The unit symbol lb denotes the international avoirdupois pound, and kg denotes the kilogram (SI base unit of mass). The lb to kg conversion is based on an exact definition linking the pound to the kilogram.
Direct rule: kg = lb × 0.45359237
Exact conversion factor
The international avoirdupois pound is defined so that:
\[ 1\ \text{lb} = 0.45359237\ \text{kg} \]
This value is exact, so converting lb to kg is a straightforward multiplication.
lb to kg formula
For a mass \(m_{\text{lb}}\) measured in pounds, the mass in kilograms \(m_{\text{kg}}\) is:
\[ m_{\text{kg}} = m_{\text{lb}}\cdot 0.45359237 \]
Worked example
Convert \(150\ \text{lb}\) to kilograms.
\[ m_{\text{kg}} = 150\cdot 0.45359237 = 68.0388555 \]
Rounding to two decimal places gives \(68.04\ \text{kg}\).
Example: 150 lb = 68.04 kg
Common conversions table
The following lb to kg values are rounded to two decimal places for practical use.
| Pounds (lb) | Kilograms (kg) | Calculation |
|---|---|---|
| 1 | 0.45 | \(1\cdot 0.45359237 = 0.45359237\) |
| 5 | 2.27 | \(5\cdot 0.45359237 = 2.26796185\) |
| 10 | 4.54 | \(10\cdot 0.45359237 = 4.5359237\) |
| 25 | 11.34 | \(25\cdot 0.45359237 = 11.33980925\) |
| 50 | 22.68 | \(50\cdot 0.45359237 = 22.6796185\) |
| 100 | 45.36 | \(100\cdot 0.45359237 = 45.359237\) |
| 200 | 90.72 | \(200\cdot 0.45359237 = 90.718474\) |
Helpful inverse relationship
Sometimes the reverse conversion is needed. Since \(1\ \text{kg} \approx 2.20462262\ \text{lb}\), a convenient inverse form is:
\[ m_{\text{lb}} = \frac{m_{\text{kg}}}{0.45359237} \approx m_{\text{kg}}\cdot 2.20462262 \]
Rounding and precision
- For everyday use, \(0.4536\) is a common rounded lb to kg factor.
- For scientific or engineering work, keep more digits or use the exact factor \(0.45359237\) to control rounding error.
- Round the final kg value to a precision appropriate for the measurement instrument (scale resolution).