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Metric vs Imperial: Complete Conversion Guide

May 26, 20269 min read

Two Systems, One World

Most of the world uses the metric system (meters, kilograms, Celsius). The United States, Liberia, and Myanmar primarily use the imperial system (feet, pounds, Fahrenheit). If you work across borders — and most developers do — you need to convert between them confidently.

Use the Unit Converter for instant, accurate conversions.

Length

MetricImperialConversion Formula
1 millimeter (mm)0.03937 inchesin = mm × 0.03937
1 centimeter (cm)0.3937 inchesin = cm × 0.3937
1 meter (m)3.2808 feetft = m × 3.2808
1 meter (m)1.0936 yardsyd = m × 1.0936
1 kilometer (km)0.6214 milesmi = km × 0.6214
1 inch (in)2.54 cmcm = in × 2.54
1 foot (ft)0.3048 mm = ft × 0.3048
1 yard (yd)0.9144 mm = yd × 0.9144
1 mile (mi)1.6093 kmkm = mi × 1.6093

Quick Estimates

  • 1 inch ≈ width of a thumb
  • 1 meter ≈ 3 feet 3 inches
  • 1 kilometer ≈ 0.6 miles (roughly two-thirds of a mile)
  • 100 km/h ≈ 62 mph

Weight / Mass

MetricImperialConversion Formula
1 gram (g)0.03527 ouncesoz = g × 0.03527
1 kilogram (kg)2.2046 poundslb = kg × 2.2046
1 ounce (oz)28.3495 gg = oz × 28.3495
1 pound (lb)0.4536 kgkg = lb × 0.4536
1 stone (st)6.3503 kgkg = st × 6.3503
1 metric tonne1.1023 US tonston = t × 1.1023

Quick Estimates

  • 1 kg ≈ 2.2 lb
  • 100 g ≈ 3.5 oz
  • A typical car weighs about 1.5 tonnes (≈ 3300 lb)

Temperature

This is the trickiest conversion because the zero points differ.

Metric (°C)Imperial (°F)Context
-40 °C-40 °FWhere both scales meet
0 °C32 °FFreezing point of water
20 °C68 °FComfortable room temperature
37 °C98.6 °FNormal body temperature
100 °C212 °FBoiling point of water

Conversion Formulas

°F = (°C × 9/5) + 32
°C = (°F - 32) × 5/9

Quick Mental Math

  • To convert Celsius to Fahrenheit: double it, subtract 10%, add 32.
    • Example: 20°C → 40 → 36 → 68°F ✓
  • To convert Fahrenheit to Celsius: subtract 32, halve it, add 10%.
    • Example: 68°F → 36 → 18 → 19.8°C ≈ 20°C ✓

Volume

MetricImperial (US)Conversion Formula
1 milliliter (mL)0.03381 fl oz (US)fl oz = mL × 0.03381
1 liter (L)0.2642 US gallonsgal = L × 0.2642
1 US fluid ounce29.5735 mLmL = fl oz × 29.5735
1 US cup236.588 mLmL = cup × 236.588
1 US pint473.176 mLmL = pt × 473.176
1 US quart946.353 mLmL = qt × 946.353
1 US gallon3.7854 LL = gal × 3.7854

Imperial vs US Liquid Measures

The UK imperial system differs from the US system:

UnitUSUK (Imperial)
Fluid ounce29.57 mL28.41 mL
Pint473 mL568 mL
Gallon3.785 L4.546 L

A UK pint is about 20% larger than a US pint — important when ordering beer.

Area

MetricImperialConversion Formula
1 sq centimeter (cm²)0.155 sq inin² = cm² × 0.155
1 sq meter (m²)10.7639 sq ftft² = m² × 10.7639
1 sq kilometer (km²)0.3861 sq mimi² = km² × 0.3861
1 hectare2.4711 acresacre = ha × 2.4711

Why Developers Need Both

Screen Sizes and DPI

CSS uses pixels, but physical dimensions often come in inches or centimeters. When designing for print or high-DPI screens, you may need to convert:

  • DPI (dots per inch) ↔ dpcm (dots per centimeter): 1 inch = 2.54 cm, so dpcm = DPI / 2.54
  • A 27-inch monitor diagonal is 68.58 cm

API Data

Many international APIs return metric values. If your frontend serves US users, you'll need to convert before display. Always store in metric and convert at the presentation layer:

function displayDistance(km: number, locale: string): string {
  if (locale === 'en-US') {
    return `${(km * 0.6214).toFixed(1)} mi`
  }
  return `${km.toFixed(1)} km`
}

Common Mistakes

  1. Confusing mass and weight. A kilogram is mass; a pound is technically force. In everyday use they're interchangeable, but in physics calculations they are not.
  2. Using US gallons for UK recipes. A UK gallon is 4.546 L vs the US 3.785 L — a 20% difference.
  3. Forgetting absolute zero. Temperature conversion must account for offset (the +32 or -32), not just the ratio.
  4. Rounding too early. Chain conversions accumulate error. Convert in one step: kg → lb not kg → g → oz → lb.