Understanding Weight & Mass Conversions
From milligrams of medication to metric tons of cargo — every weight conversion starts with a common base unit. Here's the math, the history of each unit, and the practical contexts where precision matters.
How Weight Conversion Works
All units in this converter are defined relative to the kilogram — the SI base unit of mass. To convert between any two units, the value is first expressed in kilograms, then divided by the target unit's kilogram-equivalent.
Result = Value × (source unit in kg) ÷ (target unit in kg)
For example, converting 5 pounds to grams: 5 × 0.453592 ÷ 0.001 = 2,267.96 g. The kilogram factors cancel out, leaving a clean ratio.
Conversion Factors: Every Unit Explained
Each unit's relationship to the kilogram, along with its history and primary use cases.
Kilogram (kg)
Factor: 1 (base unit)
The kilogram is the SI base unit of mass. Since 2019, it has been defined by fixing the Planck constant at exactly 6.62607015 × 10⁻³⁴ joule-seconds. Before that, it was defined by a physical platinum-iridium cylinder stored at the International Bureau of Weights and Measures in Sèvres, France — the only SI unit still based on an artifact at that time.
Gram (g)
Factor: 1 g = 0.001 kg
One thousandth of a kilogram. The gram was originally defined as the mass of one cubic centimeter of water at 4°C. It's the standard unit for nutrition labels, cooking recipes, postal weight, and laboratory measurements. Most food packaging worldwide lists contents in grams.
Milligram (mg)
Factor: 1 mg = 0.000001 kg
One millionth of a kilogram, or one thousandth of a gram. Milligrams are essential in pharmacology — drug dosages are almost always specified in mg. They're also used in nutritional science (sodium, vitamins) and in measuring trace contaminants in water (mg/L = parts per million).
Pound (lb)
Factor: 1 lb = 0.453592 kg
The avoirdupois pound is the standard "pound" in the US and UK. It equals 16 ounces. The word comes from the Latin libra pondo ("a pound by weight"), which also explains the "lb" abbreviation. The US legally defines one pound as exactly 0.45359237 kilograms.
Ounce (oz)
Factor: 1 oz = 0.0283495 kg
One sixteenth of a pound (avoirdupois). The ounce is widely used in the US for food portions, postage, and consumer products. Note that the troy ounce (31.1035 g), used for precious metals, is heavier than the avoirdupois ounce (28.3495 g). This converter uses the standard avoirdupois ounce.
Metric Ton (t)
Factor: 1 t = 1,000 kg
Also called a "tonne" (to distinguish from the US short ton of 907.185 kg and the UK long ton of 1,016.05 kg). The metric ton is the standard for bulk commodities: shipping containers, agricultural yields, industrial materials, and carbon emissions. One metric ton equals approximately 2,204.62 pounds.
Common Weight Conversions at a Glance
The conversions people look up most frequently, with formulas and examples.
| From | To | Formula | Example |
|---|---|---|---|
| Kilograms | Pounds | kg × 2.20462 | 70 kg = 154.32 lbs |
| Pounds | Kilograms | lbs × 0.453592 | 150 lbs = 68.04 kg |
| Ounces | Grams | oz × 28.3495 | 8 oz = 226.80 g |
| Grams | Ounces | g × 0.035274 | 500 g = 17.64 oz |
| Metric Tons | Pounds | t × 2,204.62 | 2 t = 4,409.24 lbs |
| Milligrams | Grams | mg ÷ 1,000 | 500 mg = 0.5 g |
Weight vs. Mass: What's the Difference?
In everyday language, "weight" and "mass" are used interchangeably — but they measure different things.
Mass
Mass is the amount of matter in an object. It's an intrinsic property — it doesn't change with location. A 70 kg person has 70 kg of mass on Earth, on the Moon, and in deep space. The SI unit of mass is the kilogram.
Mass is measured using a balance scale (comparing against known masses) and is independent of gravitational field strength.
Weight
Weight is the gravitational force acting on mass. It varies by location. A 70 kg person weighs about 686 newtons on Earth but only 114 newtons on the Moon (where gravity is roughly 1/6 of Earth's). The SI unit of weight is technically the newton, not the kilogram.
Weight is measured using a spring scale and depends on the local gravitational acceleration (9.81 m/s² on Earth's surface).
In practice, this distinction rarely matters on Earth's surface, where gravity is nearly constant. That's why bathroom scales show "kg" or "lbs" rather than newtons — they're technically measuring weight but displaying mass, relying on the assumption that gravity is standard.
Metric vs. Imperial Weight: A Brief History
The metric system's gram was originally defined in 1795 as the mass of one cubic centimeter of water at its densest temperature (4°C). This elegant definition tied mass to volume, making the system internally consistent. The kilogram became the practical base unit because a gram was too small for everyday commerce.
Imperial weight units evolved from medieval trading standards. The pound has been redefined multiple times across centuries and jurisdictions. The avoirdupois pound (the one in common use) was standardized in 1959 across English-speaking nations as exactly 0.45359237 kilograms, settling decades of slight variations between countries.
The "ton" illustrates the complexity of historical weights: the US short ton (2,000 lbs), the UK long ton (2,240 lbs), and the metric ton (1,000 kg ≈ 2,204.62 lbs) are all different. In international trade and science, the metric ton (tonne) is now the standard to avoid confusion.
Weight Conversion FAQ
? How many pounds are in a kilogram?
One kilogram equals approximately 2.20462 pounds. To convert kilograms to pounds, multiply by 2.20462. For example, 70 kg = 154.32 lbs.
? What is the difference between weight and mass?
Mass is the amount of matter in an object, measured in kilograms or grams. Weight is the gravitational force acting on that mass, technically measured in newtons. In everyday use, "weight" and "mass" are used interchangeably because gravity is roughly constant on Earth's surface.
? How many grams are in an ounce?
One ounce (avoirdupois) equals approximately 28.3495 grams. This is the standard ounce used for food, postage, and general weighing. The troy ounce, used for precious metals, equals 31.1035 grams.
? How do you convert metric tons to pounds?
One metric ton (tonne) equals 1,000 kilograms or approximately 2,204.62 pounds. Multiply the metric ton value by 2,204.62 to get pounds. A metric ton is slightly heavier than a US short ton (2,000 pounds) and lighter than a UK long ton (2,240 pounds).