🔄Unit Converter — Length, Weight, Temperature, Volume & Speed Conversion (Metric/Imperial)

Every modern unit conversion ultimately resolves to a chain of multiplications and additions against the seven base units of the International System of Units (SI), maintained by the Bureau International des Poids et Mesures (BIPM): the metre for length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for thermodynamic temperature, the mole for amount of substance, and the candela for luminous intensity. Since 2019 each of these base units has been redefined in terms of fixed numerical values of fundamental physical constants — the speed of light c, the Planck constant h, the elementary charge e, the Boltzmann constant k, the Avogadro constant N_A, and the caesium hyperfine transition frequency Δν_Cs. That reform decoupled the SI from physical artefacts (the historical 'kilogram prototype' in Sèvres) and made every base unit reproducible in any sufficiently equipped laboratory worldwide.

Derived units such as the newton (N = kg·m·s⁻²), joule (J = N·m), pascal (Pa = N/m²), watt (W = J/s) and volt (V = J/C) are built by combining the base units according to the relevant physical law. The converter does not need to know any physics for these — once the input is reduced to base SI, dimensional algebra produces every derived unit automatically. The hard part is the legacy of non-SI units that science and trade refuse to abandon. Inches, feet, miles, pounds, ounces, fluid ounces, gallons, BTUs, calories, horsepower, atmospheres, bars and Torr all survive in industry, aviation, US consumer life and chemistry, and each requires an exact (or, occasionally, a 'best-known') conversion factor that the calculator stores as a constant.

The most important factors used here are exact by international agreement and are reproduced verbatim in NIST Special Publication 811. One inch is exactly 25.4 millimetres — fixed since the 1959 international yard and pound agreement signed by the US, UK, Canada, Australia, New Zealand and South Africa, which harmonised earlier slightly-different national definitions. From that single equivalence everything else in the imperial length family derives: 1 foot = 12 in = 0.3048 m, 1 yard = 3 ft = 0.9144 m, 1 statute mile = 1760 yd = 1609.344 m. One avoirdupois pound is exactly 0.45359237 kilograms (also from 1959), so 1 ounce = 1/16 lb = 28.349523125 g. The US gallon is exactly 3.785411784 litres (defined as 231 cubic inches), the UK imperial gallon is 4.54609 L exactly. A US fluid ounce is 1/128 of a US gallon ≈ 29.5735 ml; a UK fluid ounce is 1/160 of an imperial gallon ≈ 28.4131 ml. These distinctions matter: a US pint is roughly 473 ml while a UK pint is 568 ml, so a recipe that calls for 'a pint of milk' yields a 20% different volume depending on which side of the Atlantic the cookbook was printed.

Temperature is the only common case where conversions are affine rather than purely multiplicative, because the scales have different zero points. Celsius and Kelvin share the same step size — both incrementing by exactly one when the thermodynamic temperature rises by one — so K = °C + 273.15 and °C = K − 273.15. Fahrenheit, by contrast, was historically pinned to a brine freezing point (0 °F) and human body temperature (~96 °F in Fahrenheit's own 1724 scheme), so its step is 5/9 of a Kelvin and the conversion is °F = °C × 9/5 + 32 with the inverse °C = (°F − 32) × 5/9. The Rankine scale, still used in some US thermodynamics, is Fahrenheit shifted to absolute zero: °R = °F + 459.67.

Area, volume and density compose unit factors algebraically. One square metre equals (1 m)² = (100 cm)² = 10 000 cm², because the conversion factor itself is squared. One cubic foot equals (0.3048 m)³ ≈ 0.0283168 m³ = 28.3168 litres. The most common kitchen pitfall is confusing volume and mass: cooks routinely ask 'how many grams in a cup of flour?' but the answer depends on what the cup contains, because mass = volume × density. The converter's density-aware mode handles this correctly. One US cup is 236.588 ml; if it holds water (ρ ≈ 1.000 g/ml) it weighs 236.6 g, but the same cup of all-purpose flour (ρ ≈ 0.59 g/ml) weighs about 140 g, butter (ρ ≈ 0.91 g/ml) about 215 g, granulated sugar (ρ ≈ 0.85 g/ml) about 200 g, and honey (ρ ≈ 1.42 g/ml) about 336 g. Using a one-size-fits-all conversion of '1 cup ≈ 240 g' will silently introduce 30–40% error in a recipe.

All factors used here are tracked against primary metrology sources: the BIPM SI Brochure (current 9th edition, 2019), NIST Special Publication 811 (Guide for the Use of the International System of Units), the ISO 80000 series for quantities and units, and CODATA 2022 for the recommended values of physical constants. If you need to verify a specific conversion, those four documents are the original source of truth — every reputable encyclopaedia and engineering handbook ultimately cites back to them.