The factor
kg = lb × 0.45359237. That is the definition of the pound, agreed in 1959, so there is no rounding beyond the digits shown. Halving the pounds and taking off a tenth of the result gets close: 200 lb halved is 100, less 10 is 90, against the exact 90.72 kg.
Worked examples
Rounded to two decimal places:
| Pounds | Kilograms |
|---|---|
| 1 lb | 0.45 kg |
| 5 lb | 2.27 kg |
| 10 lb | 4.54 kg |
| 45 lb | 20.41 kg |
| 100 lb | 45.36 kg |
| 150 lb | 68.04 kg |
| 180 lb | 81.65 kg |
| 200 lb | 90.72 kg |
| 250 lb | 113.4 kg |
A 45 lb gym plate is 20.41 kg, close to a 20 kg plate but not the same.
Questions
How do I convert pounds and ounces?
Divide the ounces by 16 and add them to the pounds: 7 lb 8 oz is 7.5 lb, which is 3.4 kg. Type 7.5 here.
How many kg is a stone?
6.35 kg, since a stone is 14 lb. The Weight Converter has stone in its list.