Unit Converter
Free unit converter for length, weight, temperature, volume, area, and speed. Switch between metric and imperial instantly.
What Is a Unit Converter?
A unit converter translates measurements between different systems — primarily metric (SI), imperial (US customary), and other specialized systems — without manual calculation. The most common conversions include length (miles ↔ kilometers), weight (pounds ↔ kilograms), temperature (°F ↔ °C), volume (gallons ↔ liters), area (square feet ↔ square meters), and speed (mph ↔ km/h).
Unit conversion matters in cooking, travel, engineering, medicine, and international commerce. The US is one of three countries (with Myanmar and Liberia) still primarily using imperial units. Everyone else uses the metric system. For anyone working globally, converting between systems is a daily necessity.
Common Conversion Scenarios
Unit conversions come up repeatedly in the same real-world contexts. Here are the most common scenarios and the specific conversions each requires:
| Scenario | What to Convert | Key Formula | Example |
|---|---|---|---|
| Cooking (US → metric) | Cups to mL, oz to g, °F to °C | 1 cup = 236.6 mL; 1 oz = 28.3 g | 350°F = 175°C (standard bake) |
| Road trip abroad | Miles to km, mph to km/h | × 1.609 | 60 mph = 96.6 km/h |
| Buying property | Sq ft to m², acres to hectares | sq ft × 0.0929; acres × 0.4047 | 1,500 sq ft = 139.4 m² |
| Fitness tracking | lb to kg, miles to km | lb × 0.4536; mi × 1.609 | 165 lb = 74.8 kg |
| Fuel economy | MPG (US) to L/100km | 235.2 ÷ mpg | 30 MPG = 7.8 L/100km |
| Weather abroad | °F to °C | (°F − 32) × 5/9 | 77°F = 25°C |
| Medication dosing | lb to kg (for weight-based dosing) | lb × 0.4536 | 150 lb = 68.0 kg |
| DIY/construction | Inches to cm, feet to meters | in × 2.54; ft × 0.3048 | 6 ft = 182.9 cm |
Key Concepts: SI vs. Imperial, Temperature Scales
SI (Système International) is the modern metric system used globally. It is built on seven base units: meter (length), kilogram (mass), second (time), ampere (current), kelvin (temperature), mole (amount), and candela (luminosity). All other SI units are derived from these. The metric system uses powers of 10 (kilo=10³, milli=10⁻³, micro=10⁻⁶) making scaling trivially easy.
US Imperial (Customary) units lack a consistent base — 12 inches to a foot, 3 feet to a yard, 5,280 feet to a mile. Volume uses teaspoons, tablespoons, cups, pints, quarts, and gallons with non-decimal relationships. This makes multi-step conversions error-prone and a common source of mistakes when working across systems.
Temperature scales. Celsius (°C) is based on water's freezing (0°) and boiling (100°) points. Fahrenheit (°F) uses 32° for freezing and 212° for boiling (a 180° span vs. Celsius's 100°). Kelvin (K) starts at absolute zero (−273.15°C) and uses the same degree size as Celsius. K = °C + 273.15. Kelvin is used in physics and thermodynamics.
Tips for Unit Conversions
Memorize a few anchor conversions. Rather than looking up every conversion, memorize: 1 inch = 2.54 cm (exact), 1 mile ≈ 1.6 km, 1 kg ≈ 2.2 lb, 1 liter ≈ 1.057 qt, °F = °C × 9/5 + 32. These anchors let you estimate quickly and verify calculator results for reasonableness.
Watch for US vs. UK gallons. The US gallon (3.785 L) and the UK (Imperial) gallon (4.546 L) differ by about 20%. UK pints and fluid ounces are also different. Always specify which system — especially for fuel consumption (mpg differs between US and UK vehicles by the same ratio).
Temperature “feels like” vs. actual. When converting temperatures for weather or cooking, remember that body perception differs from thermometer readings. A 100°F/37.8°C day feels extremely hot partly because of humidity. Oven temperatures in recipes often list both °F and °C — a common baking conversion is that 350°F ≈ 175°C (fan oven: 160°C).
Frequently Asked Questions
How do you convert between Fahrenheit and Celsius?
To convert °F to °C: °C = (°F − 32) × 5/9. Example: 98.6°F = 37°C. To convert °C to °F: °F = (°C × 9/5) + 32. Example: 100°C = 212°F. Useful anchors: 0°C=32°F, 20°C=68°F, 37°C=98.6°F, 100°C=212°F. Quick mental check: subtract 30 and halve for a rough °F→°C estimate.
How do you convert pounds to kilograms?
1 pound = 0.453592 kilograms (exact). Multiply pounds by 0.4536 for kg. Quick check: divide by 2.2 (since 1 kg ≈ 2.2046 lb). Example: 165 lb ÷ 2.2 ≈ 75 kg. For kilograms to pounds, multiply by 2.2046.
What is the difference between a US gallon and UK gallon?
US gallon = 3.785 liters. UK (Imperial) gallon = 4.546 liters. The UK gallon is about 20% larger. This affects fuel economy comparisons — a car achieving 40 US mpg gets about 48 UK mpg on the same trip. Always specify US or UK when working with gallons, pints, or fluid ounces internationally.
How do you convert square feet to square meters?
1 square foot = 0.092903 square meters. Multiply sq ft by 0.0929. Example: a 1,500 sq ft apartment = 1,500 × 0.0929 = 139.4 m². Reverse: m² × 10.764 = sq ft. Quick mental rule: sq ft ÷ 10 ≈ m² (within about 8%).
How do you convert mph to km/h?
Multiply mph by 1.60934. Example: 60 mph = 96.56 km/h. Reverse: km/h × 0.6214 ≈ mph. Common references: 30 mph = 48 km/h, 60 mph = 97 km/h, 100 km/h = 62 mph.
What is a metric ton vs. a US ton?
A metric ton (tonne) = 1,000 kg = 2,204.6 lb. A US (short) ton = 2,000 lb = 907.2 kg. A UK (long) ton = 2,240 lb = 1,016 kg. The metric ton is about 10% heavier than the US short ton. In international shipping and commodities, metric tons are standard.
Why does the US still use imperial units?
The US is one of only three countries (with Myanmar and Liberia) not officially using the metric system. A 1975 Metric Conversion Act made adoption voluntary, and public resistance meant it never took hold. Scientific and pharmaceutical fields in the US use metric; consumer and construction sectors largely stayed imperial. This is why unit conversion remains a daily practical necessity for Americans working globally.
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