What is a Kilogram?

The kilogram is the base SI unit of mass. Since May 2019, it has been defined by fixing the numerical value of Planck's constant to exactly 6.62607015 × 10⁻³⁴ joule-seconds — a major shift from the previous definition based on a physical platinum-iridium artifact (the International Prototype Kilogram or 'Le Grand K') stored at the BIPM near Paris since 1889. This redefinition links the kilogram to a fundamental constant of nature, making it reproducible by any sufficiently equipped laboratory. The kilogram is the global standard for body weight (most countries quote weight in kilograms), grocery sales, scientific measurements, and engineering. One liter of pure water at 4°C has a mass of almost exactly 1 kg — a coincidence designed into the original 1795 definition. The kilogram relates to the gram (1,000 g = 1 kg), the metric ton (1,000 kg = 1 t), and the pound (1 kg ≈ 2.205 lb).

Visual reference for how the kilogram relates to other weight units. Each row links to the full converter for that pair.

• Human body weight in most of the world
• Grocery and retail food quantities in Europe
• Scientific and engineering mass measurements

• ✓1 kg ≈ 2.205 lb. For quick estimates, double the kg figure and add 10%.
• ✓Body weight is best measured first thing in the morning, after using the bathroom but before breakfast.
• ✓The kg is the only SI base unit whose name contains a prefix — a historical quirk of the metric system.

• ✗Confusing mass (kg) with weight-force (newtons) in physics problems.
• ✗Using 2 instead of 2.205 when converting kg to pounds — the 2.5% error matters for airline luggage.
• ✗Writing Kg instead of kg — the correct SI abbreviation is lower-case kg.