What Is the Kilogram? Definition, History, and the SI System
The kilogram is the SI base unit of mass. Learn how it was once a physical artifact, how it is now defined by Planck's constant, and how it links to pounds.
The Kilogram Is the Unit of Mass
The kilogram is the base unit of mass in the International System of Units (SI), the modern metric system used almost everywhere for science and trade. Unlike the pound, which is a unit of weight in the US customary system, the kilogram measures mass, the amount of matter in an object, and it stays the same regardless of gravity.
One kilogram equals 1000 grams, and it is the only SI base unit whose name carries the prefix "kilo". To connect it to everyday US units, 1 kilogram = 2.20462262 pounds exactly. Our kg to lbs converter applies that factor to any value.
From a Physical Cylinder to a Constant
From 1889 until 2019, the kilogram was defined by a single object: the International Prototype of the Kilogram (IPK), a small cylinder of platinum-iridium kept near Paris, France. National standard masses around the world were calibrated against it. The obvious problem was that a physical artifact can gain contaminants or lose mass over time, drifting the definition itself.
The 2019 Redefinition
On May 20, 2019, the kilogram was redefined using a fixed value of the Planck constant, a fundamental constant of nature (h = 6.62607015 x 10^-34 joule-seconds). Because the joule is defined in terms of the meter and the second, the kilogram is now tied to unchanging physics rather than a metal cylinder. A Kibble balance can realize the kilogram from this constant.
Why It Matters
A constant-based definition means the kilogram is stable and reproducible anywhere, without shipping a reference object. It also aligns mass with the other SI base units, most of which were already defined by physical constants.
Kilograms and Pounds in Daily Life
Body weight, groceries, and shipping are often quoted in pounds in the US but in kilograms almost everywhere else. Because 1 kg = 2.20462262 lb, a 70 kg adult is about 154 lb. For a full walkthrough of that math, see our kg to lbs guide.
Quick Reference
1 kg = 1000 g. 1 kg = 2.20462262 lb. 1 lb = 0.45359237 kg. These are exact by international definition, so they never change.
Frequently Asked Questions
What is a kilogram in pounds?
One kilogram equals exactly 2.20462262 pounds, based on the international definition of the pound as 0.45359237 kg.
How was the kilogram defined before 2019?
From 1889 to 2019 it was defined by the International Prototype of the Kilogram, a platinum-iridium cylinder stored in France.
Why did the kilogram change in 2019?
It was redefined using the fixed Planck constant so the unit is stable, reproducible, and based on physics rather than a single physical object.
Is the kilogram a measure of weight or mass?
Mass. It measures the amount of matter and does not change with gravity, unlike weight which depends on local gravity.
How many grams are in a kilogram?
Exactly 1000 grams make up one kilogram.
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Quick Summary
The kilogram is the SI base unit of mass, equal to 1000 grams and exactly 2.20462262 pounds. Since 2019 it has been defined by the fixed Planck constant rather than a physical artifact, making the definition stable and universal.
Related Guide
Kg to Lbs: The Complete Kilograms to Pounds Conversion Guide
Master kg to lbs conversion with the exact 2.20462262 factor. Convert any kilograms to pounds for fitness, medical, travel, and shipping use.
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100 kg to lbs: Convert 100 Kilograms to Pounds for Lifting and Logistics
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60 kg to lbs: Convert 60 Kilograms to Pounds for Fitness and Travel
Convert 60 kg to lbs with the exact formula. 60 kilograms equals 132.28 pounds, a common weight for fitness tracking, body weight, and luggage.
50 kg to lbs: Convert 50 Kilograms to Pounds for Health and Shipping
Convert 50 kg to lbs with the exact formula. 50 kilograms equals 110.23 pounds, a common weight for body weight, fitness, and shipping limits.
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Disclaimer
Unit conversions on this site use internationally recognized, fixed conversion factors (for example, 1 inch = 2.54 cm, 1 pound = 0.45359237 kg, 1 °F = (°C × 9/5) + 32). They are provided for general, informational, and educational use. For applications where accuracy is critical — such as medical dosing, engineering, scientific research, trade, or legal compliance — always verify results against authoritative sources such as the U.S. National Institute of Standards and Technology (NIST) and the official International System of Units (SI) brochure, and follow any applicable local standards.
Exact Constants & Authoritative Sources
Every conversion on this page uses the fixed, internationally agreed constants below — drawn from NIST and the SI Brochure. No values are estimated or rounded as if exact. See the full list on our Sources & Standards page.
- EXACT1 pound (lb, avoirdupois) = 0.45359237 kilograms (exact)
- EXACT1 ounce (oz, avoirdupois) = 28.349523125 grams (exact, 1/16 lb)
- EXACT1 stone (st) = 6.35029318 kilograms (exact, 14 lb)
- EXACT1 short ton (US) = 907.18474 kilograms (exact, 2000 lb)
Common Pitfalls & How to Avoid Them
Mass vs. weight
The pound and kilogram are units of MASS, not force. On Earth, mass and "weight" read the same number in practice, but a 70 kg mass weighs less on the Moon. UnitFig converts mass; for scientific force you need newtons (1 lbf = 4.4482216152605 N, exact).
Avoirdupois vs. troy ounce
Cooking and body weight use the avoirdupois ounce (28.349523125 g). Gold and precious metals use the troy ounce (31.1034768 g). Never mix them — the error is about 10%.
Rounding the factor
Using 2.2 instead of the exact 2.20462262 overstates small errors at large values. For medical dosing or competition weight classes, keep the full factor or use a converter.
A unit-conversion reference site has no manufacturer-specific or device calibration parameters, so "layered tuning" does not apply here. The only real error-handling is input validation: the converter rejects non-numeric, empty, or out-of-range entries and shows a clear message, then falls back to exact constants above. No estimated or rounded figures are ever presented as exact.
Authoritative sources for this page
- NIST (U.S. National Institute of Standards and Technology) — NIST SP 330 — The International System of Units (SI), 2019 edition
Last Updated 2026 | This article was reviewed for accuracy