Weight Converter — Convert kg, lbs, Grams, Ounces & More
Our free weight converter instantly converts between all common mass and weight units — from milligrams to metric tonnes, ounces to pounds, and everything in between. Whether you're converting recipe ingredients, checking shipping weights, tracking fitness goals, or working on engineering calculations, this tool provides accurate results with clear unit relationships explained.
Mass vs. Weight: The Scientific Distinction
In everyday language, "weight" and "mass" are used interchangeably, but physics distinguishes them precisely. Mass is the quantity of matter in an object, measured in kilograms (SI unit). It's an intrinsic property that doesn't change with location. Weight is the gravitational force acting on mass, measured in newtons (N). An 80 kg person has the same mass on Earth and the Moon, but weighs 784 N on Earth and only 130 N on the Moon (where gravity is 1/6 as strong).
For terrestrial applications (which is everything this converter handles), the distinction is academic. We use "weight" colloquially to mean mass, and scales calibrated on Earth give consistent readings. Just know that when you "weigh" yourself in kilograms, you're technically measuring mass.
Metric Weight Units
Milligram (mg) — 0.001 g: Used for pharmaceutical dosages (ibuprofen: 200-400 mg), nutritional measurements (vitamin C RDA: 65-90 mg), and chemical analysis. Precision balances measuring to 0.1 mg are standard in laboratories.
Gram (g) — base metric unit: The gram is the everyday metric unit for food (a slice of bread: 25-30 g), letters (standard envelope: 5-10 g), and cooking (1 cup of flour ≈ 125 g). Originally defined as the mass of 1 cm³ of water at 4°C.
Kilogram (kg) — 1,000 g: The SI base unit of mass. Used for body weight, grocery quantities, luggage limits (airline: typically 23 kg), and general commerce worldwide. Defined since 2019 by fixing Planck's constant, replacing the physical platinum-iridium prototype.
Metric Tonne (t) — 1,000 kg: Used for vehicle weights (average car: 1.4 t), cargo shipping, agricultural yields, and industrial quantities. Global trade volumes are measured in tonnes. The metric tonne is also called "megagram" (Mg) in strict SI usage.
Imperial Weight Units
Ounce (oz) — 28.35 g: The avoirdupois ounce used for food packaging, postal weight, and everyday measurement in the US/UK. Not to be confused with the troy ounce (31.1 g) used for precious metals, or the fluid ounce (a volume measure). There are 16 ounces in a pound.
Pound (lb) — 453.6 g: The primary weight unit in the United States for body weight, food quantities, and commerce. Derived from the Roman "libra" (hence the "lb" abbreviation). One pound = 16 ounces = 7,000 grains.
Stone (st) — 6.35 kg / 14 lbs: Used primarily in the UK and Ireland for body weight. A person might say "I weigh 11 stone 3" meaning 11 stone + 3 pounds (157 lbs / 71.2 kg). Rarely used outside body weight contexts in modern times.
US (Short) Ton — 2,000 lbs / 907.2 kg: Used in the United States for large quantities. Often seen in construction (concrete: sold by the short ton), waste management, and mining.
UK (Long) Ton — 2,240 lbs / 1,016 kg: The imperial ton, historically used in the UK and Commonwealth. Close to the metric tonne (only 1.6% heavier). Still referenced in shipping and naval contexts.
Essential Conversion Factors
The most frequently needed conversions: 1 kg = 2.20462 lbs (multiply kg by 2.2 for quick estimate). 1 lb = 0.453592 kg (divide lbs by 2.2). 1 oz = 28.3495 g. 1 stone = 6.35029 kg = 14 lbs. 1 metric tonne = 2,204.62 lbs = 1.10231 short tons.
For quick mental math: kg to lbs — double the number and add 10%. (Example: 70 kg → 140 + 14 = 154 lbs, actual: 154.3 lbs). Lbs to kg — halve the number and subtract 10%. (Example: 150 lbs → 75 - 7.5 = 67.5 kg, actual: 68.0 kg).
Weight in Different Contexts
Cooking and Nutrition: Metric recipes use grams for precision. American recipes often use volume measures (cups) for dry ingredients, introducing variability. Nutritional information uses grams and milligrams (protein: 50g daily, sodium: 2,300mg daily limit).
Shipping and Logistics: International shipping uses kilograms. Domestic US shipping uses pounds. Airlines set luggage limits in kilograms (typically 23 kg checked, 7 kg carry-on). Freight is often billed by "volumetric weight" — calculated from package dimensions if they exceed actual weight.
Fitness and Health: Body weight tracking, food logging, and supplement dosing all require weight conversions. A person might weigh themselves in kg at the gym but need to communicate in pounds to their US doctor. Protein intake recommendations (1.6-2.2 g/kg body weight) require knowing mass in kilograms.
Precious Metals: Gold, silver, and platinum use troy ounces (31.1035 g), not regular ounces. One troy pound = 12 troy ounces (unlike 16 regular ounces per regular pound). A troy ounce is heavier than an avoirdupois ounce, but a troy pound is lighter than an avoirdupois pound. Always verify which system is being used.
Historical Context
The kilogram was originally defined in 1795 as the mass of one liter of water at 4°C. A physical platinum-iridium cylinder stored in Paris served as the reference standard from 1889 until 2019, when the kilogram was redefined using Planck's constant — making it the last SI base unit to be freed from a physical artifact. The pound was legally defined in terms of the kilogram since 1959: exactly 0.45359237 kg.
Frequently Asked Questions
How many pounds are in a kilogram?
One kilogram equals approximately 2.205 pounds. For quick mental math, double the kg value and add 10%. Example: 80 kg → 160 + 16 = 176 lbs (actual: 176.4 lbs).
What is the difference between mass and weight?
Mass is the amount of matter (measured in kg), constant regardless of location. Weight is gravitational force on that mass (measured in newtons), varying with gravity. On Earth, they're used interchangeably in everyday context.
How many grams are in an ounce?
One avoirdupois (common) ounce = 28.35 grams. One troy ounce (precious metals) = 31.10 grams. Unless specified otherwise, "ounce" refers to the avoirdupois ounce.
What is a stone in kilograms?
One stone = 6.35029 kg = 14 pounds. Used mainly in the UK/Ireland for body weight. A 70 kg person weighs approximately 11 stone. It's rarely used for anything other than expressing human body weight.
What is the difference between metric tonnes and tons?
Metric tonne = 1,000 kg (2,204.6 lbs). US short ton = 2,000 lbs (907.2 kg). UK long ton = 2,240 lbs (1,016 kg). The metric tonne and long ton are within 1.6% of each other; the short ton is about 10% less.
How do I convert body weight from kg to lbs?
Multiply kg by 2.205. Examples: 60 kg = 132.3 lbs, 70 kg = 154.3 lbs, 80 kg = 176.4 lbs, 90 kg = 198.4 lbs, 100 kg = 220.5 lbs. Divide lbs by 2.205 for the reverse.
What is a milligram used for?
Milligrams measure small quantities: medicine doses (aspirin 325mg), nutritional content (caffeine in coffee: 95mg), chemical analysis, and gem weights. One milligram is one-thousandth of a gram.
Why does the US use pounds instead of kilograms?
The US inherited British Imperial units and never completed metrication. While science and medicine use metric, everyday commerce uses pounds. Mandatory metric conversion proposed in the 1970s was met with public resistance and was made voluntary.