Weight Converter

What this converter does

It converts between eleven mass units spanning nine orders of magnitude — from the microgram of a vitamin dose to the long ton of a ship’s displacement — in any combination. It is the general tool of this cluster: where the dedicated pages each serve one direction well, this one serves every pairing at once, shows your value in all eleven units simultaneously, and exports that table as CSV. If you convert kg to lb daily, the dedicated page is quicker; if you juggle grains, stones and tonnes in one sitting, this is the page.

The units, and where each belongs

The metric side runs microgram, milligram, gram, kilogram, tonne — each step a clean power of ten, which is the entire genius of the system. The other side is the avoirdupois chain the English-speaking world built: the grain (64.79891 mg, the smallest legal unit and the foundation of the rest), the ounce of 437.5 grains, the pound of 16 ounces, the stone of 14 pounds, and two rival tons — the US short ton of 2,000 lb and the imperial long ton of 2,240 lb. Between the two systems sits one bridge, defined exactly in 1959: one pound equals 0.45359237 kilograms. Every cross-system conversion in this tool passes over that bridge, which is why every one of them is exact.

How the arithmetic stays exact

Each unit is stored as its exact decimal multiple of the kilogram, and conversions are computed as exact ratios in decimal arithmetic — not floating point. The practical consequence: 1 stone reads as exactly 14 pounds, a value converted out and back returns identically, and the displayed digits at any precision are the true decimal expansion. Floating-point converters show 13.999999999999998 lb in the same spot, then hide it with rounding; the difference only surfaces when you push precision, which is exactly when you need it not to.

Worked examples across the range

A 500 µg vitamin B12 tablet: 0.5 mg, 0.0000005 kg, or 0.0077 grains — the grain being the unit old pharmacy used, at 64.79891 mg each. A 340 g jam jar: 12 ounces on the nose. A 70 kg adult: 154.32 lb or 11 st 0.3 lb. A 907.18 kg pallet: exactly one US short ton, and 0.907 metric tonnes. A 5,000 kg truckload: 5 tonnes, 5.51 short tons, 4.92 long tons — three “ton” figures for one load, which is the whole argument for naming tons precisely.

The three tons, spelled out

The metric tonne is 1,000 kg. The US short ton is 2,000 lb = 907.18474 kg. The imperial long ton is 2,240 lb = 1,016.0469088 kg. The long ton and the tonne differ by only 1.6%, which is why they get confused; the short ton is 9.3% lighter than the tonne, which is why the confusion costs money. In this tool the three are separate entries — a deliberate design decision so a “ton” can never silently change nationality between the From and To boxes.

Which precision to use

The Auto display carries twelve significant digits and switches to scientific notation past the readable range in either direction. For real-world weights, fixed two decimals is nearly always right — no scale outside a metrology lab justifies more. The higher settings exist for auditing other converters, spreadsheets and firmware against true values, a job this page is particularly suited to because its own arithmetic is exact.

Mass and weight, one more time

Every unit here is a mass unit, including the pound as commonly used. Weight — the force a mass exerts under gravity — is a different quantity measured in newtons, and it varies with location, which mass does not. This converter never asks where you are because the question is irrelevant to mass. The one context where the distinction bites is engineering force calculations, and for those the Mass Converter page (with the slug, the unit built for exactly that job) plus an explicit gravity value is the honest route.

Common mistakes

Choosing the wrong ton, as above — the costliest error available here. Feeding troy figures to avoirdupois units: precious-metal ounces are 31.103 g, not 28.350 g, and this tool deliberately excludes them so the mistake cannot happen silently; the troy pages carry those constants. Entering “1,5” and hoping the comma means a decimal point — this tool rejects it with an explanation rather than guessing, because guessing across locales is how a dose or a dimension goes wrong by a factor of ten. And converting grams to cups or millilitres: those are volume, the conversion depends on density, and no honest weight converter will do it.

Benchmarks across the whole span

1 grain = 64.8 mg — an old aspirin was five of these. 1 gram = 15.43 grains. 1 ounce = 28.35 g. 1 pound = 453.59 g. 1 stone = 6.35 kg. 1 hundredweight (US) = 45.36 kg. 1 short ton = 907.18 kg. 1 tonne = 1,000 kg = 2,204.62 lb. 1 long ton = 1,016.05 kg. Memorise 2.2, 28, 454 and 6.35 and you can sanity-check almost any everyday conversion in your head.

Accuracy and limits

Exact factors, exact arithmetic, single display rounding, scientific notation for the extremes, and honest refusal of ambiguous input. The limits are scope: mass only (no force, no volume), everyday units only (troy, carat and slug live on their own pages), and no judgement about whether a number is physically sensible — it will convert the mass of the Moon to grains without complaint, carrying every digit faithfully.

Using this page to audit other tools

Because the arithmetic here is exact, this converter doubles as a reference for checking any other one — a spreadsheet, a shipping system, a firmware display. The method: pick anchors whose true values are definitional, convert them in the tool under test, and compare. Good anchors: 1 lb must give exactly 0.45359237 kg; 1 st exactly 14 lb; 1 short ton exactly 2,000 lb; 5 ct exactly 1 g; 7,000 gr exactly 1 lb. A tool that returns 13.999999999999998 for the stone test is doing binary floating point and rounding late — harmless at two decimals, but you now know where its digits stop being real. A tool that fails the pound anchor outright is carrying a truncated constant and will be wrong in proportion everywhere.

A one-paragraph history of why two systems exist

The metric system was built in revolutionary France precisely to end the chaos this page mediates: thousands of regional pounds and stones that varied by town and trade. It won most of the world by decimal simplicity — every step a power of ten — while the avoirdupois chain survived through Britain’s trading sphere and industrial momentum, and in the United States to this day. The 1959 agreement did not merge the systems; it nailed a single exact plank between them, the 0.45359237 kg pound, and every cross-system conversion in this tool crosses on that plank. That is why those results are exact rather than approximate: the bridge is a definition, not a survey.