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Unit Converter
Convert between measurement units instantly. Free unit converter for length, weight, volume, temperature, area, and speed with metric and imperial support.
Unit Converter
Convert between 100+ units across 12 categories including length, mass, temperature, area, volume, speed, time, data, pressure, energy, and more.
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📚 In-Depth Guide
This calculator is part of a comprehensive guide
Reviewed by CalculatorApp.me Engineering & Science Team
The Complete Guide to Measurement Unit Conversion
SI, imperial, temperature, data storage — every unit and conversion factor explained.
7
SI base units
100+
Units supported in this converter
1875
Year of the Metre Convention
3 countries
Still not fully metric (USA, Liberia, Myanmar)
What Is a Unit Converter?
A unit converter applies dimensional analysis to transform a measurement expressed in one unit into its equivalent in another unit of the same physical quantity. Every conversion multiplies the value by a conversion factor — the ratio of the two units relative to a common base.
The International System of Units (SI) defines 7 base units (metre, kilogram, second, ampere, kelvin, mole, candela) from which all other units are derived. Imperial and US customary units remain common in everyday life in the USA, UK (partially), and a handful of other countries.
Unit conversion errors have had catastrophic consequences in history — including the loss of the NASA Mars Climate Orbiter ($327M) in 1999 due to a metric/imperial confusion between engineering teams.
Key Conversion Formulas
°F to °C: C = (F − 32) × 5/9 °C to °F: F = (C × 9/5) + 32 °C to K: K = C + 273.15 K to °C: C = K − 273.15 Absolute zero: −273.15 °C = 0 K = −459.67 °F
Temperature is the only common conversion requiring both addition and multiplication (affine, not linear).
1 inch = 2.54 cm (exact) 1 foot = 30.48 cm (exact) 1 mile = 1.60934 km 1 yard = 0.9144 m (exact) 1 nautical mi = 1.852 km (exact) 1 lightyear = 9.461 × 10¹⁵ m
The inch and foot are now defined exactly via the metric system.
1 pound = 453.592 grams 1 ounce = 28.3495 grams 1 stone = 14 pounds = 6.35 kg 1 US ton = 2000 lbs = 907.185 kg 1 metric tonne = 1000 kg = 2204.6 lbs
'Weight' (force) and 'mass' differ in physics but are used interchangeably in everyday unit conversion.
1 KB = 1,024 bytes (IEC kibibyte) 1 MB = 1,048,576 bytes 1 GB = 1,073,741,824 bytes 1 TB = 1,099,511,627,776 bytes (SI: 1 KB = 1,000 bytes) Difference at 1 TB: ~73.7 GB
SI uses powers of 1,000 (kB, MB). IEC uses powers of 1,024 (KiB, MiB). Hard drive makers use SI; operating systems use IEC.
SI vs. Imperial / US Customary Units
| Quantity | SI Unit | Imperial / US | Conversion Factor | Notes |
|---|---|---|---|---|
| Length | metre (m) | inch / foot / mile | 1 m = 3.2808 ft | Defined via speed of light |
| Mass | kilogram (kg) | pound (lb) / ounce | 1 kg = 2.2046 lb | Defined via Planck constant (2019) |
| Temperature | kelvin (K) | Fahrenheit (°F) | See formula | Affine conversion (not pure multiplication) |
| Volume | litre (L) | gallon / fluid oz | 1 L = 0.2642 US gal | US and UK gallon differ (3.785 vs 4.546 L) |
| Speed | m/s | mph / knots | 1 mph = 1.60934 km/h | Knots = nautical miles per hour |
| Area | m² | sq ft / acres | 1 m² = 10.7639 sq ft | 1 acre = 4,046.86 m² |
History of Measurement Standards
Charlemagne standardizes weights
Charlemagne decreed uniform weights and measures across the Holy Roman Empire — one of the first major attempts at measurement standardization.
Metric system created in France
The French Revolution produced the metric system — a rational, decimal-based framework. The metre was defined as 1/10,000,000 of the Earth's meridian quadrant.
Metre Convention (Treaty of the Metre)
17 nations signed the treaty establishing the International Bureau of Weights and Measures (BIPM), creating the first global measurement authority.
SI formally defined
The 11th General Conference on Weights and Measures formalized the International System of Units (SI) with 6 base units (candela added in 1967).
NASA Mars Climate Orbiter lost
A $327M spacecraft was lost because Lockheed Martin sent thrust data in pound-force seconds while NASA expected newton-seconds — a historical unit conversion failure.
SI redefined via fundamental constants
All 7 SI base units were redefined in terms of fundamental physical constants (Planck h, Boltzmann k, elementary charge e, etc.) — eliminating all physical artefacts.
Key References
BIPM / SI Brochure
The International System of Units (9th Edition)
Official SI definitions of all 7 base units, 22 derived units, and prefixes — the authoritative reference for all scientific measurement.
NIST
Guide for the Use of SI in the US
NIST SP 811 provides official US guidance on SI notation, conversion factors, and units for American scientists and engineers.
NASA / JPL
Mars Climate Orbiter Mishap Investigation
The official crash investigation report detailing the £327M failure caused by a pound-force vs. newton unit mismatch.
ISO 80000
Quantities and Units Standard
ISO 80000 defines physical quantities and units for engineering applications — complementing SI with domain-specific notation.
Unit Conversion Myths vs. Facts
The metric and imperial systems are completely incompatible.
They are fully interoperable — every imperial unit has an exact metric equivalent. The inch is defined as exactly 2.54 cm by international agreement.
1 kilogram = 1 pound.
1 kilogram ≈ 2.205 pounds. The kilogram is a unit of mass; the pound is technically a force unit in physics, though used as mass in everyday contexts.
°F and °C differ only by a simple multiplication factor.
Temperature conversion requires both multiplication and addition (affine transformation): °C = (°F − 32) × 5/9. You cannot simply multiply.
1 GB = 1,000 MB.
In binary (IEC) convention, 1 GiB = 1,024 MiB. Hard drive makers use SI (1 GB = 1,000 MB), which creates the apparent 'missing space' when you buy a 1TB drive.
Frequently Asked Questions
What is dimensional analysis?▼
How do I convert Celsius to Fahrenheit?▼
What are the 7 SI base units?▼
Why does my 1 TB hard drive show as ~931 GB in Windows?▼
Is a US gallon the same as a UK gallon?▼
What is a nautical mile?▼
How do I convert pounds to kilograms?▼
What is the difference between mass and weight?▼
What units does the USA not use in metric?▼
How were units standardized historically?▼
What is ppm in unit terms?▼
Can I convert between energy and mass?▼
References
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Browse Engineering Tools →Key Takeaways
- ✓1 mile = 1.60934 kilometers; 1 kg = 2.20462 pounds
- ✓Temperature: °F = (°C × 9/5) + 32; °C = (°F - 32) × 5/9
- ✓1 gallon (US) = 3.78541 liters; 1 inch = 2.54 centimeters
- ✓The metric system is used by every country except the US, Liberia, and Myanmar