Unit Converter

Every conversion this tool performs ultimately leans on the International System of Units (SI), a set of seven base units — the metre, kilogram, second, ampere, kelvin, mole, and candela — from which everything else (newtons, joules, litres, volts) is derived. What most people never hear is that the definitions behind these units were quietly revolutionized within living memory. The metre began in the 1790s as one ten-millionth of the distance from the North Pole to the equator, then became a platinum-iridium bar kept in Paris, then a count of krypton-86 wavelengths. Since 1983 it has been defined as the distance light travels in a vacuum in exactly 1/299,792,458 of a second — which means the speed of light is no longer measured; it is fixed by definition, and the metre inherits its precision from atomic clocks. The second itself is defined by counting exactly 9,192,631,770 oscillations of radiation from a caesium-133 atom. The kilogram held out the longest. Until 2019 it was literally a golf-ball-sized platinum-iridium cylinder — the International Prototype Kilogram, nicknamed Le Grand K — stored under three bell jars near Paris. The problem: compared with its official copies, its mass drifted by roughly 50 micrograms over a century. An object that IS the kilogram cannot be wrong, so when the artifact changed, the definition of mass for the entire planet changed with it. On 20 May 2019, the SI abandoned artifacts entirely: the kilogram is now defined by fixing the Planck constant at exactly 6.62607015 × 10⁻³⁴ joule-seconds. Nothing changed for your kitchen scale — the redefinition was engineered to be seamless — but now, for the first time, every unit humans use is anchored to constants of nature rather than to objects that can be scratched, stolen, or slowly gain dust.

The most dangerous conversions are not metric-to-imperial — they are imperial-to-imperial, because "gallon", "pint", and "ton" name different quantities depending on which side of the Atlantic you are on. The UK reorganized its measures in 1824 (the imperial system) while the newly independent United States kept the older definitions (US customary), and the two have disagreed ever since. Volume is where it bites hardest. A US gallon is 3.785 litres; an imperial gallon is 4.546 litres — 20 percent bigger. That single fact silently distorts fuel economy comparisons: a car achieving 40 miles per imperial gallon is doing only about 33 miles per US gallon. The nesting is stranger still: an imperial pint holds 20 fluid ounces while a US pint holds 16 — yet the US fluid ounce (29.57 mL) is slightly LARGER than the imperial one (28.41 mL). Bigger ounces, but fewer of them, and a smaller pint overall; a "pint" of beer in London (568 mL) genuinely outmeasures one in New York (473 mL). Weight adds a third trap: the ton. A US short ton is 2,000 pounds (907.2 kg), a British long ton is 2,240 pounds (1,016 kg), and the metric tonne is exactly 1,000 kg. Three units, one word, up to 12 percent apart — which is why shipping contracts and engineering documents always spell out which ton they mean, and why this converter uses the metric ton. When a recipe, spec sheet, or fuel figure crosses the Atlantic, check which system it was written in before you convert.

Same word, different size:

  gallon:   US 3.785 L   vs  imperial 4.546 L   (+20%)
  pint:     US 473 mL (16 fl oz)  vs  UK 568 mL (20 fl oz)
  fl oz:    US 29.57 mL  vs  imperial 28.41 mL  (US bigger!)
  ton:      short 907 kg | long 1,016 kg | metric 1,000 kg

Fuel economy trap:
  40 mpg (imperial) = 40 x 3.785/4.546 = ~33 mpg (US)

Length, weight, and volume conversions are pure multiplications: double the metres and the feet double too, and zero means the same nothing in every unit. Temperature is the one everyday quantity where this logic breaks, because Celsius and Fahrenheit do not share a zero. Their conversion — F = C × 9/5 + 32 — has an offset term, making it an affine transformation rather than a proportional one, and that single +32 invalidates every ratio-based shortcut you might try. Concretely: 20 °C is not "twice as hot" as 10 °C. Convert both and the illusion collapses — 68 °F versus 50 °F is a ratio of 1.36, not 2, and on the absolute Kelvin scale (293.15 K vs 283.15 K) the true ratio of thermal energy is a mere 1.035. Ratios of temperatures are only meaningful in kelvin, where zero really is zero: −273.15 °C, the absence of thermal motion, which is also why kelvin takes no degree sign. The offset also creates a subtle trap with temperature differences. A change of 10 °C corresponds to a change of 18 °F — you multiply by 9/5 but do NOT add the 32, because the offsets cancel when you subtract two temperatures. Weather reports mix these up routinely: "tomorrow will be 5 degrees warmer" translates to 9 °F, not 41 °F. For a fast mental estimate, double the Celsius and add 30 (25 °C → about 80 °F; the exact value is 77) — good enough for a weather forecast, not for a lab.

Temperatures need the offset:
  37 C  ->  37 x 9/5 + 32 = 66.6 + 32 = 98.6 F

Temperature DIFFERENCES do not:
  a rise of 10 C  ->  10 x 9/5 = 18 F   (no +32!)

Ratios only work in kelvin:
  20 C / 10 C  "= 2x"?   68 F / 50 F = 1.36x
  293.15 K / 283.15 K = 1.035x  <- physical truth

The scales cross at -40:  -40 C = -40 F

A surprising number of unit relationships are not measured approximations but exact by international agreement. Since the International Yard and Pound Agreement of 1959, one inch equals 25.4 millimetres exactly, one foot 0.3048 metres exactly, and one pound 453.59237 grams exactly — the trailing digits do not continue; they are the definition. So when a conversion looks "off", the culprit is rounding, not the factor. Rounding damage compounds when conversions are chained. Convert 10 metres to feet and round to two decimals: 32.81. Convert that back and you get 9.9989 m — a millimetre lost to a single round trip. Do this through three or four unit hops, or in a spreadsheet that truncates each intermediate cell, and errors creep into the visible digits. The professional rule: convert once, directly from source unit to target unit, and round only the final displayed answer. This tool follows that rule internally — full floating-point precision throughout, rounding only at display time (six decimal places). Silently wrong conversions have real-world stakes. In 1999, NASA's 327-million-dollar Mars Climate Orbiter disintegrated in the Martian atmosphere because a contractor's software reported thruster impulse in pound-force seconds while the navigation team's software expected newton seconds. The trajectory drifted roughly 170 kilometres too low. No conversion is "small" when nobody notices it is wrong.

Exact by definition (1959 agreement):
  1 inch = 25.4 mm        1 ft = 0.3048 m
  1 lb   = 453.59237 g    (no more digits — that IS the value)

Round-trip drift from premature rounding:
  10 m -> 32.808399 ft -> round to 32.81
  32.81 ft back to metres = 9.99845 m   (1.5 mm lost)

Rule: convert once, round only the final answer.

The conversions most likely to ruin your evening are not scientific — they are culinary and domestic, because kitchen and workshop units carry unlabeled national assumptions. Start with the cup, the most treacherous unit in cooking. A US customary cup is 236.6 mL, the US "legal" cup used on nutrition labels is 240 mL, the metric cup used in Australia and New Zealand is 250 mL, and a Japanese cup is only 200 mL. Follow an American baking recipe with a Japanese cup and every volume ingredient silently shrinks by 15 percent. Spoons diverge too: a US tablespoon is 14.8 mL, a metric tablespoon 15 mL — harmless — but an Australian tablespoon is 20 mL, a full third larger, which matters for salt, yeast, and baking powder. The deeper kitchen error is confusing weight ounces with fluid ounces. An ounce of flour (about 28 grams) and a fluid ounce of flour (about 30 mL of volume) are unrelated quantities that happen to share a name. Volume measurement of powders is inherently unstable anyway — a "cup of flour" weighs anywhere from 120 to 150 grams depending on how it is scooped and settled, a ±20 percent swing. This is why serious baking recipes give grams, and why a 10-dollar kitchen scale beats any conversion chart. The workshop has its own folklore: a "2×4" stud actually measures 1.5 by 3.5 inches (38 × 89 mm) after drying and planing, and oven instructions cross between 180 °C, 350 °F, and UK gas mark 4 as rough equivalents. The universal lesson: a number without a clearly identified unit — and a clearly identified national convention behind it — is not yet information.

"One cup" is not one thing:
  US customary 236.6 mL | US legal 240 mL
  metric (AU/NZ) 250 mL | Japan 200 mL | rice go 180 mL

Tablespoons: US 14.8 mL | metric 15 mL | Australia 20 mL

Flour trap (volume vs weight):
  1 cup of flour = 120-150 g depending on packing (+/-20%)
  1 oz (weight) = 28.35 g  !=  1 fl oz (volume) = 29.57 mL

Oven equivalents: 180 C ~ 350 F ~ gas mark 4
Lumber: a "2x4" actually measures 1.5 x 3.5 inches