Weight Converter
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How Weight Conversion Works
Weight conversion is the process of expressing a mass measurement in one unit as an equivalent value in another unit. The kilogram is the SI base unit of mass, and since May 2019 it has been defined by fixing the numerical value of the Planck constant to 6.62607015 x 10-34 J-s, according to the International Bureau of Weights and Measures (BIPM). This replaced the previous definition based on a physical platinum-iridium cylinder stored in a vault near Paris since 1889.
The metric system uses milligrams, grams, kilograms, and metric tons, each differing by a factor of 1,000. The imperial system uses ounces, pounds, stones, and tons. According to NIST, the international pound is defined as exactly 0.45359237 kilograms, established by the 1959 International Yard and Pound Agreement between six English-speaking countries. There are 16 ounces in a pound and 2,000 pounds in a US short ton. Our BMI Calculator uses these same conversion factors for body weight.
Understanding weight conversions is essential for international shipping (where duties depend on weight), cooking (where precision affects results), healthcare (where drug dosages are weight-dependent), and fitness tracking. The stone (14 pounds) remains uniquely popular in the UK and Ireland for stating personal body weight -- a convention that puzzles visitors from metric countries and the US alike.
The Weight Conversion Formula
The formula for converting between weight units is:
Target Weight = Source Weight x (Source Unit in Grams / Target Unit in Grams)
Each unit's exact value in grams:
- Milligram (mg) = 0.001 grams
- Gram (g) = 1 gram (base reference)
- Kilogram (kg) = 1,000 grams
- Metric Ton (t) = 1,000,000 grams
- Ounce (oz) = 28.349523125 grams
- Pound (lb) = 453.59237 grams (exactly)
- Stone (st) = 6,350.29318 grams
- US Ton (short ton) = 907,184.74 grams
Worked example: Convert 175 pounds to kilograms. Since 1 lb = 453.59237 g and 1 kg = 1,000 g: 175 x (453.59237 / 1,000) = 79.38 kg. Quick shortcut: divide pounds by 2.2 for an approximate kg value.
Key Terms You Should Know
- Mass vs Weight -- Mass measures the amount of matter in an object (in kilograms), while weight measures gravitational force (in newtons). On Earth's surface, they are proportional and used interchangeably in everyday language.
- Avoirdupois System -- The system of weights used in the US and UK for most purposes (16 oz = 1 lb). Distinct from the troy system used for precious metals, where 12 troy ounces = 1 troy pound.
- Metric Ton (Tonne) -- Equal to 1,000 kg or approximately 2,204.62 pounds. Used globally for cargo, industrial quantities, and commodities trading.
- Short Ton vs Long Ton -- A US short ton equals 2,000 pounds (907.185 kg). A British long ton equals 2,240 pounds (1,016.05 kg). The long ton is now largely obsolete outside historical shipping records.
- Stone -- A unit of 14 pounds (6.35 kg) used primarily in the UK and Ireland for personal body weight. Not recognized in the US metric or imperial systems for commercial use.
Common Weight Units Compared
The table below shows quick-reference conversions between the most commonly needed weight units. All values are based on the exact definitions maintained by NIST.
| From | To | Multiply By | Example |
|---|---|---|---|
| Pounds | Kilograms | 0.45359 | 150 lb = 68.04 kg |
| Kilograms | Pounds | 2.20462 | 70 kg = 154.32 lb |
| Ounces | Grams | 28.3495 | 8 oz = 226.8 g |
| Stones | Kilograms | 6.35029 | 12 st = 76.2 kg |
| Metric Tons | US Tons | 1.10231 | 5 t = 5.51 US tons |
| US Tons | Metric Tons | 0.90718 | 10 US tons = 9.07 t |
Practical Examples
Body weight tracking: A person weighing 12 stone 7 pounds (UK) wants to know their weight in kilograms for a medical form. First convert to pounds: 12 x 14 + 7 = 175 lbs. Then to kg: 175 x 0.4536 = 79.38 kg. Use our BMI Calculator to then determine if this is a healthy weight for their height.
International shipping: A US company ships a 500-pound pallet to Germany. Converting: 500 x 0.4536 = 226.8 kg. International freight is typically quoted per metric ton. At a rate of $150 per metric ton, the shipping cost for this pallet would be approximately $34. Knowing the exact weight in both systems prevents billing disputes.
Cooking and baking: An American recipe calls for 8 ounces of butter. Converting: 8 x 28.35 = 226.8 grams. European recipes typically list butter in grams (225 g is a standard block in many countries). For dry ingredients, our Cooking Converter handles volume-to-weight conversions for common ingredients.
Tips for Accurate Weight Conversions
- Use the "divide by 2.2" shortcut for lb to kg: For quick mental math, divide pounds by 2.2. For example, 165 lbs / 2.2 = 75 kg (exact: 74.84 kg). This is accurate to within 1% for everyday use.
- Distinguish between mass units and force units: In science and engineering, weight (force) is measured in newtons, not kilograms. A 70 kg person weighs approximately 686 newtons on Earth. This distinction matters in physics and aerospace.
- Be aware of troy ounces for precious metals: A troy ounce (31.1035 g) is about 10% heavier than a standard avoirdupois ounce (28.3495 g). Gold, silver, and platinum prices are always quoted per troy ounce.
- Check which "ton" is meant: In international contexts, always clarify whether "ton" means metric ton (1,000 kg), US short ton (907.185 kg), or UK long ton (1,016.05 kg). The difference between a metric ton and a short ton is 10.2%.
- Use grams for cooking precision: Professional bakers measure flour, sugar, and other dry ingredients in grams rather than cups because density varies with packing. A cup of flour can weigh anywhere from 120-150 g depending on how it is scooped.
Frequently Asked Questions
How many pounds are in a kilogram?
One kilogram equals approximately 2.20462 pounds. This is derived from the exact definition: 1 pound = 0.45359237 kilograms, established by the 1959 International Yard and Pound Agreement. To convert kg to lbs, multiply by 2.20462. For quick mental math, multiply kg by 2.2. For example, 80 kg x 2.2 = 176 lbs (exact: 176.37 lbs). This conversion is the most searched weight conversion globally, with over 20 million monthly searches according to Google Trends data.
What is a stone and how is it used?
A stone is a unit of weight equal to exactly 14 pounds (approximately 6.35 kg). It is commonly used in the UK and Ireland for measuring personal body weight. For example, a person weighing 175 lbs would say they are 12 stone 7 pounds (12 x 14 + 7 = 175). The stone is not used in the US or in most other countries. In British culture, saying you weigh "11 stone" is as natural as saying "154 pounds" in the US or "70 kilograms" in continental Europe.
What is the difference between a metric ton and a US ton?
A metric ton (tonne) equals exactly 1,000 kilograms or approximately 2,204.62 pounds. A US ton (short ton) equals exactly 2,000 pounds or approximately 907.185 kilograms. The metric ton is about 10.2% heavier than a US short ton. There is also a UK long ton (2,240 lbs / 1,016.05 kg), which is now largely obsolete. In international trade and shipping, "ton" almost always refers to the metric ton unless otherwise specified.
Is weight the same as mass?
Technically, mass and weight are different physical quantities. Mass measures the amount of matter in an object and is constant regardless of location -- measured in kilograms. Weight measures the gravitational force acting on that mass and varies with location -- measured in newtons. A 70 kg person has a weight of about 686 newtons on Earth but only 114 newtons on the Moon, while their mass remains 70 kg in both places. In everyday language and this converter, the terms are used interchangeably on Earth's surface.
What is the difference between a troy ounce and a regular ounce?
A troy ounce equals 31.1035 grams, while a regular (avoirdupois) ounce equals 28.3495 grams. The troy ounce is about 9.7% heavier. Troy ounces are used exclusively for weighing precious metals -- gold, silver, platinum, and palladium. When you see gold priced at "$2,300 per ounce," that is per troy ounce. Confusingly, there are only 12 troy ounces in a troy pound (373.24 g), versus 16 avoirdupois ounces in a regular pound (453.59 g).
How did the kilogram definition change in 2019?
On May 20, 2019, the kilogram was redefined by fixing the Planck constant to exactly 6.62607015 x 10-34 joule-seconds, as decided by the BIPM General Conference on Weights and Measures. Previously, the kilogram was the last SI unit still defined by a physical artifact -- a platinum-iridium cylinder made in 1889 called the International Prototype of the Kilogram (IPK). The new definition means the kilogram can be realized by any laboratory with the right equipment, eliminating dependence on a single object that was slowly changing mass.