The Metric System (SI)
The metric system was developed in France during the late 18th century and has since become the international standard for science, engineering, medicine, and trade. Its official name is the International System of Units, abbreviated SI from the French Système International d'Unités.
The defining feature of the metric system is its base-10 structure. Every unit is related to the next by a factor of 10, 100, or 1,000. This makes calculations straightforward: converting 3.5 kilometres to metres is simply a matter of multiplying by 1,000 to get 3,500 m. No memorising that a league is 3 miles or that a chain is 66 feet.
The NIST SI Units page provides the authoritative definition of every SI unit and the 2019 redefinitions that fixed several base units to fundamental physical constants.
Metric Prefixes
Metric prefixes attach to any base unit to express multiples or fractions. The most commonly used prefixes are:
| Prefix | Symbol | Factor | Example |
|---|---|---|---|
| Giga | G | 10⁹ (1,000,000,000) | Gigabyte (GB) |
| Mega | M | 10⁶ (1,000,000) | Megawatt (MW) |
| Kilo | k | 10³ (1,000) | Kilometre (km) |
| Hecto | h | 10² (100) | Hectare (ha) |
| Deci | d | 10⁻¹ (0.1) | Decilitre (dL) |
| Centi | c | 10⁻² (0.01) | Centimetre (cm) |
| Milli | m | 10⁻³ (0.001) | Millilitre (mL) |
| Micro | μ | 10⁻⁶ | Micrometre (μm) |
| Nano | n | 10⁻⁹ | Nanometre (nm) |
The 7 Base SI Units
All SI units are built from seven base units. Every derived unit — speed, energy, pressure, force — is expressed as a combination of these seven:
| Quantity | Unit | Symbol |
|---|---|---|
| Length | Metre | m |
| Mass | Kilogram | kg |
| Time | Second | s |
| Electric current | Ampere | A |
| Temperature | Kelvin | K |
| Amount of substance | Mole | mol |
| Luminous intensity | Candela | cd |
The Imperial System
The imperial system evolved from traditional English units over many centuries. Unlike the metric system, the ratios between units are largely arbitrary — 12 inches in a foot, 3 feet in a yard, 1,760 yards in a mile — reflecting the historical origins of each unit (the inch approximated the width of a thumb; the foot approximated the length of a human foot).
The imperial system is the everyday standard in the United States and is still used alongside metric in the United Kingdom for certain measurements (road distances in miles, beer in pints). The Wikipedia article on the SI system provides a detailed historical overview of how metric supplanted imperial worldwide.
Length Conversions: m, ft, in, km, mi
Length is the most commonly converted quantity. Whether you are reading a product specification in inches, planning a route in miles, or measuring a room in feet, the table below covers the key conversions.
| From | To | Multiply by | Example |
|---|---|---|---|
| Metres (m) | Feet (ft) | 3.28084 | 1.8 m = 5.91 ft |
| Feet (ft) | Metres (m) | 0.3048 | 6 ft = 1.83 m |
| Inches (in) | Centimetres (cm) | 2.54 | 12 in = 30.48 cm |
| Centimetres (cm) | Inches (in) | 0.3937 | 100 cm = 39.37 in |
| Kilometres (km) | Miles (mi) | 0.62137 | 10 km = 6.21 mi |
| Miles (mi) | Kilometres (km) | 1.60934 | 5 mi = 8.05 km |
| Yards (yd) | Metres (m) | 0.9144 | 100 yd = 91.44 m |
| Nautical miles | Kilometres (km) | 1.852 | 1 nmi = 1.852 km |
Weight Conversions: kg, lb, oz, g
Mass and weight are technically different (mass is a property of matter; weight is the force of gravity on that mass), but in everyday usage they are treated as equivalent on Earth's surface.
| From | To | Multiply by | Example |
|---|---|---|---|
| Kilograms (kg) | Pounds (lb) | 2.20462 | 70 kg = 154.3 lb |
| Pounds (lb) | Kilograms (kg) | 0.45359 | 150 lb = 68.0 kg |
| Grams (g) | Ounces (oz) | 0.03527 | 500 g = 17.64 oz |
| Ounces (oz) | Grams (g) | 28.3495 | 8 oz = 226.8 g |
| Metric tonnes (t) | Short tons (US) | 1.10231 | 1 t = 1.102 short tons |
| Stone (st) | Kilograms (kg) | 6.35029 | 11 st = 69.85 kg |
Temperature Conversions: °C, °F, K
Temperature is the one quantity where conversion requires both multiplication and addition (not just scaling), because the zero points of the three main scales differ.
| Conversion | Formula | Example |
|---|---|---|
| Celsius → Fahrenheit | °F = (°C × 9/5) + 32 | 100°C = 212°F |
| Fahrenheit → Celsius | °C = (°F − 32) × 5/9 | 98.6°F = 37°C |
| Celsius → Kelvin | K = °C + 273.15 | 0°C = 273.15 K |
| Kelvin → Celsius | °C = K − 273.15 | 310 K = 36.85°C |
| Fahrenheit → Kelvin | K = (°F + 459.67) × 5/9 | 32°F = 273.15 K |
Volume Conversions: L, gal, fl oz
Volume conversions matter most in cooking, fuel, and industrial contexts. The US and UK use different definitions for gallons and fluid ounces, so always confirm which system applies.
| From | To | Multiply by | Example |
|---|---|---|---|
| Litres (L) | US gallons | 0.26417 | 10 L = 2.64 US gal |
| US gallons | Litres (L) | 3.78541 | 1 US gal = 3.79 L |
| Litres (L) | UK gallons | 0.21997 | 10 L = 2.20 UK gal |
| UK gallons | Litres (L) | 4.54609 | 1 UK gal = 4.55 L |
| Millilitres (mL) | US fl oz | 0.03381 | 250 mL = 8.45 fl oz |
| US fl oz | Millilitres (mL) | 29.5735 | 8 fl oz = 236.6 mL |
| Cubic metres (m³) | Litres (L) | 1000 | 0.5 m³ = 500 L |
Speed Conversions: km/h, mph, m/s
Speed conversions arise in driving, aviation, meteorology, and sports. The three most common units are kilometres per hour (km/h), miles per hour (mph), and metres per second (m/s).
| From | To | Multiply by | Example |
|---|---|---|---|
| km/h | mph | 0.62137 | 100 km/h = 62.1 mph |
| mph | km/h | 1.60934 | 60 mph = 96.6 km/h |
| m/s | km/h | 3.6 | 10 m/s = 36 km/h |
| km/h | m/s | 0.27778 | 100 km/h = 27.78 m/s |
| knots | km/h | 1.852 | 20 knots = 37.04 km/h |
| knots | mph | 1.15078 | 20 knots = 23.02 mph |
Why Does the US Still Use Imperial?
The United States is one of only three countries — alongside Myanmar and Liberia — that has not officially adopted the metric system as its primary system. This is not for lack of trying. The Metric Conversion Act of 1975 made the US officially metric, but the voluntary nature of the legislation meant adoption was never mandated.
Several factors perpetuate imperial use in the US:
- Infrastructure lock-in: Road signs, speedometers, construction tools, and industrial machinery are calibrated in imperial units. Replacing them represents an enormous capital cost.
- Consumer familiarity: Americans grow up measuring in feet and pounds. Recipes, body measurements, and home renovation are deeply embedded in imperial units across generations.
- Commercial labelling: Food packaging, clothing sizes, and automotive parts are labelled in imperial units. Changing this requires regulatory action across thousands of product categories.
- NASA and science: Ironically, US scientific and technical institutions use metric exclusively. The famous 1999 Mars Climate Orbiter failure was caused by a unit mismatch — one team used metric, another used imperial — resulting in the loss of a $327 million spacecraft.
For more targeted conversion help, see our guide on how to use an online unit converter for the 20 most common conversion needs.
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