How to Convert Imperial to Metric
The imperial system is used in the United States and, historically, in the United Kingdom. Most of the world, including New Zealand, uses the International System of Units (SI), which is based on the metric system. Converting between the two systems requires multiplying (or dividing) by a fixed conversion factor.
All the factors used in this converter are exact by international definition. For example, one inch is defined as exactly 25.4 millimetres, and one pound is defined as exactly 0.45359237 kilograms.
Length Conversions
| Imperial unit | Metric equivalent | Factor |
|---|
| 1 inch (in) | 25.4 millimetres (mm) | × 25.4 |
| 1 foot (ft) | 0.3048 metres (m) | × 0.3048 |
| 1 yard (yd) | 0.9144 metres (m) | × 0.9144 |
| 1 mile (mi) | 1.609344 kilometres (km) | × 1.609344 |
Weight and Mass Conversions
| Imperial unit | Metric equivalent | Factor |
|---|
| 1 ounce (oz) | 28.3495 grams (g) | × 28.3495 |
| 1 pound (lb) | 0.453592 kilograms (kg) | × 0.453592 |
| 1 stone (st) | 6.35029 kilograms (kg) | × 6.35029 |
| 1 ton (short, US) | 907.185 kilograms (kg) | × 907.185 |
Temperature Conversions
Temperature is the only category that uses a formula rather than a simple multiplication factor, because the two scales have different zero points.
| Imperial unit | Formula to Celsius | Example |
|---|
| Fahrenheit (F) | C = (F - 32) × 5/9 | 72 F = 22.2 C |
Volume Conversions
| Imperial unit | Metric equivalent | Factor |
|---|
| 1 fl oz (US fluid ounce) | 29.5735 millilitres (mL) | × 29.5735 |
| 1 cup (US) | 236.588 millilitres (mL) | × 236.588 |
| 1 pint (US) | 473.176 millilitres (mL) | × 473.176 |
| 1 quart (US) | 946.353 millilitres (mL) | × 946.353 |
| 1 gallon (US) | 3.78541 litres (L) | × 3.78541 |
Area Conversions
| Imperial unit | Metric equivalent | Factor |
|---|
| 1 square inch (sq in) | 6.4516 square centimetres (cm²) | × 6.4516 |
| 1 square foot (sq ft) | 0.092903 square metres (m²) | × 0.092903 |
| 1 square yard (sq yd) | 0.836127 square metres (m²) | × 0.836127 |
| 1 acre | 4046.86 square metres (m²) | × 4046.86 |
| 1 square mile | 2.58999 square kilometres (km²) | × 2.58999 |
Speed Conversions
| Imperial unit | Metric equivalent | Factor |
|---|
| 1 mile per hour (mph) | 1.60934 kilometres per hour (km/h) | × 1.60934 |
| 1 foot per second (ft/s) | 0.3048 metres per second (m/s) | × 0.3048 |
Imperial vs Metric: A Brief History
New Zealand adopted the metric system progressively between 1969 and 1976 as part of the Metric Conversion Act 1972. Road signs changed from miles to kilometres, weights from pounds to kilograms, and temperatures from Fahrenheit to Celsius. Today, the metric system is the legal standard in New Zealand for trade and commerce, though imperial measurements still appear in some contexts, such as screen sizes (quoted in inches) and some imported building materials.
The United States remains the primary holdout among major economies, still using the imperial system for everyday purposes. This makes imperial-to-metric conversion a practical need for anyone ordering goods from the US, following American recipes, or interpreting US specifications.
Related Calculators
Sources and method: Conversion factors from the International Bureau of Weights and Measures (BIPM) SI Brochure (9th edition) and the exact definitions established by the international yard and pound agreement (1959). Temperature formula is the standard Fahrenheit-to-Celsius relation: C = (F - 32) x 5/9.
All conversion factors are exact by international definition. Results are shown to 6 significant figures for precision and rounded to 3 significant figures for everyday use. US customary units are used for volume (US fluid ounce, US gallon) rather than imperial fluid ounces or UK gallons, as these are most commonly encountered in practice.
