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A unit converter translates measurements between different unit systems — metric (SI), imperial, and specialised engineering units — for length, weight, temperature, speed, volume, area, pressure, energy, and data. Unit conversion is fundamental in engineering, science, international trade, and everyday life, where different countries and industries use different measurement standards. Errors in unit conversion have caused real-world engineering failures, including the 1999 Mars Climate Orbiter loss due to a mix of metric and imperial units. and provide related calculation tools.

Select the measurement category (length, weight, temperature, pressure, etc.).
Enter the value in the source unit.
Select or read the converted value in the target unit.
For temperature: note that Celsius and Fahrenheit conversions are non-linear (offset + scaling); Kelvin conversions are linear offsets only.
For compound units (speed, pressure, flow rate): convert the numerator and denominator units separately if needed.

Unit conversion formulas

Linear conversion: Output = Input × conversion factor

Temperature (special cases):

°F = (°C × 9/5) + 32 | °C = (°F − 32) × 5/9 | K = °C + 273.15

Common engineering conversions: 1 bar = 100,000 Pa = 14.504 psi; 1 inch = 25.4 mm; 1 gallon (UK) = 4.546 litres; 1 gallon (US) = 3.785 litres; 1 horsepower = 745.7 W; 1 kip = 4,448.2 N.

Unit conversion context

Engineering unit systems

Three main unit systems in engineering: SI (International System) — metres, kilograms, seconds, newtons, pascals; used globally as the standard for scientific and most engineering work. Imperial/US Customary — feet, pounds, psi, BTU; still used extensively in the US, and in some UK and legacy international engineering contexts. Specialised industry units — bar (pressure), Barg (gauge pressure), mmHg (blood pressure/vacuum), kWh (energy billing), knots (maritime/aviation speed). Always confirm which system a specification uses before calculating.

Construction tips and best practices

The UK gallon and US gallon are different — 1 UK gallon = 4.546 litres vs 1 US gallon = 3.785 litres; always specify which when converting fluid volumes.
Pressure can be expressed as absolute (above zero) or gauge (above atmospheric) — always check whether a specification uses bar(g), bar(a), psi(g), or psi(a).
When converting between SI and imperial for area or volume calculations, convert linear dimensions first, then calculate area/volume — do not try to use a single area or volume conversion factor unless you are certain it is correct.

The 1999 Mars Climate Orbiter was lost at a cost of approximately $327 million due to a unit mismatch — one team used pound-force seconds, another used newton-seconds for thruster data.
One metre = 3.28084 feet = 39.3701 inches; one kilometre = 0.621371 miles — the most commonly needed length conversions between metric and imperial.
Standard atmospheric pressure = 101,325 Pa = 1.01325 bar = 14.696 psi = 760 mmHg — useful reference for pressure conversion and gauge/absolute conversion.

Common mistakes to avoid

Confusing UK and US gallons — the 20% difference is large enough to matter in fuel, liquid ingredient, and tank volume calculations.
Forgetting that temperature conversion is not a simple multiplication — °C × 1.8 gives the wrong answer; the correct formula is (°C × 9/5) + 32.
Using area conversion factors for linear measurements — 1 m² ≠ 1.094 yd²; it equals 1.094² = 1.198 yd²; always square the linear factor for area conversions.

The International System of Units (SI) is the official measurement standard for trade, science, and most engineering globally under the Metre Convention. The UK Weights and Measures Act and equivalent legislation in other jurisdictions govern legal measurements for trade. Metrication is mandatory for most commercial purposes in the UK and EU, with certain imperial units permitted for specific uses (miles on road signs, pints for draught beer). For any legal, commercial, or safety-critical measurement application, use certified measuring instruments and consult applicable standards. This calculator is for informational purposes only.