Mass Conversion Calculator
Convert mass across SI units, avoirdupois ounces and pounds, stones, and three distinct tonne units.
Content updated August 6, 2026
Ounce and pound mean avoirdupois units here. Troy, apothecaries, and fluid ounces are different units and are intentionally excluded.
Convert a nonnegative mass between metric units, avoirdupois ounces and pounds, stones, U.S. short tons, imperial long tons, and metric tonnes. The full result table makes similarly named ton units easy to compare.
Conversions pass through kilograms using published exact relationships. Ounce and pound mean avoirdupois mass units; fluid ounces and troy or apothecaries units are intentionally outside this calculator.
The Mass Conversion formula
Converted mass = entered mass × kilograms per source unit ÷ kilograms per destination unitSI prefixes and the supported avoirdupois, stone, short-ton, long-ton, and metric-tonne relationships use their stated definitions.
Worked example
One avoirdupois pound multiplied by exactly 0.45359237 kilogram per pound equals 0.45359237 kilogram, displayed as 0.45359237 kg.
Assumptions, rounding, and limitations
Assumptions
- The input is a nonnegative scalar mass.
- Ounce and pound mean avoirdupois units.
- Stone equals 14 avoirdupois pounds.
- Short ton, long ton, and metric tonne remain separate unit choices.
Rounding: The engine does not intentionally round intermediate results. Displays use up to 12 significant digits and scientific notation for values at least 1e12 or below 1e-7.
Limitations
- Does not include troy, apothecaries, carat, grain, atomic-mass, fluid-ounce, force, or local commodity units.
- Does not convert volume to mass, account for density, or convert mass to local gravitational force; floating-point representation can add tiny binary approximations.
Sources
- Handbook 133 (2026), Appendix E: General Tables of Units of Measurement — National Institute of Standards and Technology
- NIST Guide to the SI, Appendix B: Conversion Factors — National Institute of Standards and Technology
Pound and ounce need a stated system
NIST gives one avoirdupois pound as exactly 0.45359237 kilogram and one avoirdupois ounce as exactly 0.028349523125 kilogram. Sixteen avoirdupois ounces make one pound.
A troy ounce measures a different mass, while a fluid ounce measures volume. Neither can be substituted for the avoirdupois ounce in this table.
Short ton, long ton, and metric tonne are different
A U.S. short ton is 2,000 avoirdupois pounds, an imperial long ton is 2,240 pounds, and a metric tonne is 1,000 kilograms.
The calculator labels each unit explicitly and never treats the unqualified word ton as enough information for a conversion.
Frequently asked questions
▶How many kilograms are in one pound?
One avoirdupois pound is exactly 0.45359237 kilogram.
▶How many ounces are in one pound?
There are exactly 16 avoirdupois ounces in one avoirdupois pound.
▶How many pounds are in one stone?
One stone is 14 avoirdupois pounds.
▶Are short tons and long tons the same?
No. A U.S. short ton is 2,000 pounds and an imperial long ton is 2,240 pounds.
▶Does this convert fluid ounces?
No. Fluid ounce is a volume unit. Converting volume to mass also requires a substance density and conditions.
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Open calculator →Disclaimer: Mass Conversion Calculator results are estimates for general information and education only, and are not financial, tax, legal or medical advice. Verify important decisions with a qualified professional.