Weight Converter

Weight is the conversion people get wrong most often in medicine, cooking, and luggage. The US uses pounds and ounces for everything except drugs (grams and milligrams), while the rest of the world uses kilograms and grams for almost everything except newborn babies (still often reported in pounds and ounces in the UK and US). This converter handles every common unit, and every edge case below is a real story.

Pound (lb) comes from the Roman libra, which is why the abbreviation starts with L. The international avoirdupois pound was agreed in 1959 as exactly 0.45359237 kg — a definition with eight significant figures because it was a compromise between slightly different US, UK, and Canadian pounds that had drifted apart. The troy pound (used for precious metals) is 0.37324172 kg — about 18% lighter, which is why a 'pound of gold' and a 'pound of feathers' really aren't the same weight in professional contexts.

The kilogram was defined until 2019 as the mass of a specific platinum-iridium cylinder in a vault outside Paris — the last SI unit still tied to a physical object. It was replaced by a definition fixed to Planck's constant (6.62607015 × 10⁻³⁴ J·s exactly), meaning a kilogram is now defined by physics rather than by one cylinder slowly losing micrograms.

Kilograms to pounds: multiply by 2.2. Pounds to kilograms: divide by 2.2 or multiply by 0.454. For precise medical and lab work, use the full 0.45359237 factor — a 1% arithmetic error in a 60-kg patient's medication dose is a 600-mg discrepancy, which can matter for narrow-therapeutic-range drugs.

For airline baggage, the international carry-on limit is typically 7 kg (15.4 lb) and checked is 23 kg (50.7 lb). The US-specific 50-lb checked limit translates to 22.68 kg — airlines round the printed limit to 23 kg to avoid overcharging passengers by rounding error. Weigh before you leave home; airport scales read heavy more often than they read light.

• Kilograms
• Grams
• Milligrams
• Pounds
• Ounces
• Stones
• Metric Tonnes
• US Tons

• Mass vs. weight. A kilogram is mass (an invariant property); a pound is either mass (pound-mass) or force (pound-force) depending on context. On Earth they differ by a factor of standard gravity (9.80665 m/s²). Engineers working in imperial units have to watch the distinction. Cooks do not.
• Short ton, long ton, metric tonne. A US short ton is 2,000 lb (907.18 kg). A UK long ton is 2,240 lb (1,016 kg). A metric tonne is 1,000 kg (2,204.6 lb). These three differ by up to 12%. Shipping specs will use all three; always check.
• Troy ounce vs. avoirdupois ounce. A troy ounce (precious metals) is 31.1035 g. A 'regular' avoirdupois ounce is 28.3495 g. A '1 oz' gold coin contains about 9.7% more gold by mass than you'd expect from kitchen-scale logic.
• Stones. UK personal weight is reported in stones and pounds. 14 lb = 1 stone, so 10 st 6 lb = 146 lb = 66.2 kg. The decimal '10.6 stones' is 10 stones 8.4 pounds, not 10 stones 6 pounds. Do not mix the notations.
• Carats. A metric carat (diamond weight) is exactly 0.2 g. A karat (gold purity) is a fraction out of 24 and is not a weight unit at all. Same-sounding, totally different.

• Chicken breast, boneless skinless: a 'medium' one is about 170-200 g (6-7 oz). Recipes that call for '1 chicken breast' are assuming this, not a bodybuilder 350-g breast.
• One large egg, shell-off, is about 50 g (1.76 oz). Yolk is ~19 g, white is ~31 g.
• Standard cube of butter (US): 1 stick = 4 oz = 113 g.
• A can of soda: 12 fl oz water content weighs about 355 g; the packaged can is ~368 g.
• MacBook Air M3 (13-inch) is 1.24 kg (2.7 lb). An iPhone 15 is 171 g (6.0 oz).
• 50-lb checked-bag limit (most US airlines) = 22.68 kg; many airlines round to 23 kg.
• Adult male average, US: 90.6 kg (199.8 lb). Adult female average, US: 77.5 kg (170.8 lb).
• A typical house cat is 4-5 kg (9-11 lb). An African elephant bull is 5,000-6,000 kg.

• Grams over volume for baking. One cup of all-purpose flour is 120-150 g depending on how packed it is. Using grams eliminates that 25% variance.
• Weigh babies in grams. A 200 g swing in an infant's weight is diagnostically important. Pounds-and-ounces notation (5 lb 13 oz) hides the grams behind fraction arithmetic.
• Double-check luggage. Airline overweight fees are typically $100-200 per bag. A home scale that reads 23.0 kg does not guarantee the airport scale agrees — aim for 22 kg if you're close to the limit.
• For package shipping, dimensional weight often matters more than actual weight. A 5-kg box of feathers ships at the 15-kg rate if the dim-weight calculation pushes it there.

Related: Cooking Weight Converter · Density Converter · Force Converter.