Conversion Methodology & Standards
Factors based on the 1959 International Yard and Pound Agreement and NIST Special Publication 811. Every calculation in our suite routes through the SI base unit of mass (the kilogram).
1. Exact Conversion Multipliers
Under international law, imperial and US customary units of weight (mass) are legally defined by exact mathematical relationships to the metric kilogram. We never use arbitrary approximations.
| Unit | Symbol | Measurement System | Exact Factor in Kilograms | Legal / Metrological Basis |
|---|---|---|---|---|
| Pound (avoirdupois) | lb | Imperial / US Customary | 0.45359237 kg | 1959 International Yard and Pound Agreement |
| Ounce (avoirdupois) | oz | Imperial / US Customary | 0.028349523125 kg (28.349523125 g) | Exactly 1/16 of an avoirdupois pound |
| Stone | st | British Imperial | 6.35029318 kg | Exactly 14 avoirdupois pounds |
| Gram | g | International System (SI) | 0.001 kg | 1/1,000 of the SI base unit (kilogram) |
| Milligram | mg | International System (SI) | 0.000001 kg | 1/1,000,000 of a kilogram |
| Metric Ton (Tonne) | t | Non-SI unit accepted for use with SI | 1,000 kg | Exactly 1 megagram (1,000 kg) |
| US Short Ton | ton (US) | US Customary | 907.18474 kg | Exactly 2,000 avoirdupois pounds |
| Imperial Long Ton | ton (UK) | British Imperial | 1,016.0469088 kg | Exactly 2,240 avoirdupois pounds (160 stones) |
2. Rounding Policy & Precision Architecture
Computers store floating-point numbers in binary (IEEE 754), which can introduce microscopic representation errors (such as 0.1 + 0.2 = 0.30000000000000004). To ensure scientific accuracy, our engine follows three strict guidelines:
- Kg as the Universal Hub: Conversions between any two units X and Y first compute exact kilograms:
kg = X × factorX, then calculate target unit:Y = kg ÷ factorY. - Epsilon-Safe Rounding: Displayed values use an epsilon offset (
Number.EPSILON) before rounding to avoid truncation errors on floating boundaries. - Linguistic Accuracy: We reserve the term "exactly" exclusively for mathematical definitions (e.g. 1 lb = 0.45359237 kg, 1 lb = 16 oz, 1 st = 14 lb). All rounded numbers are clearly presented as "about" or "approximately".
3. Sources
Our formulas and historical factors reference official metrological bodies:
BIPM SI Brochure (9th Edition)
International Bureau of Weights and Measures definition of the kilogram via the Planck constant (h = 6.626 070 15 × 10⁻³⁴ J·s).
Visit BIPM.org →NIST Special Publication 811
National Institute of Standards and Technology Guide for the Use of the International System of Units (SI).
Visit NIST.gov SP 811 →1959 International Yard & Pound Agreement
Treaty signed by the US, UK, Canada, Australia, New Zealand, and South Africa defining the international avoirdupois pound as exactly 0.45359237 kg.
Historical Treaty Documentation →National Physical Laboratory (NPL)
The UK National Metrology Institute guidance on mass metrology and imperial stone equivalence.
Visit NPL.co.uk →4. Changelog & Version History
Added two-input mixed conversion tools (kg to lb + oz, lb + oz to kg), comprehensive printable lookup tables, updated NIST SP 811 references, removed puffery badges, and enforced strict singular/plural grammar across all 20 hub pages.
Introduced decimal precision selectors (1, 2, 4, 6 digits), floating-point safe division, and instant clipboard copying.
Initial launch of Weight Converter Hub with 8 core units and 22 primary unit pair converters.