Unit Converter
Convert any unit instantly across 15 categories — length, weight, temperature, speed, area, volume, pressure, energy, power and more. The only free converter that shows the formula and calculation. Type in either field. Saves your history and favourites.
Unit Converter Tool
15 categories, bidirectional conversion and formula display — in one free tool
Every other free unit converter splits categories into separate pages, requires a button press, and shows only the number. This tool keeps all 15 categories in one place, converts as you type in either direction, and explains the formula so you understand the math.
How to convert units
How this unit converter compares
| Feature | LazyTools ✦ | UnitConverters.net | RapidTables | Google (search) |
|---|---|---|---|---|
| All categories in one tool | ✔ 15 in one | Separate pages | Separate pages | ✔ One search |
| Formula and calculation shown | ✔ Every time | ✘ No | ✔ Some | ✘ No |
| Bidirectional live input | ✔ Both fields | One direction | One direction | One direction |
| Conversion history | ✔ Last 10 | ✘ No | ✘ No | ✘ No |
| Favourite unit pairs | ✔ Saved locally | ✘ No | ✘ No | ✘ No |
| Category search filter | ✔ Yes | ✘ No | ✘ No | ✔ Search |
| Works without internet after load | ✔ 100% local | Server-side | Server-side | Requires connection |
| No ads blocking the tool | ✔ Clean | Heavy ads | Ads | ✔ Clean |
Most common unit conversions — quick reference
| Category | Factor | Formula |
|---|---|---|
| Metres → Feet | × 3.28084 | ft = m × 3.28084 |
| Feet → Metres | × 0.3048 | m = ft × 0.3048 |
| Kilometres → Miles | × 0.621371 | mi = km × 0.621371 |
| Miles → Kilometres | × 1.60934 | km = mi × 1.60934 |
| Kilograms → Pounds | × 2.20462 | lb = kg × 2.20462 |
| Pounds → Kilograms | × 0.453592 | kg = lb × 0.453592 |
| °Celsius → °Fahrenheit | — | °F = (°C × 9/5) + 32 |
| °Fahrenheit → °Celsius | — | °C = (°F − 32) × 5/9 |
| Litres → US Gallons | × 0.264172 | gal = L × 0.264172 |
| US Gallons → Litres | × 3.78541 | L = gal × 3.78541 |
| Megabytes → Megabits | × 8 | Mb = MB × 8 |
| km/h → mph | × 0.621371 | mph = km/h × 0.621371 |
Unit Conversion Guide — Metric, Imperial and SI Units Explained
Unit conversion is one of the most frequently needed calculations in everyday life, science, engineering and cooking. Whether you are converting miles to kilometres for a road trip, kilograms to pounds for a recipe, Celsius to Fahrenheit for a weather report, or megabytes to gigabytes for data planning — having an accurate, fast and trustworthy unit converter is essential. This guide covers the major unit systems, the most important conversion factors, and the areas where mistakes are most commonly made. Understanding why different measurement systems exist and how they relate to each other also helps you catch errors and develop reliable intuition for whether a converted number is plausible.
The metric system — SI units
The metric system, formally known as the International System of Units (SI), is the world's most widely used measurement system and the official standard in scientific work globally. It is built on seven base units: metre (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), mole (amount of substance) and candela (luminous intensity). All other SI units are derived from these seven. Its defining strength is the decimal structure — all prefixes (milli-, centi-, kilo-, mega-, giga-) are powers of 10, making conversions within the metric system simple multiplication or division. Metric is used in virtually every country for official, scientific and most everyday purposes. Notable exceptions are the United States (for everyday use), Liberia and Myanmar, which still use customary units in day-to-day life.
The imperial and US customary systems
The imperial system evolved from historical British units of measurement and was formally defined in the British Weights and Measures Act of 1824. US customary units are closely related but differ in several important ways — most notably, US and UK fluid ounces, pints and gallons are different sizes. Imperial units include feet, inches, yards and miles for length; ounces, pounds and stones for weight; pints and gallons for volume; and Fahrenheit for temperature. The non-decimal relationships between imperial units (12 inches in a foot, 3 feet in a yard, 1,760 yards in a mile; 16 ounces in a pound, 14 pounds in a stone) make within-system arithmetic significantly more complex than metric.
Temperature conversion — the most common source of confusion
Temperature conversion is unique among unit conversions because it is non-linear — the formulas involve both multiplication and addition, not just a single factor. The three main temperature scales are Celsius (°C), used almost everywhere; Fahrenheit (°F), used primarily in the United States for everyday temperature; and Kelvin (K), used in scientific contexts. Celsius to Fahrenheit: F = (C × 9/5) + 32. Fahrenheit to Celsius: C = (F − 32) × 5/9. Converting Celsius to Kelvin: K = C + 273.15. Common reference points: water freezes at 0°C / 32°F / 273.15K and boils at 100°C / 212°F / 373.15K. Body temperature is 37°C / 98.6°F. Room temperature is typically 20–22°C / 68–72°F.
Length conversion — metres, feet and miles
The metre is the SI base unit of length. The metre equals exactly 3.28084 feet, 39.3701 inches, or 0.000621371 miles. Each kilometre (1,000 metres) equals 0.621371 miles — close enough to 0.6 for rough mental conversion, meaning 10 km is approximately 6 miles. Each mile equals 1.60934 kilometres. Importantly, an inch equals exactly 2.54 centimetres — a defined value, not an approximation. Equally, a foot is 30.48 centimetres. For height conversion: 6 feet is approximately 183 cm, and 5 feet 10 inches is approximately 178 cm.
Weight and mass conversion
Mass is the amount of matter in an object; weight is the force that gravity exerts on that mass. In everyday use the two terms are often used interchangeably, but in scientific contexts they are distinct. The SI unit of mass is the kilogram. The kilogram equals 2.20462 pounds or 35.274 ounces. Each pound equals 0.453592 kilograms. The stone (UK unit) equals 14 pounds or 6.35029 kilograms. The metric tonne is 1,000 kilograms or 2,204.62 pounds, while the US short ton equals 2,000 pounds or 907.185 kilograms. For medical and fitness purposes, note that body weight in the UK is often expressed in stones and pounds (e.g., 11 stone 4 pounds = 72 kg), while in the US it is expressed in pounds only.
Data storage conversion
Data storage units have two different standard definitions and this causes persistent confusion. In the strict binary (IEC) system, 1 kibibyte (KiB) = 1,024 bytes, 1 mebibyte (MiB) = 1,048,576 bytes, and so on with powers of 2. In the decimal (SI) system used by hard drive manufacturers, 1 kilobyte (KB) = 1,000 bytes, 1 megabyte (MB) = 1,000,000 bytes. This discrepancy explains why a "500 GB" hard drive appears as 465 GB in Windows (which uses binary-based units). For networking, data rates are always expressed in decimal units per second (Mbps = megabits per second), and note the distinction between bits (b, lowercase) and bytes (B, uppercase) — 1 byte = 8 bits. A 100 Mbps internet connection transfers 100 megabits, not megabytes, per second — meaning 12.5 megabytes per second of actual file transfer speed. When comparing cloud storage plans, broadband speeds or USB transfer rates, always check whether the figure is quoted in bits or bytes and in decimal or binary units, as marketing materials frequently use whichever convention makes the number appear larger.
Speed and fuel economy conversion
Speed conversion between kilometres per hour (km/h) and miles per hour (mph) uses the same factor as the length conversion — multiply km/h by 0.621371 to get mph. Common reference points: city speed limit 50 km/h = 31 mph; motorway 100 km/h = 62 mph; 120 km/h = 75 mph. For fuel economy, the UK and Australia use litres per 100 km (L/100km) where lower is better, while the US uses miles per gallon (mpg) where higher is better. To convert mpg to L/100km: divide 235.215 by the mpg value. A car achieving 30 mpg US is approximately 7.84 L/100km. Note that US and UK gallons differ — 1 UK gallon = 1.201 US gallons — so UK mpg figures are approximately 20% higher than US mpg for the same vehicle. When comparing vehicle fuel efficiency across markets, always clarify which gallon standard is being used to avoid misleading comparisons, particularly important when evaluating import vehicles or reading international automotive reviews.
Practical unit conversion tips for everyday use
A few approximate conversions are worth memorising for everyday mental arithmetic. In terms of length: multiply km by 0.6 to get miles (exact: 0.621), or multiply miles by 1.6 to get km. Temperature approximation: double the Celsius value and add 30 for a rough Fahrenheit figure (the exact formula is ×1.8 + 32) — so 20°C ≈ 70°F, 30°C ≈ 90°F. Weight: multiply kg by 2.2 to get pounds. Cooking: 1 US cup is approximately 240 mL, and 1 tablespoon is approximately 15 mL. These approximations are accurate enough for everyday estimation even though they differ slightly from the precise factors used by this tool.
Frequently asked questions
Units used in science and engineering
Scientific and engineering work uses SI base units almost exclusively, but several derived units are critical to know. Pressure is measured in pascals (Pa) — 1 Pa = 1 N/m² — but most engineering contexts use kilopascals (kPa) or megapascals (MPa). Atmospheric pressure at sea level is 101.325 kPa, equivalent to 1 atmosphere (atm), 1.01325 bar, or 14.696 psi. Energy in scientific work is the joule (J), but electrical energy is billed in kilowatt-hours (kWh) — 1 kWh = 3.6 MJ. Power in watts is the SI unit, but mechanical power is still often expressed in horsepower (hp), where 1 metric hp = 735.5 W and 1 UK hp = 745.7 W — a small but meaningful difference when specifying vehicle or pump output. Frequency in hertz (Hz) is cycles per second — processor clock speeds in gigahertz (GHz) represent billions of operations per second, radio frequencies are typically in megahertz (MHz) or GHz, and audio frequencies relevant to human hearing range from 20 Hz to 20 kHz.