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What is a kilogram?

The SI base unit of mass, redefined in 2019 via Planck's constant

Definition

The kilogram is the SI base unit of mass. Since 2019 it has been defined via the fixed numerical value of Planck's constant — a major shift from the previous definition based on a physical artifact. One litre of water at 4 °C has a mass of almost exactly 1 kg, a coincidence designed into the original 1795 definition.

Common uses

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

🌍 Real-world scale

A litre of water weighs 1 kg at 4 °C. An average adult weighs 60–90 kg. A standard bag of flour is 1 kg.

Convert from kilograms

11 conversions
Kilogram to Gram1 kg = 1,000 gKilogram to Milligram1 kg = 1,000,000 mgKilogram to Microgram1 kg = 1,000,000,000 µgKilogram to Pound1 kg = 2.20462 lbKilogram to Ounce1 kg = 35.274 ozKilogram to Metric Ton1 kg = 0.001 tKilogram to US Ton1 kg = 0.001102 ton (US)Kilogram to UK Ton1 kg = 0.0009842 ton (UK)Kilogram to Stone1 kg = 0.1575 stKilogram to Carat1 kg = 5,000 ctKilogram to Grain1 kg = 15,432.36 gr

Convert to kilograms

11 conversions
Gram to Kilogram1 g = 0.001 kgMilligram to Kilogram1 mg = 1.0000e-6 kgMicrogram to Kilogram1 µg = 1.0000e-9 kgPound to Kilogram1 lb = 0.4536 kgOunce to Kilogram1 oz = 0.02835 kgMetric Ton to Kilogram1 t = 1,000 kgUS Ton to Kilogram1 ton (US) = 907.185 kgUK Ton to Kilogram1 ton (UK) = 1,016.05 kgStone to Kilogram1 st = 6.35029 kgCarat to Kilogram1 ct = 0.0002 kgGrain to Kilogram1 gr = 6.4799e-5 kg

Tips

  • 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.

Common mistakes

  • 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.

FAQ about the kilogram

What is a kilogram?
The kilogram is the SI base unit of mass. Since 2019 it has been defined via the fixed numerical value of Planck's constant — a major shift from the previous definition based on a physical artifact. One litre of water at 4 °C has a mass of almost exactly 1 kg, a coincidence designed into the original 1795 definition.
Where is the kilogram used?
The kilogram is commonly used for: Human body weight in most of the world; Grocery and retail food quantities in Europe; Scientific and engineering mass measurements.
What is an example of a kilogram in real life?
A litre of water weighs 1 kg at 4 °C. An average adult weighs 60–90 kg. A standard bag of flour is 1 kg.
What category does the kilogram belong to?
The kilogram is a unit of weight, part of the 12-unit weight category on UnitConv.
How accurate are kilogram conversions?
All conversion factors follow NIST and BIPM international standards to 8 significant figures — the highest practical accuracy.
Explore all 12 weight units and their conversions.⚖️ Weight hub →