Weight Converter
Convert between kilograms, pounds, ounces, grams, stones, and other weight units instantly.
150 Pounds (lb) =
68.038856
Kilograms (kg)
What is Weight and Mass Conversion?
Weight conversion is arithmetic with one multiplication in it, so the interesting part is not the sum — it is knowing which unit you actually have. An ounce of gold and an ounce of flour are different masses. A ton in Britain is heavier than a ton in America, and both differ from a metric tonne.
Every conversion here goes through kilograms. The value is multiplied into kilograms and then divided out into the target unit, which means one factor per unit rather than a table of every pairing, and no accumulated rounding from chaining conversions together.
The factors are the exact definitions rather than the rounded ones you often see. The international pound has been defined as exactly 0.45359237 kilograms since 1959, and every other imperial mass unit follows from it.
The formula — how to calculate Weight and Mass Conversion
- factor(kg)
- = 1, by definition — the kilogram is the base unit
- factor(lb)
- = 0.45359237 exactly
- factor(oz)
- = 0.028349523125 — one sixteenth of a pound, exactly
Routing through a single base unit is why a value converted out and back returns the number you started with. Chained conversions — pounds to ounces to grams — accumulate rounding at every hop.
Step-by-step example
- 01Convert 150 pounds to kilograms.
- 02150 × 0.45359237 = 68.0388555 kg.
- 03Now the same weight in stones, the unit British scales still use: 68.0388555 ÷ 6.35029318 = 10.714 st, which reads as 10 stone 10 pounds.
- 04And the trap: 150 ounces of silver is 4,252.4 grams if they are ordinary ounces, but 4,665.5 grams if they are troy ounces. That is a 9.7% difference — on silver at $30 an ounce, about $124 on a 150-ounce lot.
Mass and weight are not the same thing
Strictly, mass is how much matter something contains and weight is the force gravity exerts on it. Mass is measured in kilograms; weight is a force and is properly measured in newtons.
On Earth this distinction almost never matters, because gravity is near enough constant and a kilogram of mass always weighs the same. It matters in two places: physics problems, and anywhere off the planet. The same 70 kg astronaut weighs about 686 newtons on Earth and about 114 on the Moon — the mass has not changed, only the force.
Everyday usage merged the two centuries ago and there is no point fighting it. A bathroom scale reports mass and calls it weight, and so does this converter.
One consequence is worth knowing: because scales measure force and infer mass, a scale calibrated on Earth would read wrong on the Moon, while a balance comparing two masses would still be correct anywhere.
The units that catch people out
- Troy ounce vs ounce —
- precious metals are priced in troy ounces, which are 31.1035 g against the ordinary avoirdupois ounce's 28.3495 g — about 10% heavier. A "gold price per ounce" is always troy. Confusing them misprices a transaction by roughly a tenth.
- Short ton vs long ton —
- the US short ton is 2,000 lb (907.18 kg); the UK long ton is 2,240 lb (1,016.05 kg). The long ton is 12% heavier, which matters in shipping and commodity contracts.
- Metric tonne —
- 1,000 kg, or 2,204.6 lb. It sits between the two imperial tons and is what most of the world means by "ton". Spelling it "tonne" is the convention that distinguishes it.
- Stone —
- 14 lb, used for body weight in Britain and Ireland and essentially nowhere else. British medical records use kilograms; British conversation uses stones and pounds.
- Fluid ounce —
- not a weight at all — it is a volume. An ounce of water happens to be close to a fluid ounce, which is exactly why the confusion persists and why recipes converted carelessly go wrong.
Recipes: ounces by weight or by volume?
American recipes use cups, which measure volume. A cup of flour is about 4.25 oz by weight and a cup of sugar about 7 oz — the same volume, very different masses. Converting "1 cup" to grams needs the ingredient's density, not a weight conversion. This is the single most common reason a converted baking recipe fails.
Precision, and where it goes wrong
Two things quietly destroy accuracy in unit conversion, and both are avoidable.
The first is rounded factors. Using 0.4536 for a pound instead of 0.45359237 introduces an error of about 18 parts per million — invisible on a bag of sugar, roughly 18 grams on a tonne of freight.
The second is chaining. Converting pounds to ounces, ounces to grams, grams to kilograms rounds at each step, and the errors compound. Going through one base unit in a single multiply and divide avoids this entirely.
There is also a display question that is not a precision question. A very small result — one milligram expressed in short tons is 0.0000000011 — vanishes if the display is capped at a fixed number of decimal places. Showing a flat zero for a non-zero quantity is worse than showing an awkward number, so small and large magnitudes switch to significant figures here.
Reference points worth memorising
Common equivalences
| From | To | Exact | Rough |
|---|---|---|---|
| 1 kg | lb | 2.2046226 | a bit over 2¼ lb |
| 1 lb | g | 453.59237 | just under half a kilo |
| 1 oz | g | 28.349523 | about 28 g |
| 1 st | kg | 6.3502932 | about 6⅓ kg |
| 1 tonne | lb | 2204.6226 | about 2,200 lb |
| 1 short ton | kg | 907.18474 | about 0.9 tonne |
For mental arithmetic: kilograms to pounds, double it and add 10%. 70 kg becomes 140 + 14 = 154 lb, against a true 154.3. Accurate to about a fifth of a percent.
Common mistakes to avoid
- Treating a troy ounce as an ordinary ounce when pricing precious metals — a 10% error.
- Assuming a ton is a ton. Short, long and metric differ by up to 12%.
- Converting fluid ounces as if they were a weight.
- Converting recipe volumes without the ingredient density.
- Using rounded factors, then chaining several conversions so the rounding compounds.
- Reading a very small converted value as zero because the display truncated it.
Frequently asked questions
Sources & references
Written and fact-checked by the CalcProLabs Editorial Team. Read our calculation methodology and editorial policy.
Last updated