Unit definitions
The site uses the international avoirdupois pound. One pound is exactly 0.45359237 kilograms, which also makes one pound exactly 453.59237 grams. Within the avoirdupois system, one pound contains exactly 16 ounces. One stone contains exactly 14 pounds.
The reverse kilogram-to-pound value is calculated by dividing by 0.45359237. The familiar factor 2.2046226218 is therefore an approximation, not a replacement for the exact definition.
Internal precision
Conversion constants live in one shared JavaScript module. Calculators keep the unrounded numeric result for subsequent steps and do not substitute shortened mental-math factors. Browser calculations use standard IEEE 754 double-precision arithmetic, which is more precise than the displayed result throughout the supported input range.
Display rounding
Rounding changes presentation, not the stored calculation. Primary results show up to six decimal places, with shorter unit-specific displays where that is easier to read. Precision lines retain additional digits. Static reference tables use up to six decimal places, while exact whole-unit results omit unnecessary trailing zeros.
For example, 150 × 0.45359237 equals 68.0388555 kg. At six decimal places this is displayed as 68.038856 kg.
Input parsing and validation
Both decimal points and decimal commas are supported. Clearly grouped mixed formats such as 1,234.56 and 1.234,56 are accepted. A value such as 1,234 is rejected as ambiguous instead of being silently treated as either 1234 or 1.234. Empty, negative, non-numeric, zero-disallowed, and out-of-range values receive inline guidance.
Testing
Automated tests cover published conversion values, reverse conversions, mixed pounds and ounces, parser edge cases, product unit-price comparison, barbell plate loading, dimensional weight, and round trips such as lb → kg → lb. Static tables are generated from the same defined factors and checked separately.
Quick estimates and exact results
Mental estimates such as 1 kg ≈ 2.2 lb are useful for orientation. Calculator results use the full defined relationship. The appropriate final rounding still depends on the precision of the original scale and the real-world decision.
Authoritative references
- NIST Handbook 44, Appendix B: Units and Systems of Measurement — publishes the avoirdupois pound relationship to the kilogram.
- BIPM SI Brochure — the authoritative description of the International System of Units.
- BIPM: SI base unit kilogram — describes the SI unit of mass.
- UK Weights and Measures Act 1985, section 1 — states the pound-to-kilogram relationship for UK measurement law.