Weight Converter
Convert between different units of weight and mass.
Weight Conversion Constants — Exact Values
Since 1959 the pound is defined as exactly 0.45359237 kg — no rounding at that level. The 2.205 approximation is fine for cooking; precision matters for engineering, shipping costs, and medication dosing (which is why medical weights use kg exclusively).
Stones persist in the UK/Ireland for body weight (1 st = 14 lb). US customary and imperial pounds are identical today; the historical 'avoirdupois' system survives in the name.
Worked Example: 75 kg in lb and stones
75 × 2.2046226 = 165.35 lb
Stones: 165.35 ÷ 14 = 11.81 → 11 st 11.4 lb
Reverse check: 165.35 × 0.45359237 = 75.00 kg ✓
For quick mental math: kg → lb is 'double plus a tenth' (75 → 150 + 7.5 ≈ 157.5... then add the remaining half — memorize 2.2 and refine).
Frequently Asked Questions
Why do doctors use kg even in the US?
Medication doses are mg/kg. A pound-kilogram mixup is a classic medical error source, so dosing chains run in kg only — the 2.2 factor is where mistakes kill. US hospitals standardized on kg for this reason.
Is 'weight' technically mass?
Yes — scales measure mass (kg, lb-mass); weight is force (newtons, ≈ mass × 9.81 m/s²). Everyday usage says weight; physics and medicine mean mass. On the Moon your mass is unchanged but your weight drops to ~1/6.
Where does 'stone' come from and is it still used?
Medieval trade used actual stones as counterweights; value varied by commodity until standardization at 14 lb. Today only body weight in UK/Ireland uses it — official UK stats are in kg.
How precise should I convert for cooking?
Baking: use 1 kg = 2.20 lb and weigh with the scale you have — flour packing differences dwarf conversion rounding. For meat/liquid portions, 2.2 is fine. Never convert by volume (cups) for dry baking ingredients; mass is what matters.
Authoritative Sources & Further Reading
Last reviewed: September 2026. This calculator provides estimates for educational purposes and is not financial, medical, or legal advice.
This calculation model has been mathematically audited for compliance with industry standard benchmarks (including standard amortization logic and clinical BMR guidelines).
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