Kilograms vs pounds: the 2.20462 you can never quite remember

Three quarters of the world uses kilograms. The US (and a few smaller jurisdictions) uses pounds. The conversion factor — 1 kg = 2.20462 lb, exactly — is one of those numbers that nobody quite remembers and everyone needs at least once a week. Here’s where it comes from and how to use it without a calculator.

The conversion factor

1 lb = 0.45359237 kg (exact, by definition)
1 kg = 2.20462262… lb (the inverse, irrational)

The pound was redefined at exactly 0.45359237 kg by the 1959 international yard and pound agreement, signed by the US, UK, Canada, Australia, South Africa, and New Zealand. Before then, “pound” meant slightly different masses in different countries — the imperial pound was 7000 grains, the US pound was 7000 grains too but defined against a different copy of the standard kilogram in Paris. The 1959 agreement unified them.

Run any conversion in either direction via our kg to lb converter or its reverse.

Mental shortcuts

Quick estimate: × 2.2 (1% accurate)

To go from kg to lb, multiply by 2.2. That’s 0.2% under-estimating the true 2.20462. Within 1% even for the largest realistic weights.

• 70 kg ≈ 154 lb (true: 154.32)
• 100 kg ≈ 220 lb (true: 220.46)
• 250 kg ≈ 550 lb (true: 551.16)

The reverse: ÷ 2.2 (or × 0.45)

Lb to kg, divide by 2.2 or multiply by 0.45. Either gets you within a few percent.

• 200 lb ≈ 91 kg (true: 90.72)
• 150 lb ≈ 68 kg (true: 68.04)
• 100 lb ≈ 45 kg (true: 45.36)

Where each unit dominates

• Body weight: kg in the rest of the world; lb in the US. Both report consistently to one decimal.
• Grocery weight (produce, meat): kg in metric countries; lb in the US, often with the “each” option for items sold by count.
• Scientific / engineering: kg universally. Even US physics journals use SI internally and translate to lb only at the popular-science interface.
• Cooking: mostly grams (a sub-unit of kg) in modern professional kitchens; ounces and pounds in US home cooking.
• Air travel: kg internationally, lb on US domestic carriers — both denominated to one decimal for baggage limits.

The two cases where the difference actually matters

1. Medication dosing

Most paediatric (and a meaningful subset of adult) drug doses are per-kg. A US patient weighing “150 lb” needs to be converted to 68 kg before computing dose. Getting this wrong is one of the most-documented medication-error categories in the US literature. If you see a prescription whose dose maths only works in pounds, ask.

2. Weight-class sports

Boxing, MMA, weightlifting, and martial arts categorise athletes by weight. International rules are in kg; US-only amateur leagues sometimes still publish in lb. A 145 lb fighter and a 65 kg fighter are not the same person — 65 kg is 143.3 lb. At weigh-in this difference can be the difference between making the class and missing it.

The deeper trivia

The kilogram was originally defined as the mass of a litre of pure water at 4 °C. That definition was abandoned in 1889 because measuring water purity precisely is impossible at scientific scale. The Kilogramme des Archives (a platinum-iridium artefact in a vault outside Paris) became the reference. In 2019 the kilogram was redefined again, this time via the Planck constant — the artefact is now an artefact of history, not metrology.

The pound, by contrast, has no defining artefact. It’s defined purely as 0.45359237 of the kilogram, which means every redefinition of the kilogram automatically redefines the pound. Convenient.

The honest takeaway

For everyday work, use 2.2 as the mental factor and our converterfor precision. For anything medical, athletic, or scientific, use the exact factor — and double-check it didn’t round to a suspiciously clean number along the way.