Weight Converter

How to use the weight converter

Type a value, pick the unit you have on the left and the unit you want on the right, and the answer updates as you type. Any pairing works: grams to ounces, kilograms to stone, pounds to metric tons, milligrams to pounds if a dosage label demands it. The swap control flips the two dropdowns and keeps your number, which is the quick way to check a conversion in both directions.

Results show six significant figures, so a milligram figure keeps its decimals and a tonnage figure does not sprawl. Nine units are available — milligram, gram, kilogram, metric ton, ounce, pound, stone, US ton and long ton — and every one of them is priced in kilograms internally.

The imperial half of that list comes from one exact number. The International Yard and Pound Agreement, signed on 1 July 1959, defined the avoirdupois pound as exactly 0.45359237 kilograms, and everything else divides or multiplies out from it: the ounce is a sixteenth of a pound, so 28.349523125 g exactly; the stone is 14 pounds, so 6.35029318 kg exactly; the two tons are 2,000 and 2,240 pounds.

Pound-to-kilogram conversions therefore carry no measurement uncertainty, only display rounding. The stone is the one to recognize when reading British or Irish sources: it was dropped from the units legal for trade by the Weights and Measures Act 1985, but it is still how most people there state body weight, written as 11 st 4 lb rather than as a decimal.

Strictly, this is a mass converter. The kilogram measures mass — how much matter is present, unchanged whether the object sits on Earth or the Moon — while weight is the force gravity exerts on that mass, measured in newtons. Everyday English calls both of them weight, and for anything you put on a bathroom scale that is harmless, because the scale is calibrated for Earth gravity.

It matters in engineering, where the pound splits in two: pound-mass (lbm), which is the 0.45359237 kg pound defined above, and pound-force (lbf), the force that mass experiences under standard gravity of 9.80665 m/s², equal to 4.44822 newtons. If a spec sheet says lbf, this is not the converter for it.

Which ton does your document mean?

The most common weight-conversion error is treating a ton as a ton. Three of them are in ordinary use and they are all different. The metric ton, or tonne, is 1,000 kg exactly. The US ton, also called the short ton, is 2,000 pounds, which is 907.18474 kg — about 9.3 percent lighter than a metric ton.

The long or imperial ton is 2,240 pounds, or 1,016.0469088 kg, about 1.6 percent heavier than a metric ton. A freight quote, a scrap price or an emissions figure that says only "ton" is ambiguous by up to 12 percent, since a long ton is exactly 12 percent heavier than a short ton. This tool names all three, so choose deliberately rather than by habit.

The formula, worked line by line

Nine units, one calculation. Each unit declares how many kilograms it is worth, so any pairing converts by going into kilograms first and back out into the target second. Ounces to stone, milligrams to pounds, short tons to metric tons — the same two steps every time, with different numbers.

Every factor in the table above is an exact definition, so the pivot adds no error of its own. The kilogram itself has been defined since 20 May 2019 by a fixed value of the Planck constant rather than by the platinum-iridium prototype held in France, but that changed how the unit is realized in a laboratory, not its size.

kilograms = value × (kilograms per from-unit)
result = kilograms ÷ (kilograms per to-unit)
result = value × from.perBase ÷ to.perBase
lb → st: result = value × 0.45359237 ÷ 6.35029318

Two exact factors do most of the work: a pound is 0.45359237 kg and an ounce is 28.349523125 g. The reverse figures are not exact — a kilogram is 2.20462262 pounds, a gram is 0.0352739619 ounces — so any shortened version is an approximation with a known cost. Using 2.2 instead of 2.204623 understates a 100 kg load by about 0.46 pounds, which is nothing on luggage and real on a pallet.

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