Lbs to Kg Converter — Convert Pounds to kilograms
Convert pounds to kilograms with the exact factor, a common-values table, and the formula shown.
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Lbs to Kg Converter
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Pounds to Kilograms — the Quick Answer
One pound equals 0.45359237 kilograms exactly. To convert any weight in pounds to kilograms, multiply by 0.45359237 — or equivalently, divide by 2.20462. That factor is not rounded: the avoirdupois pound has been defined as exactly 0.45359237 kg since the international yard-and-pound agreement of 1959, making this conversion permanently exact.
For quick mental math, 1 lb ≈ 0.454 kgis accurate to three decimal places. Halving your pound weight gets you close: 200 lb ÷ 2 = 100 kg (overestimates by about 9%), so the better shortcut is “divide by 2.2” — 200 ÷ 2.2 = 90.9 kg vs the exact 90.718 kg. For medical dosing, drug calculations, and international shipping, use the full 0.45359237 factor to avoid compounding rounding error.
The Lbs-to-Kg Formula
Pounds → Kilograms
kilograms = pounds × 0.45359237equivalent: kilograms = pounds ÷ 2.20462
The factor 0.45359237 is the legally-defined mass of the avoirdupois pound. Because the kilogram is the SI primary unit and the pound is derived from it, this is an exact relationship — not a rounded approximation. For 180 lb: 180 × 0.45359237 = 81.647 kg.
Source:NIST Special Publication 811 — Guide for the Use of the International System of Units· National Institute of Standards and Technology
The inverse page — kg to lbs converter — gives the full history of the 1959 international agreement. In brief: before 1959 the US and UK pounds differed by about 2 parts per million, causing silent errors in trade and engineering. The agreement fixed a single pound definition for both nations: exactly 0.45359237 kg. Today every lbs-to-kg calculation on Earth uses that constant.
Common Pounds-to-Kilograms Values
The table below covers body weights, weight-plate denominations, and everyday package sizes. Values use the exact 0.45359237 factor, rounded to three decimal places.
Exact factor 0.45359237, rounded to 3 decimal places
Common pounds to kilograms conversions
| Scenario | Pounds (lb) | Kilograms (kg) | Everyday reference |
|---|---|---|---|
| 1 lb | 0.454 kg | a can of soup | |
| 5 lb | 2.268 kg | a bag of flour / a dumbbell plate | |
| 10 lb | 4.536 kg | a toddler at birth / a bowling ball | |
| 45 lb | 20.412 kg | an Olympic barbell plate (45 lb ≈ 20 kg) | |
| 150 lb | 68.039 kg | lighter adult body weight | |
| 180 lbRecommended | 81.647 kg | average US adult male | |
| 200 lb | 90.718 kg | heavier adult / athlete weight-class |
To convert a value not in the table, multiply the pounds by 0.45359237. For example, 165 lb × 0.45359237 = 74.843 kg; 250 lb × 0.45359237 = 113.398 kg.
Worked Examples
Example 1
Convert 180 lb to kilograms
- Weight
- 180 lb
- Factor
- 0.45359237 kg/lb
Multiply pounds by the exact definition of the pound.
180 × 0.45359237 = 81.6466Round to a sensible precision for body weight.
≈ 81.647 kg
180 lb = 81.647 kg.
Using the divide-by-2.2 shortcut: 180 ÷ 2.2 = 81.818 kg — an overestimate of about 170 g. For a gym weigh-in that is fine; for drug-dosing calculations it can matter.
Example 2
Convert 7 lb 6 oz (typical newborn) to kilograms
- Weight
- 7 lb 6 oz
Convert ounces to pounds: 6 oz ÷ 16 oz/lb = 0.375 lb.
7 + 0.375 = 7.375 lbMultiply by the exact pound definition.
7.375 × 0.45359237 = 3.345Round to grams precision used in neonatal care.
≈ 3.345 kg (3,345 g)
7 lb 6 oz = 3.345 kg. This is within the healthy newborn range of 2.5–4.5 kg (5 lb 8 oz – 9 lb 15 oz).
Hospital birth records in the US often display pounds-and-ounces; the WHO growth charts use kilograms. This two-step conversion (oz to fractional lb, then × 0.453592) is the standard clinical method.
How to Use This Converter
- Enter the weight in pounds in the field above.
- The result shows the equivalent in kilograms instantly, using the exact 0.45359237 factor.
- Need the reverse direction? Visit the kg to lbs converter or use the all-in-one unit converter for any weight pair.
Common Mistakes
- Dividing by 2 instead of 2.2. Halving gives a 10% overestimate — 200 lb ÷ 2 = 100 kg, but the correct answer is 90.7 kg. Always divide by 2.2 (or multiply by 0.4536) for a practical answer.
- Mixing up avoirdupois pounds and troy pounds. Everyday goods — food, body weight, gym equipment — use avoirdupois pounds (0.45359237 kg each). Precious metals use troy pounds (373.242 g each, lighter per pound but heavier per ounce). This converter uses avoirdupois exclusively.
- Forgetting to convert ounces first. If your scale shows 7 lb 4 oz, convert the ounces to a decimal before multiplying: 4 oz ÷ 16 = 0.25 lb, so the weight is 7.25 lb × 0.45359237 = 3.288 kg. Skipping this step understates the weight.
- Using lbs-to-kg for force rather than mass.Pounds and kilograms both describe mass in everyday use. In engineering, a “pound-force” (lbf) is different from a pound-mass (lbm); similarly, a kilogram-force (kgf) differs from a kilogram. This converter handles mass only.
Background
Why the Pound Is Exactly 0.45359237 Kilograms
The pound is one of humanity's oldest named weights. Its Latin precursor, the libra, gave us both the abbreviation lb and the British pound sterling symbol £ — because a pound of silver once equalled a pound of money. Medieval England had multiple competing pounds (merchant, Tower, Troy, and others), which caused persistent trade disputes. By the 16th century the avoirdupois pound (from the French avoir de poids, 'goods of weight') had become the commercial standard in England, fixed at 7,000 grains where a grain was the weight of a dry barleycorn.
The kilogram arrived with the metric system in the 1790s, defined as the mass of one cubic decimeter of water at maximum density. A physical platinum prototype was made in 1799. The US adopted metric units as legal in 1866 but kept customary units for commerce; the UK did the same. Both nations defined their pounds in terms of their own national prototype weights, which differed from each other by about 2 parts per million — enough to matter in precision engineering.
In 1959 six English-speaking nations (the US, the UK, Australia, Canada, New Zealand, and South Africa) signed the international yard-and-pound agreement, fixing 1 avoirdupois pound = exactly 0.45359237 kg [1]. The seven-digit constant was chosen to be exact with respect to the platinum-iridium international prototype kilogram that existed at the time. It eliminated the US/UK discrepancy permanently.
In 2019 the kilogram was itself redefined in terms of the Planck constant, so the unit is no longer anchored to a physical cylinder that could lose atoms [2]. The pound rode along — still exactly 0.45359237 of whatever the kilogram is now. From the pound's perspective nothing changed; from the kilogram's perspective the definition became more stable and reproducible anywhere in the universe.
- International yard and pound agreement — NIST Weights and Measures · National Institute of Standards and Technology · 1959
- NIST SP 811 — The redefined SI and the exact pound definition · National Institute of Standards and Technology · 2019
Weight & Mass Terms
Quick reference
Pounds, kilograms & related terms
Pound (lb)
An avoirdupois unit of mass equal to exactly 0.45359237 kg. Abbreviation lb from Latin libra.
- The avoirdupois pound = 16 ounces = 7,000 grains. It is the pound used for body weight, grocery items, and gym equipment. The troy pound (12 troy ounces = 373.242 g) is used for precious metals only and is not covered here.
Kilogram (kg)
The SI base unit of mass. Since 2019 it is defined via the Planck constant rather than a physical prototype.
- One kilogram = 1,000 grams ≈ 2.20462 lb. It is the mass of roughly one liter of water. The kilogram is the only SI base unit whose name contains a prefix (kilo-), a historical quirk from the metric system's origins.
Avoirdupois system
The everyday US and UK weight system: 16 ounces per pound, 14 pounds per stone, 2,000 pounds per short ton.
- Avoirdupois (from Old French avoir de poids, 'goods having weight') became the commercial standard in England by the 16th century. It is the system this converter uses. Troy and apothecary systems — used for precious metals and pharmacy — use different pound and ounce definitions.
Ounce (oz)
1/16 of an avoirdupois pound = 28.3495 g. To convert ounces to kg: multiply by 0.0283495.
- When converting pounds-and-ounces to kg, first turn the ounces into a decimal fraction of a pound (oz ÷ 16), then multiply the total pounds by 0.45359237. Example: 5 lb 8 oz = 5.5 lb × 0.45359237 = 2.495 kg.
Stone (st)
A UK unit of body weight: 1 stone = 14 lb = 6.350 kg. 180 lb = 12 stone 12 lb.
- The stone is still commonly used for body weight in the UK and Ireland. To convert stone to kg: multiply by 6.35029. To convert stone-and-pounds to kg: (stone × 14 + pounds) × 0.45359237.
Short ton vs metric ton
US short ton = 2,000 lb = 907.185 kg. Metric ton (tonne) = 1,000 kg = 2,204.62 lb.
- A metric ton is slightly heavier than a US short ton. International commodity prices are often quoted per metric ton; US construction and agriculture use short tons. The UK long ton is 2,240 lb (1,016 kg).
0.45359237 exactly
The pound definition fixed in 1959. All lbs-to-kg calculations are exact multiples of this seven-digit constant.
- The full value of 1 lb in kg is 0.45359237 exactly. Because it is a defined constant (not a measured one), using more digits does not improve precision — the digits after the 7th are all zeros by definition.
Related Converters and Tools
For the reverse direction, the kg to lbs converter uses the same 1959 agreement in reverse. For BMI — which takes kilograms and height in meters — visit the BMI calculator. If you want to know what your weight in pounds means for daily calorie needs, the calorie / TDEE calculator accepts pounds directly and converts internally. For other unit pairs, use the all-in-one unit converter.
Frequently Asked Questions
The most common questions we get about this calculator — each answer is kept under 60 words so you can scan.
What is the pounds to kilograms conversion factor?
1 lb = 0.453592 kg exactly. To convert, multiply your pounds value by 0.453592.What is 150 pounds in kilograms?
150 pounds = 68.04 kilograms. Multiply 150 by 0.453592 to get the result.How do I convert kilograms back to pounds?
Divide your kilograms value by 0.453592 (or multiply by the reciprocal). The page shows the exact reverse rate.Is this conversion exact or rounded?
The factor itself is exact by international standard; only the displayed result is rounded for readability. Internally full precision is kept.Is the Lbs to Kg converter free?
Yes — free, instant, no signup. All math runs in your browser; nothing is stored.