Unit Converter

ToolHub's unit converter handles nine categories of measurement in one page: length, weight, temperature, volume, area, speed, time, data (bits and bytes), and energy. Type a number into either side, and the other side updates instantly. A full table below the inputs shows your value in every unit at once, so you do not need to convert one-at-a-time.

Conversions run entirely in your browser. No data leaves your device, and the page works offline once loaded. All factors are based on the International System of Units (SI) and the US National Institute of Standards and Technology (NIST) for US customary units.

Length

Millimetres, centimetres, metres, kilometres for metric; inches, feet, yards, miles for US customary; plus the nautical mile (1,852 m) used in aviation and maritime navigation. Common conversion: 1 inch = 2.54 cm exactly.

Weight (mass)

Milligrams, grams, kilograms, metric tonnes; ounces, pounds, stones, US short tons (2,000 lb), and UK long tons (2,240 lb). Important: 1 kg = 2.20462 lb, not 2.2 - the difference matters at larger weights.

Temperature

Celsius, Fahrenheit, and Kelvin. Temperature is the only category that uses both a scale factor and an offset, so the conversion formulas are not simple multiplication: °F = °C × 9/5 + 32 and K = °C + 273.15.

Volume

Millilitres, litres, cubic metres for metric. US customary includes teaspoons, tablespoons, fluid ounces, cups, pints, quarts, and gallons. UK imperial pints and gallons are also included - these are about 20% larger than US versions. Plus cubic inches and cubic feet for engineering work.

Area

Square millimetres through square kilometres, square inches through square miles, plus the acre (43,560 sq ft) and the hectare (10,000 m²). Real estate, farmland, and construction all use these regularly.

Speed

Metres per second (SI base), kilometres per hour, miles per hour, knots (nautical miles per hour), and feet per second. Knots and km/h are the most commonly confused: 1 knot = 1.852 km/h.

Time

Milliseconds through years. The "month" unit here is the standard 30-day month and "year" is 365 days; for calendar math involving specific dates, use a dedicated date calculator.

Data

Decimal (KB, MB, GB, TB, PB - powers of 1,000) and binary (KiB, MiB, GiB, TiB - powers of 1,024) units. Hard drive marketing uses decimal. RAM and operating systems usually use binary, which is why "500 GB" drives show as "465 GiB" in Windows.

Energy

Joules, kilojoules, calories (small calorie = 4.184 J), kilocalories (food Calories), watt-hours, kilowatt-hours, BTU, and foot-pounds. Watch out: the "Calorie" on a nutrition label is actually a kilocalorie.

Different sources use slightly different conversion factors, often due to rounding at the third or fourth decimal. ToolHub uses the official factors:

• 1 inch = 2.54 cm exactly (international standard since 1959)
• 1 lb = 0.45359237 kg exactly (international avoirdupois pound)
• 1 US gal = 3.785411784 L exactly
• 1 acre = 4046.8564224 m² exactly
• 1 BTU = 1055.06 J (international table value)

For most everyday use the difference is irrelevant. For engineering or legal work, double-check against your standard of reference (ASTM, ISO, BIPM).

What is the difference between weight and mass?

In physics, mass is the amount of matter in an object (kilograms), while weight is the force gravity exerts on that mass (newtons). Everyday speech uses "weight" to mean mass. Our converter follows everyday usage: when you convert "pounds" to "kilograms" you are converting mass.

Why is there a difference between US and UK gallons?

The US gallon (3.785 L) was based on the old English wine gallon. The UK switched in 1824 to the imperial gallon (4.546 L), defined as 10 pounds of water at 62°F. The US kept the older definition. A US pint is 16 US fluid ounces; a UK pint is 20 UK fluid ounces. Recipes from either country need careful unit checking.

How is 1 second defined?

Since 1967, the second has been defined as 9,192,631,770 periods of radiation from the cesium-133 atom. This is the most precisely defined unit in the SI system.

Are kilometres always 1,000 metres?

Yes. All SI prefixes are exact powers of ten: kilo = 1,000, mega = 1,000,000, milli = 0.001, micro = 0.000001. This is unlike data, where "kilobyte" sometimes means 1,000 bytes and sometimes 1,024 (KiB).

How accurate is the temperature conversion?

The Celsius-Fahrenheit and Celsius-Kelvin formulas are exact. The only error comes from your input precision. We display up to 10 significant digits.

The measurement conversions people look up most often, with the official factors already worked out. Type any of these into the converter above to see every related unit at once.

Need feet to meters, celsius to fahrenheit, or litres to gallons in reverse? Every unit converts both directions, so you can start from either column.

Two conversions trip people up far more than the rest: gallons and data sizes. Both look like a plain unit swap, but each hides a change in definition that can throw your answer off by a wide margin if you use the wrong one.

US gallon vs UK gallon

A US gallon is 3.785 litres. A UK (imperial) gallon is 4.546 litres, about 20% larger. That gap matters in real life: "40 miles per gallon" quoted in the UK is a different efficiency from "40 miles per gallon" in the US, and a "5 gallon" fuel figure or recipe means different volumes on each side of the Atlantic. Always confirm which gallon your source is using before you convert.

KB vs KiB

A kilobyte (KB) is 1,000 bytes. A kibibyte (KiB) is 1,024 bytes. The 24-byte gap looks trivial at this scale, but it compounds as the units grow. At the gigabyte level a decimal GB is 1,000,000,000 bytes while a binary GiB is 1,073,741,824 bytes, roughly 7% more. That 7% is exactly why a "500 GB" hard drive shows up as about 465 GiB in your operating system: the drive maker counts in decimal, Windows counts in binary.

Almost every conversion is a single multiplication. You take your value and multiply it by a fixed conversion factor, which is the number of target units contained in one source unit. Because 1 mile equals 1.609 kilometres, converting 5 miles is just:

5 miles × 1.609 = 8.045 kilometres

To go the other way you divide instead of multiply: 8.045 km ÷ 1.609 = 5 miles. That is all a converter does internally: it looks up the factor and multiplies or divides.

What is the difference between a US and UK gallon?

A US gallon is 3.785 litres and a UK (imperial) gallon is 4.546 litres, so the UK gallon is about 20% bigger. They are not interchangeable: converting fuel economy, recipe volumes, or tank capacities between the two systems without allowing for this gap gives the wrong answer. The converter lists both, so pick the gallon that matches your source.

What is the difference between KB and KiB?

KB (kilobyte) means 1,000 bytes and KiB (kibibyte) means 1,024 bytes. Storage sizes and network speeds are usually quoted in decimal units (KB, MB, GB), while computer memory and many operating systems report binary units (KiB, MiB, GiB). The converter keeps the two families separate so you never accidentally mix a 1,000-based value with a 1,024-based one.

How do I convert Celsius to Fahrenheit?

Multiply the Celsius value by 9/5 (that is 1.8) and add 32: °F = °C × 9/5 + 32. For example, 20 °C becomes 20 × 1.8 + 32 = 68 °F, and 37 °C body temperature becomes 98.6 °F. To reverse it, subtract 32 first, then divide by 1.8.

Does this work offline?

Yes. Once the page has loaded, every conversion runs in your browser with no server calls, so it keeps working with no internet connection. Your inputs never leave your device.