger.
  • The pound-force is a unit of force, whereas the kilogram is a unit of mass. On Earth's surface, 1 kg experiences roughly 9.80665 Newtons of gravitational force.
  • Scientific work should always use the SI base units: meters, kilograms, seconds, and Kelvin.
  • For quick estimation while driving: 100 km/h is approximately 62 mph; 50 km/h is approximately 31 mph.

Frequently asked questions

Why is 1 inch exactly 2.54 cm?

In the 1959 International Yard and Pound Agreement, the United States, United Kingdom, Canada, Australia, and South Africa officially defined the international yard as 0.9144 meters, making 1 inch exactly 25.4 millimeters.

Why does the temperature formula require adding or subtracting 32?

Daniel Gabriel Fahrenheit set 0°F at the freezing point of a concentrated brine solution, which puts the freezing point of pure water at 32°F, creating a 32-degree offset from Celsius.

What is a nautical mile?

A nautical mile is based on the circumference of the Earth (one minute of latitude) and is internationally standardized at exactly 1,852 meters (approx. 1.1508 statute miles).

How do I convert square meters to square feet?

Multiply square meters by 10.7639 (because (3.28084 ft)² ≈ 10.7639 ft²). For example, an 80 m² apartment is 80 × 10.7639 ≈ 861 sq ft.

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