Weight and Mass Converter
Weight (or more precisely, mass) is measured in a variety of units depending on context and location. The metric system uses milligrams, grams, kilograms, and metric tonnes, with the kilogram as the SI base unit of mass, redefined in 2019 in terms of the Planck constant for greater precision. The imperial and US customary systems use ounces, pounds, stones, and tons, with some differences between the US ton (short ton, 2,000 pounds) and the imperial ton (long ton, 2,240 pounds). These systems persist side by side in everyday life: body weight is commonly reported in kilograms in most countries but in pounds and stones in the United States and United Kingdom, while food ingredients may be measured in grams for precision or in ounces for convenience. All conversion factors used here are the exact legally defined values from NIST Special Publication 330: one pound equals exactly 0.45359237 kilograms, one ounce equals exactly 0.028349523125 kilograms, and one stone equals exactly 6.35029318 kilograms. This calculator converts simultaneously among nine units: milligrams, grams, kilograms, metric tonnes, ounces, pounds, stone, US short tons, and imperial long tons. Enter a value in any field and all others update immediately, making it straightforward to convert body weight, ingredient measurements, shipping weights, or any other mass quantity between any combination of units.
NIST conversion factors
All conversions below use standard NIST-defined factors, with all units expressed relative to the kilogram.
| Unit | Symbol | Kilograms |
|---|---|---|
| Milligram | mg | 0.000001 |
| Gram | g | 0.001 |
| Kilogram | kg | 1 |
| Metric tonne | t | 1,000 |
| Ounce | oz | 0.028349523 |
| Pound | lb | 0.45359237 |
| Stone | st | 6.35029318 |
| US ton | short ton | 907.18474 |
| Imperial ton | long ton | 1,016.0469 |
Weight and mass converter: frequently asked questions
What is the difference between mass and weight?
Mass is the amount of matter in an object and does not change with location. Weight is the force of gravity on that mass and changes with the gravitational field. In everyday use, weight and mass are used interchangeably. This calculator converts between mass units.
What is the difference between a US ton and an imperial ton?
A US ton (short ton) is 2,000 pounds or 907.18 kilograms. An imperial ton (long ton, used in the UK) is 2,240 pounds or 1,016.05 kilograms. A metric tonne is 1,000 kilograms. Always check which ton is meant in your context.
What is a stone?
A stone is a UK unit equal to 14 pounds or 6.35 kilograms. It is commonly used in the United Kingdom for body weight measurements.
Should I use imperial or metric units?
Metric units (grams, kilograms, tonnes) are standard for science, medicine, and commerce worldwide. Imperial units (pounds, ounces, stones, tons) are used primarily in the United States and some Commonwealth countries for everyday purposes. Use the unit appropriate to your audience and context.
Official sources
- NIST SP 330: The International System of Units.
- NIST Handbook 44: Specifications, Tolerances, and Other Technical Requirements.
Reviewed by the CalculatorHub team, edited by James Graham, 13 June 2026. See our methodology.