Pounds to Kilograms Converter

How to convert pounds to kilograms (the formula)

To convert pounds to kilograms, multiply by 0.45359237: kg = pounds × 0.45359237. That factor is exact, the international pound has been defined as precisely 0.45359237 kilograms since 1959. Dividing by 2.20462 gives the same result.

Two examples: 150 lb × 0.45359237 = 68.04 kg, and 200 lb × 0.45359237 = 90.72 kg. Because a pound is lighter than a kilogram, the kilogram value is always the smaller number, an easy check that you multiplied rather than divided.

This page converts live in both directions, so you can enter pounds and read kilograms, or the reverse. That is handy when a US bathroom scale reads pounds but a medical form, gym program or airline form asks for kilograms.

Why one pound is exactly 0.45359237 kilograms

The value 0.45359237 is not a rounded measurement, it is the legal definition of the international pound, agreed in 1959 by the United States, United Kingdom, Canada, Australia, New Zealand and South Africa. The same agreement fixed the yard at exactly 0.9144 meters, which is why the inch is exactly 2.54 cm. The US National Bureau of Standards published the terms as Letter Circular 1033.

Because the pound is now defined in terms of the kilogram, the kilogram is the base unit and the pound is derived. Since 2019 the kilogram has been defined by fixing the Planck constant, so the entire chain, kilogram to pound to inch, rests on fundamental constants rather than on physical artifacts.

Because the definition has many significant digits, converting pounds to kilograms is exact for any realistic input. The only rounding you ever apply is the one you choose when you write the answer down. NIST SP 811 lists 1 lb = 0.45359237 kg as an exact factor.

Pounds to kilograms conversion chart

Both directions for the most common values. The pound-to-kilogram column is rounded to four decimal places.

Anchor to remember: 1 pound is a little under half a kilogram, so 2.2 pounds is about 1 kg, and 220 pounds is about 100 kg.

Worked examples: body weight, dumbbells and luggage

Four pound-to-kilogram conversions US users ask for most, worked with the exact factor.

• Body weight: 150 lb × 0.45359237 = 68.04 kg, a common weight-goal conversion.
• Dumbbells: a 45 lb plate is 45 × 0.45359237 = 20.41 kg, close to a metric 20 kg plate, but not identical.
• Luggage: a 50 lb bag is 50 × 0.45359237 = 22.68 kg, just under the common 23 kg international limit.
• Shipping: a 100 lb parcel is 100 × 0.45359237 = 45.36 kg.

The dumbbell example is worth noting: a US "45 lb" plate and a metric "20 kg" plate differ by about 0.4 kg (roughly 0.9 lb). If you are matching plates across a mixed set, convert rather than assuming they are the same.

Pounds to kilograms in fitness, medicine and shipping

Three fields lean on the pound-to-kilogram conversion constantly.

Fitness. Gym programs from outside the US often prescribe loads in kilograms, while US plates are marked in pounds. A 135 lb barbell is 61.23 kg, and a 225 lb deadlift is 102.06 kg. Knowing the conversion keeps a program accurate across mixed equipment.

Medicine. Drug doses are frequently calculated per kilogram of body weight, so a US patient's weight in pounds must be converted precisely. A 180 lb adult is 81.65 kg. Rounding here can change a dose, so use the exact factor and full precision.

Shipping and travel. Carriers set limits in either unit. A 70 lb parcel is 31.75 kg; a 40 lb carry-on is 18.14 kg. Converting before you pack avoids surprises at the counter. In all three cases, the direction matters: pounds to kilograms makes the number smaller.

Pounds-to-kilograms mistakes to avoid

These are the recurring errors in pound-to-kilogram conversions.

• Multiplying by 2.2 instead of dividing. That is the kg-to-lb direction. Going lb to kg makes the number smaller, not larger.
• Using 0.5 as the factor. It is close but not exact; 0.5 overstates the kilograms by about 10%. Use 0.45359237.
• Rounding in medical contexts. Weight-based dosing needs the exact value. Keep every digit until the final step.
• Confusing pounds-mass with pounds-force. In everyday use they are treated the same, but in engineering the pound-force is a different quantity requiring the gravitational constant.

Quick check: 1 lb is a bit under half a kilogram, so 200 lb should land near 90 kg. If your answer is near 440 kg, you multiplied by 2.2 instead of dividing.

Pounds to kilograms for travel and shipping limits

Airlines, couriers and freight carriers mix pounds and kilograms constantly, and the direction of the conversion decides whether you pay an overweight fee.

The key reference points: 1 lb = 0.45359237 kg, and the two limits people hit most are the 50 lb / 23 kg checked-bag rule and the 40 lb / 18 kg carry-on rule. A 50-pound bag is 22.68 kg, safely under a 23 kg limit. But a bag that weighs 51 pounds is 23.13 kg, over the 23 kg line, so the same bag passes a US 50 lb scale and fails a metric 23 kg scale. Converting before you leave home avoids that.

• 40 lb = 18.14 kg (carry-on)
• 50 lb = 22.68 kg (checked bag)
• 70 lb = 31.75 kg (heavy parcel)
• 100 lb = 45.36 kg (freight)
• 150 lb = 68.04 kg

For parcels, volumetric weight complicates things further, but the mass conversion itself is always this single multiplication. When a carrier quotes a limit, check which unit its scale uses, convert your package with the exact factor, and leave a small margin, a half-kilogram cushion covers scale calibration differences.

One more place this conversion matters is cooking and portioning imported goods. A recipe that calls for a two-pound roast is asking for 0.907 kg, so a metric butcher scale set to kilograms should read just under one. Similarly, a five-pound bag of flour is 2.268 kg. Whenever a US label and a metric scale disagree by a fraction, convert with the exact 0.45359237 factor before assuming the scale is broken.

Sources: BIPM SI Brochure (the official definition of the SI).

Where these conversion factors come from

The relationship between pounds and kilograms is fixed by international agreement rather than by local convention. The values used on this page follow the NIST guide to the SI (US Department of Commerce) and the BIPM SI Brochure, the two documents that define the exact values used worldwide.

Because these factors are exact by definition rather than measured, converting a number and converting it back always returns the number you started with, there is no rounding drift to worry about.