Conversion Calculator
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Whether you are calculating engineering tolerances, cooking recipes, international travel baggage, or scientific physics equations, converting between the International System of Units (Metric SI) and the US Customary / British Imperial systems is essential.
Our free online Unit Conversion Calculator serves as your master conversion hub, providing instant calculations across all major measurement categories:
- Length & Distance: Meters, Kilometers, Inches, Feet, Yards, Miles, Nautical Miles, and Millimeters.
- Weight & Mass: Kilograms, Grams, Milligrams, Metric Tonnes, Pounds (lbs), Ounces (oz), and Stones (st).
- Speed & Velocity: Miles per Hour (mph), Kilometers per Hour (km/h), Meters per Second (m/s), and Knots (kn).
- Temperature: Celsius (°C), Fahrenheit (°F), and Kelvin (K).
- Power & Energy: Kilowatts (kW), Mechanical Horsepower (hp), Metric Horsepower (PS), and Watts (W).
- Angle & Trigonometry: Degrees (°) and Radians (rad, π rad).
Topical Conversion Hub: Complete Category Reference
Use the master table below to navigate to dedicated, high-precision tools for specific unit pairs:
| Category | Base Metric Unit | Base US / Imperial Unit | Key Conversion Formula | Dedicated Converters |
|---|---|---|---|---|
| Weight & Mass | Kilogram (kg) | Pound (lb) | 1 kg = 2.20462 lbs |
kg to lbs • lbs to kg • lbs to oz |
| Power & Engines | Kilowatt (kW) | Horsepower (hp) | 1 kW = 1.34102 hp |
kW to HP • HP to kW |
| Speed & Velocity | Kilometer/hour (km/h) | Mile/hour (mph) | 1 mph = 1.60934 km/h |
mph to kph • kph to mph |
| Angles & Circles | Radian (rad) | Degree (°) | 1 rad = 180 / π ≈ 57.3° |
rad to deg • deg to rad |
| Length & Distance | Meter (m) | Foot (ft) / Inch (in) | 1 in = 25.4 mm |
inches to mm • cm to inches |
| Temperature | Celsius (°C) | Fahrenheit (°F) | °F = (°C × 1.8) + 32 |
°C to °F • °F to °C |
Understanding Measurement Systems: Metric vs. Imperial
The Metric System (SI)
The International System of Units (SI) is the worldwide scientific standard built upon seven base decimal units (meter, kilogram, second, ampere, kelvin, mole, and candela). Because every metric unit scales by powers of ten using standard prefixes (milli- $10^{-3}$, centi- $10^{-2}$, kilo- $10^{3}$, mega- $10^{6}$), mathematical scaling requires simply shifting decimal points.
US Customary & British Imperial Systems
Derived from traditional British units, the US Customary System utilizes historical, non-decimal scaling factors:
1 Foot = 12 Inches1 Yard = 3 Feet = 36 Inches1 Statute Mile = 5,280 Feet = 1,760 Yards1 Pound (lb) = 16 Ounces (oz)1 Stone (UK) = 14 Pounds (lbs)
Master Conversion Factors Quick Reference Table
| Starting Unit | Target Unit | Exact Multiplier | Calculation Formula |
|---|---|---|---|
| Inches (in) | Millimeters (mm) | 25.4 (exact) | mm = in × 25.4 |
| Feet (ft) | Meters (m) | 0.3048 (exact) | m = ft × 0.3048 |
| Miles (mi) | Kilometers (km) | 1.609344 (exact) | km = mi × 1.609344 |
| Pounds (lbs) | Kilograms (kg) | 0.45359237 (exact) | kg = lbs × 0.45359237 |
| Kilograms (kg) | Pounds (lbs) | 2.20462262 | lbs = kg × 2.20462 |
| Horsepower (hp) | Kilowatts (kW) | 0.74569987 | kW = hp × 0.7457 |
| Kilowatts (kW) | Horsepower (hp) | 1.34102209 | hp = kW × 1.34102 |
| Radians (rad) | Degrees (°) | 57.29577951 | deg = rad × (180 / π) |
| Degrees (°) | Radians (rad) | 0.01745329 | rad = deg × (π / 180) |
Frequently Asked Questions (FAQ)
Why are there differences between US and UK gallons?
A US liquid gallon equals 3.78541 liters (231 cubic inches), whereas an Imperial (UK) gallon equals 4.54609 liters (the volume of 10 pounds of water at 62°F). When converting liquid volume, always specify whether US or Imperial standards apply.
How do you convert Celsius to Fahrenheit in your head?
Double the Celsius temperature and add 30 for a quick estimate. For example: 20°C × 2 = 40 + 30 = 70°F (exact is 68°F). The exact formula is (°C × 1.8) + 32.
What is the SI base unit for mass?
The kilogram (kg) is the only SI base unit that includes a metric prefix (kilo-) in its name. It is defined using Planck’s fundamental physical constant ($h = 6.62607015 \times 10^{-34} \text{ kg}\cdot\text{m}^2/\text{s}$).