Kilograms vs Pounds (kg vs lb)

The kilogram (kg) is the SI base unit of mass. The pound (lb), more precisely the avoirdupois pound, is the unit of mass in the US customary and British imperial systems.

Since the international yard and pound agreement of 1959, the pound is defined through the kilogram: 1 lb = 0.45359237 kg exactly. Every pound conversion follows from this one number, so the result is the same in every country that uses the agreement.

• Pounds to kilograms: multiply by 0.45359237.
• Kilograms to pounds: divide by 0.45359237, which is the same as multiplying by about 2.2046226.
• Example: 150 lb × 0.45359237 = 68.038856 kg.
• Example: 80 kg ÷ 0.45359237 = 176.37 lb.

Keep the full factor during the calculation and round only the final answer. For body weight or groceries, one decimal place is usually enough.

To estimate pounds from kilograms, multiply by 2.2. The real factor is about 2.20462, so the shortcut comes out about 0.2% low. For 100 kg it gives 220 lb, while the exact value is 220.462 lb.

To go the other way, divide pounds by 2.2. For 200 lb that gives about 90.9 kg, close to the exact 90.7185 kg. The error is small enough for everyday use, but use the exact factor for shipping labels, medicine or anything you have to declare.

A pound is divided into 16 ounces, so one ounce (oz) is exactly 0.45359237 ÷ 16 kg, about 28.3495 g. In the other direction, 100 g is about 3.5274 oz.

• 1 lb = 16 oz (exact)
• 8 oz ≈ 226.8 g
• 1 kg ≈ 35.274 oz

These are avoirdupois ounces, used for food and everyday goods. Precious metals use the troy ounce, which is a different, heavier unit.

Kilograms and pounds both measure mass, the amount of matter in an object. Weight is a force: the pull of gravity on that mass. A scale measures force and shows mass by assuming normal gravity, which is why the two words are used loosely in daily life.

In engineering, force has its own units. The pound-force (lbf) is the weight of a one-pound mass under standard gravity, about 4.44822 newtons. Writing lb for mass and lbf for force avoids confusion in technical work.

• Multiplying when you should divide. A value in pounds is always a larger number than the same mass in kilograms.
• Rounding the factor to 0.45 or 2.2 in a calculation that needs precision, then rounding the result again.
• Mixing up the pound (mass) with the pound-force (force) in technical specifications.