Unit Converter — Convert Any Unit Online Free

How to Use the Unit Converter

1. Open the unit converter.
2. Select a category (length, weight, volume, area, speed, temperature, pressure, digital storage).
3. Enter a value in any unit field — other units update instantly.
4. Use the swap button to reverse the direction of a specific conversion pair.
5. Conversion formulas are shown below the results so you understand how the calculation works.

Unit Categories and Key Conversions

Metric System Prefixes

Metric vs. Imperial: Why Two Systems Exist

The metric system (SI — Système International) was developed in France in the late 18th century and adopted globally for science, engineering, and trade. It is based on powers of 10, making conversion between units trivial (multiply or divide by 10, 100, 1000).

Imperial units (feet, pounds, gallons) developed organically in Britain from historical measures — a foot was originally the length of a human foot; an acre was the amount of land one ox could plough in a day. The British Empire spread these units globally; the US retained them after independence.

Only three countries have not officially adopted metric as the primary measurement system: the United States, Myanmar, and Liberia. The UK uses a hybrid — metric for most official purposes (road distances are a notable exception: UK roads still use miles and mph), while everyday speech mixes both ("I'm 5'10" but "I weigh 75 kg").

Common Conversion Mistakes

Unit confusion has caused real-world errors:

• Mars Climate Orbiter (1999): NASA lost a $327 million spacecraft because one engineering team used metric units (Newton-seconds) and another used imperial (pound-force seconds) for thruster data. The probe entered Mars atmosphere at the wrong angle and was destroyed.
• US/UK gallon confusion: The US gallon (3.785 L) and UK gallon (4.546 L) differ by about 20%. Fuel economy conversions (mpg) between countries require specifying which gallon is meant — US mpg and UK mpg for the same car are significantly different numbers.
• Gimli Glider (1983): Air Canada Flight 143 ran out of fuel mid-flight because the fuel load was calculated in pounds instead of kilograms. The plane had about half the fuel needed for the flight.
• Binary vs. decimal GB: Hard drive manufacturers use 1 GB = 1,000,000,000 bytes (decimal). Operating systems traditionally displayed storage using 1 GB = 1,073,741,824 bytes (binary). A "500 GB" hard drive appears as ~465 GB in Windows — not a defect, but a definition difference.

Quick Reference: Most Common Conversions

• 1 mile = 1.609 km (exact: 1.60934 km)
• 1 kg = 2.205 lbs = 2 lbs 3.3 oz
• 1 stone = 14 lbs = 6.350 kg
• 1 inch = 2.54 cm (exact)
• 1 foot = 30.48 cm (exact)
• 1 US gallon = 3.785 litres
• 1 UK pint = 568 ml; 1 US pint = 473 ml
• 1 acre = 0.4047 hectares = 4,047 m²
• 100 km/h = 62.1 mph
• 1 bar = 14.5 PSI = 100 kPa

Unit Conversion in Cooking

Cooking recipes use different measurement systems depending on country of origin — US recipes use cups, tablespoons, and ounces; UK and European recipes use grams and millilitres. Key cooking conversions:

• 1 cup (US) = 240 ml
• 1 tablespoon (US) = 14.8 ml; 1 tablespoon (UK/AU) = 15 ml; 1 tablespoon (AU) = 20 ml (different!)
• 1 teaspoon (US) = 4.9 ml; 1 teaspoon (UK/AU) = 5 ml
• 1 oz (weight) = 28.35 g; 1 fl oz (US) = 29.57 ml; 1 fl oz (UK) = 28.41 ml
• 1 stick of US butter = 113 g = 8 tablespoons = ½ cup

Weight measurements (grams) are more accurate than volume measurements (cups) for baking — a cup of flour can vary by 20–30% depending on how tightly it is packed.

Common Questions

Why does converting km/h to mph give different results from mpg conversion?

Speed conversion (km/h to mph) uses 1 mile = 1.60934 km. Fuel economy conversion (L/100km to mpg) requires knowing which gallon — US (3.785 L) or UK (4.546 L). A car achieving 7 L/100km = 40.4 UK mpg but only 33.6 US mpg. When comparing fuel economy across countries, always specify which mpg standard is being used or convert all values to L/100km to avoid ambiguity.

What is the difference between weight and mass?

Mass is the amount of matter in an object (kilograms, grams, pounds) — it is constant regardless of location. Weight is the force exerted by gravity on that mass (Newtons, pound-force) — it varies with gravitational field strength. On Earth, for practical purposes, we use these interchangeably: a 70 kg person weighs 686 Newtons on Earth but only 113 Newtons on the Moon. The unit converter uses mass (kg, lbs) since that is what scales measure and what everyday "weight" refers to.

What is a nautical mile and why is it used?

A nautical mile is defined as 1 arc-minute of latitude — roughly 1/21,600th of the Earth's circumference through the poles. 1 nautical mile = 1.852 km = 1.15078 statute miles. Using nautical miles makes navigation calculations simpler: if you travel 60 nautical miles, you have moved exactly 1 degree of latitude. It is used in aviation and maritime navigation for this geometric relationship with Earth coordinates. A knot is 1 nautical mile per hour.