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Sources & Standards

Every number on UnitFig comes from fixed, internationally agreed definitions — chiefly the U.S. National Institute of Standards and Technology (NIST) and the International System of Units (SI) maintained by the BIPM. Below is the complete list of exact constants we apply, grouped by category, each traceable to its source. Nothing here is estimated or presented as exact when it is not.

Authoritative Sources

NIST (U.S. National Institute of Standards and Technology)

NIST SP 330 — The International System of Units (SI), 2019 edition — https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.330-2019.pdf

NIST (U.S. National Institute of Standards and Technology)

NIST SP 811 — Guide for the Use of the International System of Units (SI), 2008 — https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication811.pdf

BIPM (International Bureau of Weights and Measures)

SI Brochure — The International System of Units (9th edition) — https://www.bipm.org/en/publications/si-brochure

NIST (U.S. National Institute of Standards and Technology)

NIST Metric Program — SI Units and Metric Conversion — https://www.nist.gov/pml/weights-and-measures/metric-si

NIST (U.S. National Institute of Standards and Technology)

NIST — SI Units: Temperature — https://www.nist.gov/pml/weights-and-measures/si-units-temperature

IEC (International Electrotechnical Commission)

IEC 80000-13 — Quantities and units for information science and technology (binary prefixes) — https://www.iso.org/standard/61316.html

Weight Loss & Fitness

  • EXACT1 pound (lb, avoirdupois) = 0.45359237 kilograms (exact)NIST
  • EXACT1 ounce (oz, avoirdupois) = 28.349523125 grams (exact, 1/16 lb)NIST
  • EXACT1 stone (st) = 6.35029318 kilograms (exact, 14 lb)NIST
  • EXACT1 short ton (US) = 907.18474 kilograms (exact, 2000 lb)NIST

Weather & Comfort

  • EXACT°F = °C × 9/5 + 32 (exact linear relation)NIST
  • EXACT°C = (°F − 32) × 5/9 (exact inverse relation)NIST
  • EXACTKelvin (K) = °C + 273.15 (exact offset)NIST
  • EXACTWater freezes at 0 °C = 32 °F and boils at 100 °C = 212 °FNIST

Height & Construction

  • EXACT1 inch (in) = 25.4 millimetres (exact)NIST
  • EXACT1 foot (ft) = 0.3048 metres (exact, 12 in)NIST
  • EXACT1 yard (yd) = 0.9144 metres (exact, 3 ft)NIST
  • EXACT1 mile (mi) = 1609.344 metres (exact, 5280 ft)NIST
  • EXACT1 centimetre (cm) = 0.01 metre (definition)BIPM

Cooking & Baking

  • EXACT1 US gallon (gal) = 3.785411784 litres (exact, 231 in³)NIST
  • EXACT1 US fluid ounce (fl oz) = 29.5735295625 mL (exact, 1/128 gal)NIST
  • EXACT1 US cup = 236.5882365 mL (exact, 8 fl oz)NIST
  • EXACT1 US pint (pt) = 473.176473 mL (exact, 16 fl oz)NIST
  • EXACT1 US quart (qt) = 946.352946 mL (exact, 32 fl oz)NIST
  • EXACT1 litre (L) = 1000 millilitres (definition)BIPM

Driving & Travel

  • EXACT1 mile per hour (mph) = 1.609344 km/h (exact, 1 mi/h)NIST
  • EXACT1 mph = 0.44704 metres per second (exact)NIST
  • EXACT1 km/h = 1/3.6 m/s (exact reciprocal)BIPM
  • EXACT1 knot = 1.852 km/h (exact, 1 nautical mile/h)NIST

Real Estate & Land

  • EXACT1 acre = 4046.8564224 square metres (exact, 43,560 ft²)NIST
  • EXACT1 hectare (ha) = 10,000 square metres (definition)BIPM
  • EXACT1 square foot (ft²) = 0.09290304 m² (exact)NIST
  • EXACT1 square mile = 2.589988110336 km² (exact)NIST

Tires & HVAC

  • EXACT1 standard atmosphere (atm) = 101,325 pascals (exact)NIST
  • EXACT1 bar = 100,000 pascals (exact, definition)BIPM
  • EXACT1 psi = 6894.757293168 pascals (exact, 1 lbf/in²)NIST
  • EXACT1 kilopascal (kPa) = 1000 pascals (definition)BIPM

Fuel Economy

  • EXACT1 US MPG → L/100 km: L/100 km = 235.214583 ÷ MPG (exact, US gal = 3.785411784 L, 1 mi = 1.609344 km)NIST
  • EXACT1 US MPG = 0.425143707 km per litre (exact)NIST
  • EXACT1 Imperial MPG (UK) uses a 4.54609 L gallon — not interchangeable with US MPGNIST

Digital Storage

  • EXACT1 kibibyte (KiB) = 1024 bytes (exact, binary, IEC)IEC
  • EXACT1 kilobyte (kB, decimal) = 1000 bytes (exact, SI)IEC
  • EXACT1 mebibyte (MiB) = 1,048,576 bytes; 1 megabyte (MB) = 1,000,000 bytesIEC
  • EXACT1 gibibyte (GiB) = 1,073,741,824 bytes; 1 gigabyte (GB) = 1,000,000,000 bytesIEC

Power & Energy

  • EXACT1 mechanical horsepower (hp) = 745.6998715823 watts (exact, 550 ft·lbf/s)NIST
  • EXACT1 British thermal unit, IT (Btu) = 1055.05585262 joules (exact)NIST
  • EXACT1 Btu/h = 0.293071070172 watts (exact, Btu ÷ 3600 s)NIST
  • EXACT1 watt (W) = 1 joule per second (definition)BIPM

Accuracy & Responsibility

These constants are exact by international definition. For applications where errors are costly — medical dosing, engineering, trade, or legal compliance — always verify against the primary sources linked above and follow any applicable local standards. Unit conversions on this site are provided for general, informational, and educational use.

Last Updated 2026 | Reviewed against NIST SP 330 and the SI Brochure