Weight is one of the oldest measured quantities and one of the most fragmented — carats for gems, grains for ammunition and pharmacy, hundredweights for agriculture, tonnes for shipping, and three different 'tons' for whoever you're negotiating with. Getting the conversion right matters for medical dosing, precious metals trade, international freight and any technical field where a decimal error becomes a lawsuit.

The base of the modern system is the kilogram, redefined in 2019 from the Planck constant (replacing the last physical artifact standard). Everything else derives from it: 1 lb = 0.45359237 kg exactly; 1 oz = 28.349523125 g; 1 grain = 64.79891 mg; 1 carat = 200 mg exactly. These are internationally defined constants, not measurements — the calculator uses full precision to the 6th decimal.

The ton problem catches shippers regularly. US 'short ton' = 2000 lb (907.185 kg). UK 'long ton' = 2240 lb (1016.047 kg). Metric tonne = 1000 kg = 2204.62 lb. A cargo quoted at '1000 tons' could vary by 10% depending on which ton. Contracts should always specify — the calculator distinguishes all three.

In medicine, precision is life-critical. Pediatric drug doses are calculated per kilogram; a rounding error on a 6 kg infant is 10× a rounding error on a 60 kg adult. Modern electronic prescribing systems require kg with at least one decimal (2 decimals for neonates). Never use lb for medical dosing — the conversion introduces avoidable error at the point of care.

For everyday life, the two conversions worth memorizing: 1 kg ≈ 2.2 lb (or divide lb by 2.2 for kg), and 1 stone = 14 lb (UK). The calculator handles everything else — plus grains for reloaders, carats for jewelers, hundredweights for farmers, and metric tonnes for the freight forwarder in your inbox. One tool covers 15+ units so you never have to open five browser tabs to move numbers between systems.