Why use this calculator
Mass is where rounded conversion factors do the most damage, because the numbers are usually multiplied by something. A recipe scaled for a commercial batch, a shipping manifest priced per kilogram, or a medication dose calculated per kilogram of body weight all amplify a small factor error into a real one.
Many converters still carry the old rounded values of 453.6 grams to the pound or 28.35 to the ounce; the exact defined figures are 453.59237 and 28.349523125, and this tool uses those. It also shows the three notations people actually speak in: kilograms for medicine and sport, pounds for North America, and stones and pounds for British and Irish body weight, which no decimal figure conveys correctly.
How to use it
- Enter the number exactly as your source gives it, whether that is a bathroom scale reading, a recipe quantity, a product label or a freight document.
- Choose the unit that number is already in from the dropdown, checking the abbreviation carefully because lb and oz are easy to transpose on a packaging label and a metric tonne is not the same as a US ton.
- Every other unit is calculated at once and the ones you did not choose are highlighted.
- For body weight in Britain or Ireland read the stones-and-pounds row rather than the decimal stones figure, since 10.71 stones is 10 st 10 lb, not 10 st 71 lb.
What each result means
- Milligrams
- The unit used for medication doses, supplement labels and small laboratory quantities.
- Grams
- The everyday metric unit for food, postage and any recipe written by mass rather than volume.
- Kilograms
- The SI base unit of mass, used for body weight, freight and dosing almost everywhere.
- Tonnes
- One thousand kilograms, the metric unit for bulk freight, aggregates and vehicle mass.
- Ounces
- One sixteenth of a pound, used for food packaging and postage in the United States.
- Pounds
- The imperial and US customary unit of mass for body weight, produce and freight.
- Stones
- Fourteen pounds, used almost exclusively for body weight in Britain and Ireland.
- Stones and pounds
- The same body weight spoken the way it is actually said, with whole stones and the remaining pounds.
- US short tons
- Two thousand pounds, the North American ton, which is lighter than both the metric tonne and the UK long ton.
The formula
Every conversion routes through the gram, so the result is value × (grams per source unit) ÷ (grams per target unit).
The defining factors are 1 pound = 453.59237 g exactly
1 ounce = 1/16 pound = 28.349523125 g
1 stone = 14 pounds = 6,350.29318 g
1 US short ton = 2,000 pounds = 907,184.74 g
Stones and pounds are recovered by taking the whole number of stones out of the total pounds and leaving the remainder, since one stone is exactly fourteen pounds.
How this number is used in practice
Clinical dosing is the highest-stakes use of this conversion. Paediatric and chemotherapy doses are prescribed per kilogram of body weight, but patients in North America report their weight in pounds, so the conversion sits between a stated weight and a delivered dose. Guidance in several health systems is to record weight in kilograms only, precisely to remove the conversion step, because the classic error is entering a pound figure into a field expecting kilograms and administering more than twice the intended dose.
Freight and agriculture turn the same conversion into money. Bulk commodities are contracted per tonne, per short ton or per long ton depending on the market, and the three differ by up to twelve per cent, so a quote that does not name its ton is not a quote.
Air freight prices by chargeable weight, comparing actual mass in kilograms against volumetric weight, which means a shipment quoted in pounds has to be converted before the comparison can even be made.
Baking is where precision genuinely rewards the exact factor. Professional recipes are written by mass because flour compacts and a cup is not a reliable measure, and bakers' percentages are all ratios to the flour mass. Converting a recipe from ounces to grams with a rounded factor shifts hydration by a fraction of a per cent, which is invisible in a single loaf and obvious across a commercial batch where the dough has to behave identically every day.
Worked examples
A body weight of 150 pounds
68.0389 kilograms, 10.7143 stones, or 10 st 10.0 lb.
70 kilograms converted for a North American form
154.3236 pounds and 2,469.1773 ounces.
A 1 ounce recipe quantity in metric
28.35 grams, or 0.0283 kilograms rounded to four places.
A 2 tonne vehicle mass in North American units
4,409.2452 pounds and 2.204623 short tons.
When people use it
- Converting body weight between kilograms, pounds and stones for medical or fitness records
- Rewriting a recipe from imperial ounces into grams for a digital kitchen scale
- Checking freight and postage limits quoted in a unit different from your own
- Comparing bulk commodity prices quoted per tonne against prices per short ton
- Calculating dose-per-kilogram medication from a weight recorded in pounds
Tips
- One pound is exactly 0.45359237 kilograms. Using 453.6 grams introduces an error that shows up once you scale a recipe or a shipment.
- For mental arithmetic, a kilogram is about 2.2 pounds and a pound is about 0.45 kilograms; 10 kg is close to 22 lb.
- Decimal stones are not stones and pounds. A reading of 10.71 stones is 10 st 10 lb, because the remainder is out of fourteen, not ten.
- Always name the ton. A metric tonne is 1,000 kg, a US short ton is 907 kg and a UK long ton is 1,016 kg.
- Mass and weight are different quantities. Scales report mass; weight is a force and changes with gravity, which is why only mass is converted here.
- Fluid ounces measure volume, not mass, and never convert to grams without knowing the density of what is in the container.
What it assumes, and where it stops being accurate
- All imperial units are avoirdupois, the ordinary commercial system, not troy units used for precious metals.
- The figures convert mass, not force; no gravitational field strength is applied at any point.
- Negative masses are rejected and treated as zero, since a mass below zero has no physical meaning.
- Values are capped at one billion of the source unit so a shared link cannot request an unbounded conversion.
- Displayed figures are rounded for readability, so reversing a rounded output will not always return the exact original value.
- Ton refers to the US short ton here; long tons and metric tonnes are listed separately in the reference table.
Terms used on this page
- Avoirdupois
- The ordinary commercial weight system where a pound is sixteen ounces, used for food, freight and body weight.
- Troy ounce
- A separate unit of about 31.10 grams used only for precious metals such as gold and silver.
- Stone
- Fourteen pounds, or 6.35029318 kilograms, used for body weight in Britain and Ireland.
- Tonne
- The metric ton of exactly 1,000 kilograms, distinct from both the short and long ton.
- Mass
- The amount of matter in an object, unchanged by location, as distinct from weight, which is a force.
Questions people ask
- How many kilograms are in a pound?
- Exactly 0.45359237 kilograms. This is a definition rather than a measurement, agreed internationally so that imperial and metric masses relate without uncertainty. The common shortcut of 0.4536 is accurate enough for a single bathroom-scale reading but drifts noticeably once the figure is multiplied across a batch, a shipment or a dose calculation.
- What is a stone and where is it still used?
- A stone is fourteen pounds, or about 6.35 kilograms, and it survives almost exclusively for body weight in Britain and Ireland. It is not used in the United States, where body weight is given in pounds alone, nor in commerce anywhere. Because the remainder is out of fourteen rather than ten, decimal stones are easy to misread.
- Is a ton the same everywhere?
- No, and the differences are large enough to matter in a contract. A metric tonne is 1,000 kilograms, a US short ton is 2,000 pounds or about 907 kilograms, and a UK long ton is 2,240 pounds or about 1,016 kilograms. The spread from lightest to heaviest is around twelve per cent, so any quoted ton should name which one it means.
- What is the difference between mass and weight?
- Mass is how much matter an object contains and does not change with location. Weight is the force gravity exerts on that mass and would differ on the Moon. Scales are calibrated to report mass even though people call the reading a weight. Every figure in this calculator is a mass, which is why no gravitational constant appears in the formulas.
- Can I convert fluid ounces to grams here?
- No, because a fluid ounce measures volume and a gram measures mass, and the bridge between them is density. Water is close to one gram per millilitre, so a fluid ounce of water is about 29.6 grams, but flour is roughly half that and honey almost half again as much. Use a density figure or a recipe written by mass instead.
- Why do troy ounces differ from ordinary ounces?
- The troy system is a separate scale used only for precious metals. A troy ounce is about 31.10 grams against the avoirdupois ounce of 28.35 grams, roughly ten per cent heavier, and a troy pound contains twelve troy ounces rather than sixteen. This calculator uses avoirdupois units throughout, which are the ones used for food, freight and body weight.
Use this with
Weight Converter answers one part of the question. These pick up where it stops: Length Converter, Cups to Grams Converter, Temperature Converter and BMI Calculator.
Where these figures come from
This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.
- NIST Special Publication 811: Guide for the Use of the International System of Units. Definitive table of exact conversion factors between SI and customary units
- NIST: SI base unit: kilogram. The kilogram defined from the Planck constant
- NIST Office of Weights and Measures. US legal metrology: unit definitions, trade measurement and rounding practice
Weight Converter produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.