Weight Converter

Type a weight, pick the unit you have and the unit you want, and the converted figure appears with four decimal places. The converter covers the seven units people actually mix in daily life: milligrams, grams, kilograms and metric tons on the metric side, plus ounces, pounds and stone on the imperial side.

How the conversion works

Every unit is stored as an exact multiplier relative to the kilogram, so any conversion is one multiplication and one division:

result = value × factor_from ÷ factor_to

With the defaults — 75 kilograms into pounds — that is 75 ÷ 0.45359237 = 165.3467 lb. The odd-looking divisor is not a measured approximation. On 1 July 1959 the US, UK, Canada, Australia, New Zealand and South Africa signed the International Yard and Pound Agreement, fixing the avoirdupois pound at exactly 0.45359237 kg. The rest of the imperial side follows from that one number: an ounce is a sixteenth of a pound, 28.349523125 g exactly, and a stone is fourteen pounds, 6.35029318 kg exactly.

The table rounds to six decimal places; the calculator keeps full floating-point precision and rounds only for display. Because every factor traces back to the kilogram, converting stone to ounces or milligrams to metric tons needs no special case — the kilogram sits in the middle of every route.

Where the kilogram comes from

The metric side of this converter rests on a unit that was, for most of its life, a physical object you could hold. When revolutionary France rebuilt its measures on decimal lines, the Decree of 18 Germinal Year III, 7 April 1795, defined the gram as the mass of one cubic centimetre of water at the melting point of ice, and the kilogram as a thousand of those grams. Water made a convenient anchor because anyone with a careful laboratory could in principle reproduce it. Four years later, in 1799, the definition was handed to a platinum artefact, the kilogramme des Archives, calibrated to the mass of one litre of water at its temperature of maximum density, close to 4 degrees Celsius, where water is at its densest.

The artefact approach became international after the Metre Convention of 1875. In 1889 the first General Conference on Weights and Measures adopted the International Prototype of the Kilogram, a platinum-iridium cylinder cast in 1879, kept near Paris and often called Le Grand K. For 130 years that single piece of metal defined the kilogram for the entire world, and every other mass — including the pound, through the 1959 agreement — was in the end expressed against it. The weakness was plain: a standard you can only check by physical comparison can drift, and periodic comparisons across the twentieth century suggested Le Grand K and its official copies had diverged by roughly 50 micrograms since they were made.

The fix took effect on 20 May 2019. Following a vote by the 26th General Conference on 16 November 2018, the kilogram was redefined in terms of the Planck constant, fixed at exactly 6.62607015 × 10⁻³⁴ joule-seconds. Mass is now derived from fundamental physics rather than from an object, so a properly equipped laboratory can realise it without a trip to Paris. The changeover was engineered to be invisible in everyday use: the new kilogram matches the old one, staying within about 30 parts per million of that original litre of water.

The pound, the ounce and the grain

The imperial side has a messier pedigree. The word avoirdupois comes from the Anglo-Norman aveir de peis, "goods of weight" — heavy merchandise sold on a large balance. The system took hold among wool traders in England and the Low Countries around 1300, competing with an older twelve-ounce tower pound of 5,400 grains. The avoirdupois pound's sixteen ounces gave it a practical edge, since sixteen halves cleanly four times over, and it gradually won out for general commerce. Its size settled at 7,000 grains — the figure Elizabeth I set in 1588, having started at 6,992 — with each grain now fixed at exactly 64.79891 milligrams.

That grain is the quiet common ancestor of the troy and avoirdupois systems, which is why a pound of gold and a pound of feathers are genuinely different weights: the troy pound used for precious metals is 5,760 grains, the avoirdupois pound 7,000. This converter uses avoirdupois throughout, the pound of the butcher, the gym and the bathroom scale. Before 1959 even the avoirdupois pound was not perfectly uniform — the American and British standards each differed from the eventual international value by roughly one part in ten million, irrelevant at the market but awkward in a standards laboratory. The International Yard and Pound Agreement erased that gap, and the UK wrote the new value into the Weights and Measures Act 1963, which took effect on 1 January 1964.

Stone and body weight in the UK and Ireland

Almost nobody in Britain says they weigh 75 kilograms, and few say 165 pounds either. Body weight is given in stones and leftover pounds: 11 st 11 lb, in that style. The stone has an unusually tangled past. The medieval Assize of Weights and Measures listed several at once, including an eight-pound stone for beeswax and sugar and a twelve-and-a-half-pound London stone for wool, and butchers weighed live animals in fourteen-pound stones but their carcasses in eight-pound ones. A 1350 statute of Edward III set the stone for wool and other merchandise at fourteen pounds, reaffirmed by Henry VII in 1495, and that is the value that survived. The stone was withdrawn from UK trade use by the Weights and Measures Act 1985, with Ireland acting in 1983, but it lives on for human weight on both islands. This converter returns decimal stone — 75 kg shows as 11.8105 st — so to get the traditional form, keep the whole stones and multiply the remainder by 14. Here 0.8105 × 14 gives 11.3 lb, which people round to 11 st 11 lb.

One name, three different tons

The metric ton in this converter is exactly 1,000 kg. The US short ton is 2,000 lb, or 907.18474 kg, and the older British long ton is 2,240 lb, or 1,016.0469088 kg. Because both are defined in pounds, the gap between them is exactly 12 percent, so when a source just says tons, check which country wrote it before converting anything that matters.

Mass, weight and why the difference rarely matters

Strictly speaking, kilograms and pounds measure mass, while weight is a force — mass times gravitational acceleration, measured in newtons. A 75 kg person has a mass of 75 kg everywhere, but weighs about 735 N on Earth and roughly 122 N on the Moon. A bathroom scale measures the force and is calibrated to display mass assuming Earth gravity. For any conversion you would do on this page the distinction changes nothing, which is why everyone, this site included, calls it a weight converter.

Frequently asked questions

How many pounds is 1 kg?

One kilogram is 2.2046 pounds. The quick mental version is to multiply kilograms by 2.2, which lands within a quarter of a percent — close enough for luggage allowances and gym plates. Going the other way, divide pounds by 2.2 or multiply by 0.4536.

How much is 11 stone in kg and pounds?

11 stone is exactly 154 pounds, since a stone is 14 pounds, and 69.85 kg. A typical British body weight written as 11 st 11 lb works out to 165 lb, or just under 75 kg.

Why is a pound exactly 0.45359237 kilograms?

Because six countries agreed to define it that way. The 1959 International Yard and Pound Agreement, signed by the US, UK, Canada, Australia, New Zealand and South Africa, fixed the avoirdupois pound at exactly 0.45359237 kg. Before that each country maintained its own standard and the two differed very slightly — irrelevant at the butcher's, meaningful in a standards lab.

Is a ton the same as a tonne?

No. A tonne, also called a metric ton, is 1,000 kg. A US short ton is 2,000 lb (907.18 kg) and a British long ton is 2,240 lb (1,016.05 kg). The t in this converter is the metric ton, which is what science, industry and every metric country mean by the word.

Are these fluid ounces or weight ounces?

Weight ounces — the avoirdupois ounce of 28.3495 grams, sixteen to the pound. A fluid ounce is a volume unit, 29.57 ml in the US or 28.41 ml imperial, and shares nothing but the name. For recipes that mix the two, weigh dry ingredients and measure liquids by volume.