Weight and Mass Converter
Weight and mass measurements are fundamental to commerce, science, medicine, and everyday life. The metric system, standardized internationally as the SI system, uses the gram as its base unit for mass, with larger and smaller units built on powers of 10: milligrams for pharmaceuticals and small components, kilograms for everyday objects, and metric tonnes for bulk goods and industrial quantities. The imperial and US customary systems, still used primarily in the United States, include ounces and pounds for everyday weights, and stones for body weight measurement in the United Kingdom. Modern conversion factors are defined with precision: 1 pound equals exactly 453.59237 grams, 1 ounce equals exactly 28.349523125 grams, and 1 stone equals 14 pounds or approximately 6.35 kilograms. Beyond these, larger units like the US short ton (907.185 kg) and UK long ton (1,016.047 kg) handle heavy industrial measurements, while the metric tonne (1,000 kg) is the global standard for shipping and bulk commerce. Understanding and converting between these units is essential when comparing product weights, following recipes from different regions, dosing medications, or trading internationally. This calculator displays ten common mass units simultaneously: micrograms through metric tonnes on the metric side, and ounces, pounds, stones, and tons on the imperial side. Enter a value in any field and all others update instantly with NIST-defined precision.
NIST conversion factors
All conversions below use standard NIST-defined factors, with all units expressed relative to the gram.
| Unit | Symbol | Grams |
|---|---|---|
| Microgram | mcg | 0.000001 |
| Milligram | mg | 0.001 |
| Gram | g | 1 |
| Kilogram | kg | 1,000 |
| Metric tonne | t | 1,000,000 |
| Ounce | oz | 28.349523125 |
| Pound | lb | 453.59237 |
| Stone | st | 6,350.29318 |
| US short ton | US ton | 907,184.74 |
| UK long ton | UK ton | 1,016,046.9088 |
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 remains constant. Weight is the force exerted by gravity on that mass and varies by location. For practical purposes, this calculator treats them equivalently when measuring in standard Earth gravity. Scientists distinguish the two, but in commerce and everyday use, the terms are often used interchangeably.
Why are there so many different mass units?
Different industries and regions use different units for historical and practical reasons. The metric system (grams, kilograms) is used globally in science and commerce. The imperial system (ounces, pounds, stone) is used in the United States and some Commonwealth countries. Metric tonnes (1,000 kg) are standard for bulk goods and shipping.
What is a stone?
A stone is an imperial unit equal to 14 pounds or about 6.35 kilograms. It is still commonly used in the United Kingdom for measuring body weight. Most other countries use kilograms for this purpose.
How precise are these conversions?
These are NIST-defined standard conversions. For example, 1 pound equals exactly 453.59237 grams and 1 ounce equals exactly 28.349523125 grams. All conversions shown are mathematically precise to two decimal places.
What is a metric tonne?
A metric tonne (also called a ton in SI usage) equals 1,000 kilograms or about 2,204.62 pounds. It is the standard unit for measuring large quantities of goods, from shipments to industrial production. Different from the US short ton (907.185 kg) and the UK long ton (1,016.047 kg).
Official sources
- NIST Handbook 44: Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices.
- NIST Special Publication 330: The International System of Units.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.