Why this matters
Weight and mass conversions are among the most frequently needed unit conversions in daily life, yet they are also among the most error-prone because of the confusing overlap between imperial and US customary systems. The pound is defined differently in avoirdupois, troy, and apothecary systems. The ton has three distinct definitions: the US short ton (2,000 pounds), the UK long ton (2,240 pounds), and the metric tonne (1,000 kilograms). Using the wrong ton in a shipping calculation or engineering specification can lead to significant errors in cost estimates and structural load calculations.
The distinction between mass and weight is technically important but practically negligible for everyday conversions on Earth's surface. Mass is the amount of matter in an object, measured in kilograms, and is constant regardless of location. Weight is the gravitational force on that mass, measured in newtons, and varies with altitude and planetary body. For cooking, shipping, fitness, and commerce, the terms are used interchangeably because the conversion factor between kilograms and pounds on Earth is fixed. This tool uses the internationally agreed-upon definition: 1 pound equals exactly 0.45359237 kilograms, and 1 ounce equals exactly 1/16 of a pound.
The converter covers nine units: milligrams, grams, kilograms, metric tonnes, ounces, pounds, stones, US short tons, and UK long tons. When you enter a value in any one unit, all eight equivalents are displayed simultaneously with individual copy buttons, so you can grab exactly the conversion you need without recalculating.
Reference table
| Unit | Symbol | Conversion to kilograms |
|---|---|---|
| Milligram | mg | 0.000001 kg |
| Gram | g | 0.001 kg |
| Kilogram | kg | 1 kg (base unit) |
| Metric tonne | t | 1,000 kg |
| Ounce | oz | 0.028349523125 kg (1/16 lb) |
| Pound | lb | 0.45359237 kg (exact) |
| Stone | st | 6.35029318 kg (14 lb) |
| US short ton | ton (US) | 907.18474 kg (2,000 lb) |
| UK long ton | ton (UK) | 1,016.0469088 kg (2,240 lb) |
How to use it
Enter a numeric value in the input field — this is the weight or mass you want to convert.
Select the source unit from the dropdown: milligrams, grams, kilograms, tonnes, ounces, pounds, stones, US tons, or UK tons.
All eight equivalent values appear immediately in the results panel, each with its own copy button.
Click the copy button next to any result to grab that specific conversion to your clipboard.
Testing your result
Verify the conversions against known reference values. One kilogram should convert to exactly 2.2046226218487758 pounds. One pound should equal exactly 16 ounces and 0.45359237 kilograms. One stone should equal 14 pounds. One US short ton should equal 2,000 pounds, and one UK long ton should equal 2,240 pounds. Test the reverse direction by entering the converted value with its new unit and confirming you get back the original number. If any conversion is off by more than a rounding error in the last displayed digit, the result is incorrect.
Common mistakes
Confusing US short tons and UK long tons, which differ by 240 pounds per ton — a significant error in bulk shipping.
Using the troy ounce (31.103 grams) when the tool uses the avoirdupois ounce (28.35 grams) for all conversions.
Assuming a stone is a metric unit, when it is an imperial unit used almost exclusively in the UK and Ireland for body weight.
Forgetting that the metric tonne (1,000 kg) is not the same as either the US or UK ton.
Edge cases and options
The conversion from pounds to kilograms uses the exact international avoirdupois pound definition of 0.45359237 kilograms, which has been the standard since 1959. This means the conversions are mathematically exact at the defined precision, not approximations. The stone is a uniquely British unit where 1 stone equals 14 pounds or approximately 6.35 kilograms. Body weight in the UK is conventionally expressed as a combination of stones and pounds, such as '11 stone 6' meaning 11 stones and 6 pounds. The tool converts the full value in a single unit, so you would enter the total in pounds (160 pounds for 11 stone 6) if you need to include the partial stone.
Real-world use cases
Converting ingredient weights between metric and imperial measurements for recipes sourced from different countries.
Calculating shipping weights in kilograms when the supplier provides prices per US short ton or UK long ton.
Translating body weight between pounds and kilograms for medical records or fitness tracking apps.
Converting material quantities for construction or manufacturing when specifications use mixed unit systems.
Frequently asked questions
Q: What's the difference between mass and weight?
A: Mass = amount of matter (kg, constant everywhere). Weight = force of gravity on mass (Newtons, varies with gravity). For everyday use, mass = weight on Earth's surface.
Q: Why are there US and UK tons?
A: US short ton = 2000 lb. UK long ton = 2240 lb. Metric tonne (t) = 1000 kg = 2204.62 lb. Always clarify which ton you mean.
Q: What's a stone?
A: 1 stone = 14 lb = 6.35 kg. UK body weight is commonly expressed in stones and pounds ('I weigh 11 stone 6').
Q: Are conversions exact?
A: Yes — 1 lb = 0.45359237 kg exactly (international avoirdupois pound), 1 oz = 1/16 lb exactly.
Q: Does the tool handle troy ounces?
A: No — the tool uses avoirdupois ounces (28.35 g) for all conversions. Troy ounces (31.10 g) are used for precious metals and are a different unit.
Q: Is my data uploaded?
A: No — all conversions happen in your browser.
Start using it now
Try the Weight Converter tool. See also Distance Converter and Volume Converter.
