The Exact International Standard
Under the 1959 International Yard and Pound Agreement signed by the United States, United Kingdom, Canada, Australia, New Zealand, and South Africa, the international avoirdupois pound is legally defined relative to the metric kilogram as:
Taking the mathematical reciprocal of this constant gives the multiplier for converting kilograms to pounds:
For all engineering, commercial, and fitness applications, this is rounded to $2.20462$ (or 2.205).
The Conversion Formulas
1. Kilograms to Pounds:
2. Pounds to Kilograms:
Fast Mental Math Shortcut: "Double and Add 10%"
When you do not have a calculator, you can compute kilograms to pounds in your head with less than $0.2\%$ error:
Worked Example:
Convert $75\text{ kg}$ to pounds mentally:
Mass vs. Weight: Physical Distinction
In scientific physics:
On the Moon (where gravity is 1/6th of Earth), an 80 kg human still has a mass of 80 kg, but would weigh only โ 29.4 lbs on a spring scale.
