Weight converter

Enter one weight and see it in every unit at the same time: metric from milligrams to tonnes, the avoirdupois ounce and pound, the stone, the troy ounce used for precious metals, and the Taiwanese catty, tael and qian that markets, pharmacies and gold shops in Taiwan still use. Where two units share a name and differ by ten or twenty per cent, this page keeps them separate and says so rather than picking one for you.

Mass, weight, and why the kilogram changed in 2019

Strictly, all the units on this page measure mass — how much matter something contains — rather than weight, which is the force gravity exerts on that mass. In everyday use the words are interchangeable because we all measure at the same gravity, and nothing on this page changes if you use the wrong word.

The kilogram is the only SI base unit that was defined by a physical object until recently: a platinum-iridium cylinder kept near Paris, which was found to be drifting by tens of micrograms relative to its copies. In 2019 the definition changed to fix the Planck constant instead, so the kilogram is now derived from constants of nature like the metre and the second.

The pound is defined by reference to the kilogram: exactly 0.45359237 kg since the 1959 International Yard and Pound Agreement. So every metric-to-imperial conversion here is exact, and only the display rounds.

The two ounces, and a ten per cent mistake

The avoirdupois ounce is one sixteenth of a pound: 28.349523125 g. The troy ounce, used for gold, silver, platinum and palladium, is 31.1034768 g. They differ by about ten per cent, and a bullion price quoted "per ounce" always means troy.

Both are in the unit list on this page, deliberately not merged. A converter that quietly treats them as the same unit produces a plausible-looking number that is wrong by ten per cent on exactly the kind of figure where that matters.

A third thing called an ounce is not a mass at all. The fluid ounce is a volume, and the US and imperial versions differ from each other as well — 29.57 ml against 28.41 ml. If a recipe asks for 8 fl oz, no mass converter can answer without knowing the density of what you are measuring.

Taiwanese units: catty, tael and qian

The Taiwanese catty (台斤) is exactly 600 g, and it is what traditional markets, fruit stalls and butchers in Taiwan quote. A price of NT$80 per catty is NT$80 for 600 g, which is NT$133 per kilogram — the arithmetic every shopper does in their head and every English-language converter ignores.

The mainland Chinese shijin (市斤) is 500 g. Same character for the unit, twenty per cent apart, and it is a recurring source of disputes in cross-border shopping. This tool will not silently treat one as the other: type 市斤 and it stops and explains, because a twenty per cent error on a price cannot be recovered from a plausible-looking number.

Below the catty are the tael and the qian. One catty is 16 taels, so a tael is 37.5 g; one tael is 10 qian, so a qian is 3.75 g. Chinese medicine prescriptions are written in qian, and Taiwanese gold shops price in qian rather than troy ounces — which means a gold conversion in Taiwan often needs both the troy ounce and the qian in the same calculation. That is why they are both here.

The word "ton", which means three different things

A metric tonne is 1000 kg. A US short ton is 907.18474 kg. A UK long ton is 1016.0469088 kg. The spread between the extremes is twelve per cent, and freight, steel and bulk commodity quotes use all three depending on origin.

Because "ton" on its own cannot be resolved, this page treats it as ambiguous: type it and you get an explanation of the three options rather than a number. Type "tonne" or 公噸 and you get the metric one. This is the same policy as the two ounces and the two kinds of jin — where a name is genuinely shared, an error message is more useful than a guess.

Precision, and nothing uploaded

The display defaults to six significant figures, enough that converting a value out and back returns exactly what you typed. You can fix it to a set number of decimals instead; if that would print a non-zero value as 0, the page switches to scientific notation and explains why rather than showing a misleading zero.

All of it runs in your browser. The tables are static HTML, no request carries what you type, and the site-wide Content-Security-Policy blocks connections to anywhere but our own origin. Disconnect your network and reload — the converter keeps working.