Weight Converter

How to use this calculator#

1. 1
   Check which ounce or ton you are dealing withThis converter uses the avoirdupois ounce (28.3495 g) and the US short ton (907.185 kg). Precious metals are priced by the troy ounce (31.1035 g) and shipping often quotes metric tonnes (1,000 kg) — neither is what you will get here.
2. 2
   Enter a single number, not stones and pounds togetherCollapse mixed units first. 11 st 6 lb is (11 × 14) + 6 = 160 lb. Typing 11.6 st means 11.6 × 14 = 162.4 lb, which is 2.4 lb heavier than you meant.
3. 3
   Read the result, then convert the fraction backWhen the answer is in stones, the digits after the decimal point are fourteenths, not tenths. Multiply the decimal part by 14 to get pounds: 11.81 st is 11 st and 0.81 × 14 = 11.3 lb.
4. 4
   Round in the safe direction for any limitAirline baggage, postage bands and load ratings are hard cut-offs. Round converted weights up when you are checking against a maximum and down when you are checking against a minimum.

The formula#

Pounds, kilograms and the exact factor#

Every conversion here passes through the kilogram. Since 1959 the international avoirdupois pound has been defined as exactly 0.45359237 kilograms, which makes 1 kg equal to 2.2046226 lb. The ounce is one sixteenth of a pound, exactly 28.349523125 grams, and the stone still used for body weight in Britain and Ireland is 14 pounds, or 6.35029318 kg.

The figures worth memorising: 1 kg = 2.20462 lb, 1 lb = 0.453592 kg, 1 oz = 28.3495 g, 1 g = 0.035274 oz, 1 stone = 6.35029 kg, and 1 metric tonne = 1,000 kg = 2,204.62 lb. A US short ton is 2,000 lb, or 907.185 kg, which makes it noticeably lighter than the metric tonne despite the shared name.

Everyday conversions worked out#

Body weight drives most searches. 70 kg is 154.32 lb, 80 kg is 176.37 lb and 100 kg is 220.46 lb. Going the other way, 150 lb is 68.04 kg and 200 lb is 90.72 kg. In stones, 70 kg is 11.02 st — which reads as 11 st 0.3 lb, because the fractional part of a stone has to be multiplied by 14 to turn it back into pounds.

Kitchen and postage conversions matter too. 500 g is 17.64 oz or 1.10 lb, and a US pound of butter is 453.6 g. For shipping, a parcel quoted at 2 kg is 4.41 lb, so a 5 lb carrier limit leaves room for just over 2.26 kg. When you are checking a weight against any kind of limit, always round the converted figure upward.

Mass, weight and the ounce trap#

Strictly speaking kilograms and pounds measure mass while weight is a force, but in daily use the words are interchangeable because gravity barely varies across the Earth's surface. The kilogram was defined by a platinum-iridium cylinder held near Paris until 2019, when it was redefined in terms of the Planck constant — a change that left the everyday value unchanged.

The classic pitfall is mixing up two different ounces. The avoirdupois ounce used for food, post and body weight is 28.3495 g, but the troy ounce used for gold, silver and other precious metals is 31.1035 g, roughly 10% heavier. Bullion prices are always quoted per troy ounce, so using the wrong one distorts a valuation badly. This converter uses avoirdupois throughout.

Worked examples#

Reference tables#

Common mistakes#

• Using the troy ounce for food, or the avoirdupois ounce for bullionA troy ounce is 31.1035 g against 28.3495 g for the avoirdupois ounce — about 10% heavier. Value a 100 oz silver bar with the wrong ounce and you misprice it by roughly 9.7%, which on a five-figure holding is a four-figure error.
• Treating a US ton and a metric tonne as the same thingA US short ton is 2,000 lb (907.185 kg); a metric tonne is 1,000 kg (2,204.62 lb). The tonne is 10.2% heavier. On a 20-unit freight quote that is two extra tonnes of unaccounted mass, which can push a load past an axle-weight limit.
• Reading the decimal part of a stone as pounds11.81 st is not 11 st 8.1 lb. Stones divide into fourteenths, so the decimal must be multiplied by 14: 0.81 × 14 = 11.3 lb, making it 11 st 11.3 lb. The naive reading is more than 3 lb light.
• Assuming equal volumes of two ingredients weigh the sameThis converter changes units of mass only. A cup of all-purpose flour is 125 g and a cup of honey is 340 g — nearly triple. If your starting number came from a measuring cup rather than a scale, you need a density-based cooking converter, not a weight converter.

Frequently asked questions#

Key terms#

Mass versus weight

Mass is the amount of matter in an object and does not change with location; weight is the force gravity exerts on that mass. Kilograms and pounds are used for both in everyday speech because gravity varies by less than 0.5% across the Earth's surface.

Avoirdupois

The standard commercial weight system of the English-speaking world: 16 ounces to the pound, 14 pounds to the stone, 2,000 pounds to the US short ton. Unless a source says otherwise, an ounce or pound means the avoirdupois one.

Troy ounce

Exactly 31.1034768 g, used only for precious metals and gemstones. There are 12 troy ounces in a troy pound, not 16, which is why a troy pound is lighter than an avoirdupois pound despite the heavier ounce.

Stone

Exactly 14 avoirdupois pounds, or 6.35029318 kg. Still standard for body weight in the UK and Ireland; it has no legal role in trade and is not used in the US.

Metric tonne

Exactly 1,000 kg, sometimes written 'tonne' or 'metric ton' to distinguish it from the 2,000 lb US short ton and the 2,240 lb UK long ton.

Planck-constant definition of the kilogram

Since 2019 the kilogram has been fixed by defining the Planck constant as exactly 6.62607015 × 10⁻³⁴ joule seconds, replacing the platinum-iridium prototype kept near Paris. The everyday size of a kilogram did not change.

Sources#