A pounds to kilograms calculator converts a US/UK customary weight to the metric kilogram. Enter a value in pounds and the calculator returns the kg equivalent using the exact NIST-defined conversion: 1 lb = 0.45359237 kg.
Useful for international shipping labels, scientific work that mixes units, converting US gym weights or recipe ingredients to metric, and any setting where the same number needs to be expressed in both systems. The inverse (kg to pounds) lives at our companion calculator linked below.
Key takeaway
The pound-to-kilogram factor is exact by international agreement, 1 lb = 0.45359237 kg, no rounding. That precision matters for scientific work and high-stakes commercial weighing, where small errors at scale compound. For everyday use, 1 lb ≈ 0.454 kg or even ÷ 2.2 is plenty accurate.
How it's calculated
The conversion:
kilograms = pounds × 0.45359237
The factor 0.45359237 is the international avoirdupois pound, defined by NIST and ISO. There are several other "pounds" historically (troy pound for precious metals, apothecaries' pound for medicine, several European pre-metric pounds), but the avoirdupois pound is what every modern English- speaking country uses for everyday weight.
A useful sanity check: the kilogram is bigger than the pound (just over 2×), so the kg number should always be smaller than the lb number for any positive weight. If you see a result larger than the input, you've inverted the formula.
Source: NIST exact conversion, 1 lb (avoirdupois) = 0.45359237 kg
Examples
150 lb, average US adult weight
- Weight 150 lb
150 lb converts to 68.04 kg. Roughly the average weight of an adult American woman (slightly below the male average of ~200 lb / 90.7 kg). Mental check: 150 ÷ 2.2 ≈ 68. The shortcut is within 0.1 kg of the exact answer across normal human-weight ranges.
10 lb, gym dumbbell
- Weight 10 lb
A 10 lb dumbbell is 4.54 kg. Not the same as a 5 kg dumbbell (which is 11 lb), gym equipment is sold in either system, not always with cleanly equivalent weights. The 0.5 kg gap matters less for casual lifting than for competitive lifters tracking precise progressions.
Frequently asked questions
How do I convert pounds to kilograms?
Multiply by 0.45359237 for the exact answer, or divide by 2.2 for a quick mental estimate within ~0.4% of the real value across normal weights. So 100 lb × 0.45359237 = 45.36 kg (or 100 ÷ 2.2 ≈ 45.5 kg, close, but slightly off). The "÷ 2.2" approximation slightly overstates the metric weight, by about 0.4 kg per 100 lb.
Why is the conversion factor 0.45359237?
It's the internationally agreed exact value of the avoirdupois pound, set in 1959 when the US and Commonwealth countries unified their definitions of the pound and yard. Before that, each country had slightly different "pound" values, which created international trade confusion. The 0.45359237 figure is now exact by definition, just like the inch's 25.4 mm.
What's the difference between pounds and kilograms in physics?
Kilograms measure mass (an intrinsic property of matter). Pounds as commonly used in the US measure weight (the force gravity exerts on mass), though in everyday language people use "pound" to mean mass too. The distinction matters off Earth: a 150 lb astronaut on the Moon "weighs" about 25 lb (because lunar gravity is ~1/6 of Earth's) but their mass, and the kilogram value, stays the same.
Are there different kinds of pounds?
Yes, several historical definitions. Avoirdupois (1 lb = 0.4536 kg) is the everyday pound for groceries, body weight, and shipping. Troy pound (1 lb = 0.3732 kg, 12 troy oz) is used for precious metals. Apothecaries' pound (also 0.3732 kg) was used in pharmacy until the 20th century. Most modern uses are avoirdupois unless you're trading gold, silver, or doing pre-1950s pharmacology research.