Unit Converter

Unit conversion is multiplication by a constant, with one exception that trips people up. The difficulty is not the arithmetic; it is knowing which constant applies.

The SI is defined by constants now

Since 2019, every SI base unit is derived from a fixed physical constant rather than from a physical object or a laboratory procedure.

The kilogram was the last to change. Until 2019 it was defined by a platinum-iridium cylinder held near Paris, which had measurably drifted from its official copies over a century — meaning the kilogram itself was slowly changing. Redefining it in terms of the Planck constant made it reproducible in any adequately equipped laboratory, permanently.

None of this changes any conversion you will do. It means the definitions no longer depend on an object that could be damaged.

US and imperial units differ

This is the most common source of a wrong conversion, because both are called by the same names.

Note the reversal in the last row: a US fluid ounce is larger than an imperial one, even though a US gallon is smaller. The two systems divide differently — 128 US fluid ounces to a US gallon against 160 imperial fluid ounces to an imperial gallon.

Length and mass are the same in both: an inch is 25.4 mm exactly, a pound is 0.45359237 kg exactly, both by international agreement in 1959. Only volume differs.

The practical consequence appears in recipes. An American recipe calling for a pint of cream means 473 ml; a British one means 568 ml. On a custard that is the difference between success and soup.

Indian land units

These are the least standardised units in common use anywhere, and the variation is by state and sometimes by district.

Acre and hectare are standardised: 1 acre is 43,560 sq ft, and 1 hectare is 10,000 sq m or about 2.471 acres.

A bigha in West Bengal is 1,600 sq ft. A bigha in Rajasthan is around 27,000. The ratio between the smallest and largest is seventeen to one, and both are called a bigha.

For any land transaction, use the figure in square feet or square metres from the registered document. A conversion from a local unit is an approximation, and property disputes have turned on exactly this ambiguity. Where a document states a local unit, the applicable state's definition governs — which is worth confirming rather than assuming.

Temperature is not a plain multiplication

Every other conversion on this page is a multiplication. Temperature has an offset as well, because the scales place zero in different places.

°F = °C × 9/5 + 32 °C = (°F − 32) × 5/9 K = °C + 273.15

That offset is why proportional reasoning fails. 40°C is not twice 20°C in any meaningful sense, and doubling a Celsius figure does not double anything physical — only kelvin, which starts at absolute zero, supports that kind of arithmetic.

Two reference points worth remembering: −40 is the same in both scales, and 28°C is about 82°F, which is a useful anchor for weather.

A rough mental conversion from Celsius: double it and add 30. For 20°C that gives 70 against a true 68; for 35°C it gives 100 against 95. Accurate enough to decide what to wear.

Weight and mass

They are different quantities and the distinction is usually harmless.

Mass is how much matter something contains, measured in kilograms, and it does not change with location.

Weight is the force gravity exerts on that mass, measured in newtons, and it does change — a 70 kg person weighs 687 N on Earth and about 114 N on the Moon.

Everyday use conflates them because Earth's gravity is near enough constant. A bathroom scale measures force and reports mass, which is correct everywhere anyone actually stands on one.

The distinction matters in physics, in engineering calculations, and in the occasional exam question. It does not matter at the grocer's.

Where conversions go wrong

Compound units. Converting km/h to m/s means converting both parts: × 1000 ÷ 3600, which is ÷ 3.6. Doing only one is a common error, and it is off by a factor of a thousand or of 3,600.

Squared and cubed units. A square metre is not 100 square centimetres — it is 10,000, because the conversion factor is squared. A cubic metre is a million cubic centimetres. This catches people converting areas and volumes constantly.

Rounding too early. Convert once with the full factor, then round. Rounding at each step of a chain accumulates error.

Assuming the wrong system. Gallons, pints and fluid ounces need the country specified.

Ambiguous tonnes. A metric tonne is 1,000 kg. A US short ton is 907 kg. A UK long ton is 1,016 kg.

Troy weight for precious metals. A troy ounce is 31.10 g against an ordinary ounce of 28.35 g. Gold prices are quoted in troy ounces, and using the wrong one is a 9.7% error.

Where India actually sits

India adopted the metric system in 1957 and completed the transition by 1962, so all official measurement is metric.

In practice a hybrid persists. Height is stated in feet and inches almost universally. Body weight is in kilograms. Land is in local units in rural areas and square feet in urban property. Fuel efficiency is kilometres per litre; distance is kilometres. Cloth is bought by the metre; jewellery by the gram, with tola surviving informally at 11.664 g.

This is not disorder so much as convention settling per domain, and it is stable. The only place it causes real problems is land, where the units are genuinely ambiguous.

The conversions worth memorising

A handful of factors cover most everyday needs and are close enough to do in your head.

The mile-to-kilometre factor of 1.609 is very close to the golden ratio, 1.618, which gives a curious shortcut: consecutive Fibonacci numbers approximate the conversion. 5 miles is about 8 km, 8 miles about 13 km, 13 about 21. It is a coincidence of arithmetic rather than anything meaningful, and it is accurate to within about half a percent.

Significant figures

A converted value should not carry more precision than the original had.

"About 5 miles" converted to 8.04672 km claims a precision of five decimal places from a figure that was approximate to begin with. The correct answer is "about 8 km".

The rule is that a result carries the precision of the least precise input. Measuring a room as 12 feet by 15 feet — both to the nearest foot — gives an area of 180 square feet, not 16.7225 square metres. The conversion is exact; the measurement was not.

This matters most where a converted figure is passed on and treated as measured. A specification stating 25.4 mm because someone converted "an inch" implies a tolerance the original never had.

What this tool assumes

• SI definitions and internationally agreed conversion factors, exact where the definition is exact.
• US and imperial volume units are listed separately, because they are different units with the same names.
• Indian land units use the most common regional definition, which may not be the one that applies in your district — check the registered document.
• Temperature applies the offset correctly, so it is not a simple ratio.
• Conversions are computed at full precision and rounded once for display.