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Convert any unit, instantly

This free unit converter covers the measurements people reach for every day and the specialist ones engineers, scientists and students need less often. Pick a category, choose your from and to units, type a value — the answer appears live. Nothing to install, no sign-up and no ads interrupting the conversion tool.

Whether you're converting cm to inches for a product dimension, kg to pounds for a recipe, Celsius to Fahrenheit for a weather report or km to miles for a road trip, you get a precise answer in under a second — and the swap button flips the direction in one click.

Fourteen measurement categories

Most online unit converters cover four or five categories. This tool covers fourteen: Length (nm, mm, cm, m, km, inch, foot, yard, mile, nautical mile), Mass/Weight (mg, g, kg, tonne, oz, lb, stone), Temperature (°C, °F, Kelvin), Area (cm², m², km², hectare, acre, ft², in², mi²), Volume (mL, L, m³, US/UK gallon, quart, pint, cup, fl oz, tbsp, tsp), Speed (m/s, km/h, mph, knot, ft/s), Time (second through year), and Digital storage (bit through TiB, with separate decimal and binary units).

Beyond the everyday eight, you get Energy (J, kJ, cal, kcal, Wh, kWh, MJ, BTU), Pressure (Pa, kPa, bar, PSI, atm, mmHg), Power (W, kW, MW, metric and US horsepower, BTU/h), Angle (degree, radian, gradian, arcminute, arcsecond, turn), Fuel economy (km/L, MPG US, MPG UK, L/100km), and Frequency (Hz, kHz, MHz, GHz, THz, RPM). Every long-tail unit conversion search — from fuel consumption to signal frequency — is one tab away.

Common conversions: the numbers you reach for most

The most searched conversions are available as one-click presets under each category. For length: 1 cm → in, 1 ft → m, 1 km → mi, 1 mi → km. For mass: 1 kg → lb, 1 lb → kg, 1 oz → g. For temperature: 100 °C → °F, 0 °C → °F, 32 °F → °C. Click any chip and the fields fill instantly — then change the value to your own number.

For digital storage the presets cover GB/MB and TB/GB pairs. Speed presets handle 60 mph/100 km/h for international drivers. These quick-picks demonstrate the right units to use for each conversion, so you always know you're in the correct category and using the right standard (US vs UK gallon, SI vs IEC data sizes).

Why this beats Google's converter, RapidTables, and UnitConverters.net

Google's built-in converter is convenient for simple queries but limited: it handles one conversion at a time, doesn't work offline, and shows ads alongside results on mobile. For 14 categories in a single interface with no surrounding ad noise, it falls short. RapidTables is accurate but requires a separate page for each category and has heavy advertising that makes mobile use frustrating. UnitConverters.net covers many categories but clutters results with banner ads that obscure the answer on smaller screens.

This tool puts all 14 categories in a single page with no ads on the converter itself, works offline after the first load, and is genuinely free with no usage limits, no account, and no paywalled conversions. The formula line shows exactly what was converted (e.g. 5.00 Mile = 8.047 Kilometer) so you can sanity-check or copy the statement as a reference.

Precision, offline use, and privacy

Every number you type stays in your browser. Nothing is sent to a server, no cookies track your conversions, and the tool works with no internet connection after the page has loaded. Linear units use exact internationally-defined factors — 1 inch = 0.0254 m exactly; 1 pound = 0.45359237 kg exactly per NIST. Temperature uses correct offset formulas (not a naive ratio). Fuel economy handles the L/100km ↔ km/L inversion (100 ÷ one gives the other) automatically.

Results display to 10 significant figures and switch to scientific notation for values outside the range where a decimal display would be misleading — very large astronomy figures, very small nanometre-scale measurements. There is no account to create, no email required, and no usage cap. Bookmark it and reach for it the same way you'd reach for a calculator. It will always load fast and work offline.

The SI system: seven base units that define everything

Every quantity in science and engineering ultimately traces back to just seven SI base units: the metre (length), kilogram (mass), second (time), ampere (electric current), kelvin (thermodynamic temperature), mole (amount of substance), and candela (luminous intensity). Every other unit — newtons, pascals, volts, joules — is a derived unit expressed as a product or quotient of these seven. A joule, for instance, is one kilogram times one metre squared per second squared (kg·m²·s⁻²). Once you understand that structure, any unfamiliar unit can be decomposed into something familiar.

In 2019 the entire SI system was redefined to anchor each base unit to a fixed numerical value of a physical constant rather than to any physical artifact. The kilogram, which had been defined since 1889 by the mass of a platinum-iridium cylinder locked in a vault near Paris (the International Prototype of the Kilogram), was replaced by fixing Planck's constant h to exactly 6.626 070 15 × 10⁻³⁴ J·s. Similarly, the kelvin is now defined by fixing the Boltzmann constant k to 1.380 649 × 10⁻²³ J·K⁻¹, and the mole by fixing the Avogadro constant. The practical consequence is that the kilogram can in principle be realised by any suitably equipped laboratory anywhere in the universe — it is no longer tied to one object that could be lost, contaminated, or damaged. This makes all measurement ultimately traceable to fundamental physics rather than to human-made objects.

The derived units you encounter daily — newtons (force), pascals (pressure), watts (power), hertz (frequency), volts (electric potential) — all reduce to combinations of those seven base units. Knowing this helps when converting between unit systems: a pressure in PSI and a pressure in pascals both measure force per unit area, so the conversion factor is simply the ratio of the pound-force to the newton multiplied by the ratio of inches squared to metres squared — no exotic lookup table needed.

Imperial vs US customary: they are not the same system

A common misconception is that British Imperial units and US customary units are identical. They share many names — pint, gallon, fluid ounce — but the quantities are different, and conflating them causes significant errors. The divergence is rooted in the British Weights and Measures Act of 1824, which standardised UK measures after the US had already been using its own Colonial-era values. The US never adopted the 1824 Act, so the two systems drifted apart permanently.

The differences are substantial. One US fluid ounce = 29.5735 mL while one UK fluid ounce = 28.4131 mL — a 4% gap. A US pint = 16 US fl oz = 473 mL, but a UK pint = 20 UK fl oz = 568 mL, making it about 20% larger. The gap widens further at the gallon: a US gallon = 3.785 L, whereas a UK (imperial) gallon = 4.546 L. A US gallon is therefore roughly 83% the volume of a UK gallon. For fuel economy, this means a vehicle that achieves 40 MPG (US) is actually achieving the equivalent of about 48 MPG (UK) — the same car, two different numbers, purely because of the gallon definition.

This tool lists US Gallon, UK Gallon, US Pint, UK Pint, US Fluid Ounce, and UK Fluid Ounce as separate entries precisely because treating them as interchangeable causes real errors. When you read a recipe from an American cookbook in a UK kitchen, or compare fuel economy figures across a US and European car review, you need to know which standard was used. The 1824 divergence is still causing confusion two centuries later — verifying which system you're working in takes one second; fixing a mistake caused by mixing them can take much longer.

Dimensional analysis: how to chain unit conversions without error

Dimensional analysis (also called the factor-label method or unit-factor method) is the systematic technique for converting between units by multiplying by carefully constructed fractions equal to one. Each fraction has a numerator and denominator that represent the same physical quantity in different units, so multiplying by it changes the label without changing the value. The key rule is that identical units in the numerator and denominator cancel, just like algebraic variables — and you keep multiplying until only the target unit remains.

A worked example: converting 60 mph to m/s. Start with 60 miles/hour. Multiply by (1.609344 km / 1 mile) to convert miles to kilometres — the 'miles' cancel, leaving 96.56 km/hour. Then multiply by (1000 m / 1 km) to convert kilometres to metres — the 'km' cancel, leaving 96,560 m/hour. Finally multiply by (1 hour / 3600 s) — the 'hour' cancels, leaving 96,560 / 3600 = 26.82 m/s. You can collapse the middle step: km/h to m/s always uses the factor 1/3.6, because 1000 m/km ÷ 3600 s/hour = 1/3.6. So 100 km/h = 100/3.6 = 27.78 m/s.

The power of dimensional analysis is that it makes errors visible: if your units don't cancel to give the target unit, you've either used the wrong conversion factor or set up a fraction upside down. Unit labels are not decoration — they are part of the arithmetic. Engineers who dropped them from intermediate steps have caused expensive failures. NASA's 1999 Mars Climate Orbiter was lost because one team sent thruster data in pound-force seconds while the receiving software expected newton-seconds; the $327 million spacecraft entered the Martian atmosphere at the wrong angle and disintegrated. The factor was 4.45 — the ratio of a pound-force to a newton — a small number with a catastrophic consequence. Similarly, a 1983 Air Canada Boeing 767 (the 'Gimli Glider') ran out of fuel mid-flight when ground crew calculated the required fuel load in pounds instead of kilograms, loading roughly half the necessary fuel. The crew had to glide to an emergency landing on a disused airstrip. In both cases, a consistent habit of writing and checking units at every step would have caught the error before it propagated.

SI prefixes: from yocto to quetta, and why kibibyte ≠ kilobyte

The SI prefix system extends the seven base units across an enormous range of magnitudes by attaching a multiplier prefix. The current scale runs from yocto (y, 10⁻²⁴) at the small end through milli (10⁻³), centi (10⁻²), deci (10⁻¹), kilo (10³), mega (10⁶), giga (10⁹), tera (10¹²), peta (10¹⁵), exa (10¹⁸), zetta (10²¹), up to yotta (Y, 10²⁴). In 2022 the International Bureau of Weights and Measures added two new prefixes at each extreme: ronto (r, 10⁻²⁷) and quecto (q, 10⁻³⁰) at the small end, and ronna (R, 10²⁷) and quetta (Q, 10³⁰) at the large end — driven partly by data-storage and data-transfer volumes that were beginning to approach yottabyte scale. A quettabyte is 10³⁰ bytes; the entire internet's annual traffic as of 2022 was estimated at roughly 4.8 zettabytes per year, so quetta-scale storage remains a future concern rather than a current one.

In computing, the SI prefixes caused a longstanding ambiguity. Hard drive manufacturers used 1 kilobyte = 1,000 bytes (the SI definition), while operating systems and programmers traditionally used 1 kilobyte = 1,024 bytes (powers of two that map naturally to binary addressing). A 500 GB hard drive therefore showed up as roughly 465 GB in Windows, which was counting 1,073,741,824 bytes per 'gigabyte'. The IEC 80000-13 standard (1998) resolved this by defining a separate set of binary prefixes: kibibyte (KiB) = 2¹⁰ = 1,024 bytes, mebibyte (MiB) = 2²⁰ = 1,048,576 bytes, gibibyte (GiB) = 2³⁰ bytes, and so on with the suffix '-bi' (binary) replacing the ambiguous SI prefix. This tool distinguishes them explicitly: Gigabyte (GB, 10⁹ bytes) and Gibibyte (GiB, 2³⁰ bytes) are listed as separate units so you can match the standard your specification uses.

Knowing where SI prefixes stop and binary prefixes begin prevents a class of off-by-a-few-percent errors that compound badly at large scales. A data centre ordering 100 TB of usable storage under the decimal definition would receive significantly less usable space than expected if the operating system reports capacity in TiB. At 100 TB vs 100 TiB the gap is about 9% — nearly 10 TB of 'missing' storage that was never missing, just mislabelled. The same arithmetic applies to RAM (always specified in binary GiB in hardware specs), network transfer speeds (always specified in decimal Gbps by ISPs), and file sizes reported by different operating systems. Picking the right prefix, like picking the right unit, is the entire problem.