What Is a Kilogram? The SI Unit of Mass

Introduction: The World's Standard for Mass

The kilogram (symbol: kg) is the SI base unit of mass. It is used in virtually every country on Earth for commerce, science, medicine, and daily life. From the produce you buy at the grocery store to the dosage of medication prescribed by your doctor, the kilogram is the fundamental reference. Unlike length and time, the kilogram is the only SI base unit that includes a metric prefix in its name — “kilo” means one thousand, so a kilogram is literally 1,000 grams.

The Modern Definition: Planck Constant

Since May 20, 2019 (World Metrology Day), the kilogram has been defined by fixing the value of the Planck constant at exactly 6.62607015 × 10⁻³⁴ kg·m²/s. This means the kilogram is no longer based on a physical object but on an unchanging constant of nature. Any sufficiently equipped laboratory anywhere in the world can now realize the kilogram to high precision using a Kibble balance, which relates mechanical and electrical power.

Key Conversion Formulas

• 1 kilogram = 2.20462 pounds
• 1 kilogram = 35.274 ounces
• 1 kilogram = 1,000 grams
• 1 pound = 0.453592 kilograms
• 1 stone = 6.35029 kilograms
• 1 metric tonne = 1,000 kilograms

Worked Examples

Example 1: Converting 70 kg to pounds

70 × 2.20462 = 154.32 pounds. This is a typical adult body weight, showing why knowing this conversion is useful when traveling between metric and imperial countries.

Example 2: Converting 5 pounds to kilograms

5 × 0.453592 = 2.268 kg. A 5-pound bag of flour weighs about 2.27 kilograms.

Example 3: Converting 150 pounds to kilograms

150 × 0.453592 = 68.04 kg. For a quick mental estimate, divide pounds by 2.2: 150 / 2.2 ≈ 68.2 kg.

Quick Reference Table

A Brief History

The kilogram was originally defined in 1795 as the mass of one liter of water at 4°C (its maximum density). In 1799, a platinum cylinder called the Kilogramme des Archives became the first physical prototype. In 1889, an improved platinum-iridium cylinder called the International Prototype of the Kilogram (IPK), stored at the International Bureau of Weights and Measures (BIPM) near Paris, became the global standard. For 130 years, the IPK literally was the kilogram — whatever it weighed was exactly one kilogram by definition. But comparisons with copies showed the IPK was drifting by about 50 micrograms per century, prompting the 2019 redefinition based on the Planck constant.

Real-World Applications

• Grocery shopping: Produce, meat, and bulk goods are sold by the kilogram in most countries.
• Medicine: Drug dosages are often calculated per kilogram of body weight (e.g., mg/kg).
• Shipping: Freight charges are based on weight in kilograms or metric tonnes.
• Fitness: Gym weights in metric countries are marked in kilograms. A standard Olympic barbell weighs 20 kg.
• Aviation: Aircraft payload, fuel, and baggage limits are specified in kilograms internationally.

Convert between kilograms and any other weight unit using our weight converter.

Frequently Asked Questions

How many pounds are in a kilogram?

One kilogram equals approximately 2.20462 pounds. For quick mental math, multiply kilograms by 2.2.

Why was the kilogram redefined in 2019?

The physical platinum-iridium prototype stored in Paris was slowly changing mass over time due to surface contamination and atomic-level changes. The 2019 redefinition ties the kilogram to the Planck constant, a fixed value of nature, ensuring perfect long-term stability.

What is the difference between mass and weight?

Mass is the amount of matter in an object, measured in kilograms. Weight is the gravitational force acting on that mass, measured in newtons. On Earth's surface, a 1 kg mass weighs about 9.81 newtons. On the Moon, the same 1 kg mass weighs only about 1.62 newtons, but its mass remains 1 kg.