Weight Converter
Convert between kilograms, pounds, ounces, stones, tonnes and other units of mass.
How to use this calculator
- 1Enter the value you want to convert.
- 2Pick the unit you are converting from and the one you want.
- 3The table below shows the same value in every other supported unit.
How the calculation works
result = (value × factor(from) ) / factor(to)- factor
- How many kilograms one unit represents
Every value is converted to kilograms first, then out to the target unit. This means n units need n factors rather than n² conversion pairs.
The pound has been defined as exactly 0.45359237 kilograms since 1959, making every imperial mass conversion exact.
Strictly these are units of mass, not weight. Weight is a force and depends on gravity; everyday usage conflates the two, and this converter follows everyday usage.
Worked example
70 kilograms to pounds
- 1.One kilogram is 1 kilograms, so 70 kg is 70 kilograms.
- 2.One pound is 0.4535924 kilograms, so divide: 70 ÷ 0.4535924 = 154.32358.
- 3.Put another way, 1 kg = 2.2046226 lb.
Result: 154.32358 lb
Mass versus weight
In everyday speech "weight" and "mass" are the same thing, and this converter follows that convention. Physically they are not: mass is how much matter an object contains, while weight is the force gravity exerts on that mass, and it changes depending on where you are. A person who is 70 kg on Earth still has 70 kg of mass on the Moon, but they would weigh about a sixth as much there, because lunar gravity is weaker.
The kilogram is the SI unit of mass. Since 2019 it has been defined by fixing the value of the Planck constant, replacing the old approach of comparing objects against a single physical cylinder of platinum-iridium kept in France — a definition that, embarrassingly for a reference standard, could drift by a few micrograms over time as the metal picked up contamination or wore during cleaning.
How the everyday units relate
Once you leave the metric system, mass units splinter by both region and use case. None of the groupings below is arbitrary — each grew out of a specific trade or industry that needed its own convenient scale.
- Metric — grams, kilograms and tonnes, scaled by powers of ten. Used almost everywhere for almost everything, from a recipe's ingredient list to a cargo ship's displacement.
- Imperial / US customary — ounces, pounds and stones, built on the old avoirdupois system. The US uses pounds and ounces for retail and body weight; the UK still commonly reports body weight in stones and pounds.
- Shipping tons — the US short ton (2,000 lb) and UK long ton (2,240 lb) diverged centuries ago and were never reconciled, which is why freight and bulk commodity contracts always need to state which one applies.
- Precious-metal and gemstone units — troy ounces for gold and silver, and carats for gemstones, both survive from historical trading conventions because switching an entire commodities market to new units after centuries of contracts, benchmarks and pricing habits would be its own kind of costly.
Why the stone survives in the UK
The stone (14 lb) was a standard unit of weight across medieval England, used for weighing wool, produce and eventually people. It was formally dropped from UK trade in favour of metric units, but it never left everyday conversation the way other imperial units did — most British people still describe body weight in stones and pounds rather than kilograms, even though a bathroom scale sold in the UK today is calibrated in kilograms by default.
This is a good example of how unit systems change unevenly. A country can be legally and scientifically metric while a specific, culturally sticky measurement — body weight, beer in pints, road distances in miles — keeps its old unit indefinitely.
Common conversion pitfalls
A handful of mix-ups account for most of the confusion when converting mass.
- Troy ounces are not avoirdupois ounces — a troy ounce (used for gold, silver and other precious metals) is about 10% heavier than the ordinary ounce used for food and body weight — a "one-ounce" gold coin contains noticeably more metal than a kitchen ounce would suggest.
- Short tons, long tons and tonnes are three different things — a US short ton is 2,000 lb, a UK long ton is 2,240 lb, and a metric tonne is 1,000 kg (about 2,204.6 lb) — close enough to cause errors if assumed interchangeable, far enough apart to matter in bulk trade.
- "Weight" in cooking usually means mass — a recipe calling for "8 oz of flour" means mass, not the fluid ounce used for liquids — the two units share a name but measure different things entirely.
What this assumes, and where it stops
Assumptions
- Conversion factors follow the internationally agreed definitions of each unit.
Limitations
- Results are computed in double-precision floating point, which is exact to about 15 significant figures.
- Very large or very small values are displayed in scientific notation to avoid misleading precision.
Common questions
What is the difference between a troy ounce and a regular ounce?
A troy ounce (31.1035 g) is about 10% heavier than an avoirdupois ounce (28.3495 g). Troy weight is used exclusively for precious metals, so a "one ounce" gold coin contains more metal than the name suggests to anyone thinking in kitchen ounces.
Why are US and UK tons different?
The US short ton is 2,000 pounds (907.18 kg) and the UK long ton is 2,240 pounds (1,016.05 kg). The metric tonne sits between them at 1,000 kg. Shipping and commodity contracts must specify which one they mean.
Sources
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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