The answer to how many kg in a pound is exactly 0.45359237, or 453.59237 grams. That number is a definition, not a measurement. Since July 1, 1959, the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa have used the same pound, fixed by its relation to the kilogram.
Going the other way, 1 kg is 2.2046226218... lb, a decimal that never ends. That is why 2.2 and 2.20462 give different answers. The sections below trace the definition to its source, show what changed in 1959, check whether the 2019 redefinition of the kilogram touched the pound, and set out rounding rules.
How many kilograms are in a pound?
One pound (lb) is 0.45359237 kg or 453.59237 g. In other words, 1 pound in kg is an exact figure, not a rounded one. The abbreviation "lb" comes from the Roman unit libra, and "lbs" is simply the plural. You can convert in either direction with two formulas:
- Pounds to kg: kg = lb × 0.45359237
- Kg to pounds: lb = kg ÷ 0.45359237
For 150 lb, the lb to kg step is 150 × 0.45359237 = 68.0388555, which you would write as 68.04 kg. To check the other direction with 75 kg, 75 ÷ 0.45359237 = 165.3467 lb. The exact answer to how many pounds in a kilogram is 1 ÷ 0.45359237 = 2.2046226218... lb.
Why a pound is exactly 0.45359237 kg
The pound is defined through the kilogram. The United States set that definition in 1959 through the National Bureau of Standards, which is now called NIST.
The 1959 agreement
The Bureau's notice of June 30, 1959 says that from July 1, 1959, 1 yard equals exactly 0.9144 meter and 1 pound (avoirdupois) equals exactly 0.45359237 kilogram. It adds that the national standards laboratories of Australia, Canada, New Zealand, South Africa and the United Kingdom would use the same equivalents. The units were named the international yard and the international pound, and the arrangement is usually called the International Yard and Pound Agreement.
The same notice fixed the inch at exactly 25.4 mm. The length converter handles inch and centimeter conversions. The United Kingdom now writes the figure into its own law. Section 1 of the Weights and Measures Act 1985 says the pound "shall be 0·453 592 37 kilogram exactly".
What the pound was before 1959
According to the same notice, the US pound was set in 1893 at 1 ÷ 2.20462 kg. In 1894 it was adjusted to 1 ÷ 2.20462234 kg, based on a new determination of the British Imperial pound. That is about 0.4535924277 kg. The notice says small but troublesome discrepancies had built up between groups at home and abroad.
The new pound is smaller than the 1894 US pound by about 1 part in 10 million, as the notice puts it. We calculated the gap between the two printed values: 0.0000000577 kg per pound, or 0.0577 milligram. On a million pounds of cargo that is about 57.7 grams. The notice says the change had no effect on ordinary trade and mattered only in precision machining and science. It does not give a number for the British pound before 1959, so this article does not either.
Why these digits
The notice points out that the new factor is exactly divisible by 7, which gives the grain an exact value. An avoirdupois pound is 7,000 grains, so 453.59237 g ÷ 7,000 = 0.06479891 g. One grain is therefore exactly 64.79891 milligrams. The notice describes this property but does not list it as the reason for the choice.
The survey foot exception, and why there is no survey pound
The 1959 notice kept a second definition for one unit only, the foot used in geodetic surveys. That "U.S. survey foot" kept its 1893 value. NIST Handbook 44 says it was retired on December 31, 2022 and should be avoided after January 1, 2023, except for historic and legacy uses. Neither the notice nor the NIST documents describe a second definition for the pound.
Did the 2019 SI redefinition change the pound?
No. Since May 20, 2019, the kilogram has been defined by fixing the Planck constant at 6.62607015 × 10⁻³⁴ J s. The BIPM lists the kilogram, ampere, kelvin and mole as the four base units whose definitions changed. The pound is still defined through the kilogram. Handbook 44 states it plainly: 1 avoirdupois pound = 0.45359237 kilogram.
The size of the kilogram did not change either. Handbook 44 explains that the Planck constant's value was chosen so the kilogram equaled the mass of the old international prototype at the moment of adoption. So the pound now rests indirectly on the Planck constant, and the factor stayed the same.
2.2 or 2.20462? Rounding rules
The pounds to kg factor has 8 digits. The kg to pounds factor never ends. To see what rounding costs, we converted the same 200 lb with several factors. Differences are measured against the exact result, 90.71847 kg.
| Factor used | Result for 200 lb (kg) | Difference (kg) | Difference (%) |
|---|---|---|---|
| 0.45 | 90.00 | −0.72 | −0.79 |
| 0.454 | 90.80 | +0.08 | +0.09 |
| 0.4536 | 90.72 | +0.0015 | +0.002 |
| 0.453592 | 90.7184 | −0.00007 | −0.0001 |
| 0.45359237 (exact) | 90.71847 | 0 | 0 |
The table covers only 200 lb. The absolute difference grows with the weight, while the percentage stays the same. Halving the pounds is a quick mental shortcut, and it is off by about 10 percent: 200 lb becomes 100 kg.
The reverse direction behaves the same way. For 100 kg, the 2.2 factor gives 220.0 lb against an exact 220.462 lb, a gap of 0.46 lb or about 0.21 percent. The 2.20462 factor gives 220.462 lb and misses by less than 0.0003 lb. These four rules keep the error small:
- Use 0.45 or lb ÷ 2.2 only for a quick estimate, and do not quote the result as exact.
- Do the calculation with the full factor and round only the final answer.
- Report the result at the precision the task needs. Body weight rarely needs more than 0.1 kg, baggage 0.01 kg, and a recipe 1 gram.
- When a threshold applies, such as a baggage limit, round in the direction that keeps you under it.
Pounds to kg conversion chart
The values below were computed by multiplying the pound figure by 0.45359237. The exact column is the product, not a measurement. What a scale shows depends on the scale's own precision.
| Pounds (lb) | Kilograms (exact) | Kilograms (2 decimals) |
|---|---|---|
| 1 | 0.45359237 | 0.45 |
| 2 | 0.90718474 | 0.91 |
| 5 | 2.26796185 | 2.27 |
| 10 | 4.5359237 | 4.54 |
| 25 | 11.33980925 | 11.34 |
| 45 | 20.41165665 | 20.41 |
| 50 | 22.6796185 | 22.68 |
| 100 | 45.359237 | 45.36 |
| 135 | 61.23496995 | 61.23 |
| 225 | 102.05828325 | 102.06 |
For kilograms to pounds, divide by 0.45359237. The next table shows the result next to what the 2.2 rule gives.
| Kilograms (kg) | Pounds (lb, 2 decimals) | Multiplied by 2.2 (lb) |
|---|---|---|
| 1 | 2.20 | 2.2 |
| 10 | 22.05 | 22.0 |
| 20 | 44.09 | 44.0 |
| 23 | 50.71 | 50.6 |
| 32 | 70.55 | 70.4 |
| 70 | 154.32 | 154.0 |
| 100 | 220.46 | 220.0 |
For a batch of values, the weight converter does the arithmetic. This article does not repeat that page's unit overview. It covers the definition and the rounding.
Where the rounding shows up
Checked bags
The International Air Transport Association (IATA) says guidelines recommend that a single checked bag not exceed 23 kg (50 lb). It lists 32 kg (70 lb) as the maximum for a single checked bag in many regions, including the EU and the United States. The pairs are rounded. Exactly 50 lb is 22.68 kg, and a 23 kg bag weighs 50.71 lb, which is 0.71 lb over a 50 lb limit.
Airline tables round the same way. Delta's excess and overweight baggage page pairs 51 lb with 23 kg and 71 lb with 32 kg. The exact values are 23.13 kg and 32.21 kg. We read the page on October 28, 2026, and each airline's own rule applies to your flight. For parcels, the dimensional weight calculator covers volume-based pricing.
Gym plates
A 45 lb plate is 20.41 kg, and a 20 kg plate is 44.09 lb. The 0.41 kg gap per plate adds up. As a hypothetical example, a 45 lb bar with a 45 lb plate on each side weighs 135 lb, which is 61.23 kg.
Ounces, stone and troy ounces
An ounce is 1/16 of a pound, so exactly 28.349523125 grams. Handbook 44 lists 16 ounces to the pound. The troy ounce used for gold and silver is a different unit at 31.1034768 grams. It is about 9.7 percent heavier than the avoirdupois ounce.
The stone is a British unit equal to 14 pounds, so exactly 6.35029318 kg. Schedule 1 of the Weights and Measures Act 1985 lists it that way. There are 14 pounds in a stone. A 70 kg weight is 154.32 lb, which is 11 stone and about 0.32 lb.