Unit Converter

Convert length, weight, temperature, area, volume, speed and digital data instantly.

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What Is a Unit Converter?

A unit converter allows you to quickly convert between different units of measurement. Whether it's length, weight, temperature, area, volume, speed or digital data sizes — simply enter a value and get the result instantly. All calculations happen directly in your browser without sending data to a server.

Supported Categories

This unit converter supports 7 categories: Length (km, m, cm, mm, mile, yard, foot, inch, nautical mile), Weight (tonne, kg, g, mg, pound, ounce, stone), Temperature (Celsius, Fahrenheit, Kelvin), Area (km², m², cm², hectare, acre, ft², in², mi²), Volume (liter, ml, m³, US/UK gallon, fl oz, cup, pint, quart), Speed (km/h, m/s, mph, knots, ft/s) and Digital Data (bit, byte, KB, MB, GB, TB, PB, KiB, MiB, GiB, TiB).

Tips for Unit Conversion

  • For digital data, pay attention to the difference between decimal (KB = 1000 bytes) and binary units (KiB = 1024 bytes).
  • Temperature conversions use special formulas — they are not based on simple multiplication.
  • Use the quick overview table to compare all units of a category at a glance.

SI vs. imperial: why two worlds exist

The International System of Units (SI), formally defined in the 9th edition of the SI brochure (BIPM 2019), forms the basis of nearly all scientific measurement. It comprises seven base units: metre (length), kilogram (mass), second (time), ampere (current), kelvin (temperature), mole (amount of substance) and candela (luminous intensity). Since 2019 all seven base units are defined exclusively via natural constants — the kilogram, for instance, through the Planck constant (h = 6.62607015 x 10^-34 Js) rather than the Paris prototype kilogram. This definition is universally reproducible; a lab in Tokyo arrives at exactly the same kilogram as a lab in Berlin.

The imperial system (US, UK) by contrast is based on historical standards: a foot has been exactly 0.3048 metres since 1959, a pound exactly 0.45359237 kilograms. Before 1959 the American and British foot differed by a few micrometres — the international foot reconciled them. In practice the system is uneven: the UK gallon (4.54609 L) and US gallon (3.785411784 L) differ by nearly 20%, leading to classic mistakes in car ads and fuel billings. Pints are similarly uneven (UK: 568 mL, US: 473 mL). When publishing globally, always specify which pint you mean.

An important distinction is SI vs. binary prefixes in computing. A kilobyte (KB) under the SI standard is 1000 bytes, a kibibyte (KiB) is 1024 bytes — the gap grows exponentially: TB is 10^12 bytes, TiB is 2^40 = 1.0995 x 10^12 bytes. A hard drive advertised as 1 TB has 10^12 = 1,000,000,000,000 bytes, but the OS shows 931 GiB — hence the common question "Why is my disk smaller than advertised?". The IEC standardized the binary prefixes Ki, Mi, Gi, Ti in 1998, yet in daily use they're rarely seen. Our tool supports both.

Key conversion formulas at a glance

These five formulas cover the most common everyday cases:

  • Celsius ↔ Fahrenheit: F = C x 9/5 + 32 and C = (F - 32) x 5/9. Example: 20 C = 68 F.
  • km ↔ miles: 1 mile = 1.609344 km. Rule of thumb: 5 mi = 8 km (error < 1%).
  • kg ↔ pound: 1 lb = 0.45359237 kg. Rule of thumb: 2.2 lb = 1 kg.
  • Litre ↔ US gallon: 1 gal US = 3.785411784 L. UK gallon differs: 1 gal UK = 4.54609 L.
  • km/h ↔ m/s: m/s = km/h / 3.6. Example: 100 km/h = 27.78 m/s.

Real-world examples

Typical conversions that come up in daily life:

  • US recipe (cup to millilitre): 1 cup = 236.59 mL — a US cup is smaller than the metric cup (250 mL).
  • Height 5'10" to centimetres: 5 x 12 + 10 = 70 inch = 177.8 cm.
  • Oven temperature 350 F to Celsius: (350 - 32) x 5/9 = 176.7 C — pick 175 or 180 on most ovens.
  • Aircraft speed 500 knots to km/h: 500 x 1.852 = 926 km/h — typical cruise speed.
  • Hard drive 2 TB advertised vs. 1.82 TiB in the OS: 2 x 10^12 / 2^40 = 1.819 TiB — not defective, just different prefixes.

Accuracy and limits of conversion

Conversion factors are exact or high-precision values. 1 inch = 0.0254 m has been exactly defined since 1959 (no rounding error), as has 1 pound = 0.45359237 kg. Our tool shows up to 10 significant figures — enough for almost all everyday uses. But: for financially or legally relevant quantities (fuel deliveries, pharma filling, construction plans) specific rules apply for significant figures and rounding. The SI brochure recommends banker's rounding and respecting input measurement precision — measuring 10 m to 2 significant figures and reporting 10.97 yards to 4 decimal places is fake precision. Temperature conversion has another pitfall: 0 C is not 0 F (it is 32 F), and 0 K is absolute zero (-273.15 C). Temperature differences behave differently from absolute values.

Frequently asked questions

Why is my 1 TB USB stick shown as 931 GB?
Manufacturers use the SI prefix (TB = 10^12 bytes), operating systems often use binary (GiB = 2^30 bytes). 1 TB SI = 0.909 TiB binary, which Windows shows as roughly 931 GiB. Not a defect — just different standards.
US gallon or UK gallon — which do I need?
In the US, parts of Canada and Latin America the US gallon (3.785 L) is standard. In the UK, Australia and former colonies the imperial gallon (4.546 L). For car fuel economy, always check the source.
Why is 0 K = -273.15 C and not -273?
Absolute zero is at -273.15 C, measured with high precision since the Cordovillo conference and defined exactly since 2019 via the Boltzmann constant k = 1.380649 x 10^-23 J/K. The 0.15 comes from the exact scale offset.
How accurate are the factors in the tool?
We use exact definitions per the SI brochure 9th edition (BIPM 2019) and NIST. E.g. 1 lb = 0.45359237 kg exactly, 1 ft = 0.3048 m exactly. Display up to 10 significant figures is enough for >99% of all uses.
Are values sent to a server?
No. Calculation runs entirely in your browser via JavaScript — no server round-trips, no tracking, no logging of inputs.
When baking, can I just use 1 cup = 250 mL?
Only for metric recipes. US recipes use 236.59 mL, Australian 250 mL. For precision baking (yeast dough, macarons) weigh in grams rather than measuring volume — flour density varies by 20%.

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