Convert between pounds, kilograms, ounces, grams, and stone instantly.
The most frequently needed weight conversion is between pounds (used in the US) and kilograms (used everywhere else and in all scientific contexts). The exact relationship: 1 pound = 0.45359237 kilograms, or 1 kilogram = 2.20462262 pounds.
Quick mental math: multiply kilograms by 2.2 for a close approximation. A 70 kg person weighs about 154 lbs (actual: 154.3 lbs). Divide pounds by 2.2 to get approximate kg: 165 lbs ÷ 2.2 ≈ 75 kg (actual: 74.8 kg). For most everyday purposes, this approximation is accurate enough.
The stone, used primarily in the UK and Ireland for body weight, adds another layer: 1 stone = 14 pounds = 6.35 kg. A person weighing "11 stone 4 pounds" in UK terms is 11 × 14 + 4 = 158 pounds = 71.7 kg. Stone is rarely used in formal contexts — it appears mainly in casual conversations about body weight in British and Irish contexts.
In everyday language, "weight" and "mass" are used interchangeably. Scientifically, they are different. Mass is the amount of matter in an object — constant everywhere. Weight is the gravitational force on that mass — varies by location. A 70 kg person has 70 kg of mass on Earth, on the Moon, and in space. Their weight in Newtons (686 N on Earth, 114 N on the Moon) depends on local gravity.
The instruments we use to "weigh" things in daily life actually measure mass (calibrated for Earth's gravity). A kitchen scale reading "200 grams" means the object has a mass of 200 grams. This distinction matters in aerospace engineering and physics but is irrelevant for everyday unit conversion.