Weight Converter

Converts between kilograms, pounds, ounces, grams, and stone. Internal arithmetic uses the international avoirdupois pound (0.45359237 kg exactly) and the international troy ounce, with avoirdupois ounce as default.

Stone (14 lb) is still common in UK body-weight contexts. Imperial and US customary pounds match exactly; the divergence appears only in larger units like the ton.

The 1959 International Yard and Pound Agreement, signed by the US, UK, Canada, Australia, New Zealand, and South Africa, defined the international avoirdupois pound as exactly 0.45359237 kilograms. Before that, the US and UK used slightly different definitions that diverged at the seventh decimal place, enough to matter in precision physics but not in commerce. The avoirdupois system underlies almost all everyday weighing; the troy system survives for precious metals (one troy ounce ≈ 31.1035 g, distinct from the avoirdupois ounce of 28.3495 g) and the apothecaries' system remains in pharmaceutical compounding for some legacy formulations.

A worked example: a person weighing 80 kg corresponds to 176.37 lb, 12 stone 8.4 lb, or 2,821.92 ounces avoirdupois. Converting between metric and imperial body weights is among the most common everyday uses of this kind of tool, particularly for visitors to or from the UK where stone-and-pound notation persists in casual conversation despite metric being the legal standard. Recipe scaling, shipping cost calculations, and freight invoices are the next most common contexts.

Limitations cluster around units the tool deliberately omits. Tons are not enabled by default because the term is overloaded: short ton (US, 2000 lb ≈ 907 kg), long ton (UK, 2240 lb ≈ 1016 kg), and metric ton or tonne (1000 kg exactly) differ by up to 12%. Picking the wrong one in a shipping quote or carbon-footprint calculation is a recurring error. The grain (1/7000 of a pound, ≈ 64.8 mg) is omitted; it survives mostly in firearms (bullet weight) and pharmacology. The mass-versus-weight distinction (mass is intrinsic, weight depends on local gravity) is collapsed for everyday use; the converter operates in mass units even when users colloquially say 'weight'. On the Moon, a 1 kg mass weighs about 1.62 N rather than 9.81 N, but the kilogram count itself is unchanged.

For precision contexts (laboratory, jewelry, postal), specifying the exact unit family matters more than the conversion factor. A '5 ounce' gold bar at troy weight is 155.5 g; at avoirdupois weight it would be 141.7 g, an 8.7% difference and a several-thousand-dollar discrepancy at typical gold prices.