Pounds and ounces use two systems
The metric units here scale by powers of ten: 1 kg = 1,000 g = 1,000,000 mg = 1,000,000,000 mcg. The avoirdupois pound has 16 ounces; the separate troy pound has 12 troy ounces. A troy ounce (31.1034768 g) is about 9.7% heavier than an avoirdupois ounce. For precious-metal sales based on weight, NIST Handbook 130 §2.17.2 states that the units "shall be either troy weight or SI units". This difference matters when a weight is stated without its system: a one-ounce gold coin and a one-ounce bag of flour do not have the same mass. An avoirdupois pound divides into whole-ounce halves, quarters, and eighths (8, 4, and 2 oz). A 12-ounce troy pound divides into whole-ounce halves and quarters (6 and 3 oz), while an eighth is 1.5 troy ounces.
The 1959 agreement among the standards laboratories of the US, UK, Canada, Australia, New Zealand, and South Africa set the international pound at exactly 0.45359237 kg. It replaced small differences among the earlier national standards, letting this converter use one figure for the avoirdupois pound. A US "short ton" (2,000 lb, the one this tool uses) and a British "long ton" (2,240 lb) are different units; neither equals a metric tonne (1,000 kg). Check which system a source document uses before comparing ton values.
The exact factors this converter uses
Micrograms, milligrams, grams, and kilograms convert to each other by exact powers of ten (1 kg = 1,000 g = 1,000,000 mg = 1,000,000,000 mcg), since that is how the SI system is built. The imperial bridge is fixed by legal definition rather than by measurement: 1 lb = 453.59237 g exactly (NIST Handbook 44, Appendix C, marks this "exact"). Neither the ounce nor the US ton has its own independent definition. Both are derived arithmetically from that same fixed pound: 1 oz = 1 lb ÷ 16 = 28.349523125 g, and 1 US ton = 2,000 lb = 907,184.74 g.
Because grams and pounds share no common round factor, pound conversions often need several decimal places: 1 kg is 2.204623 lb, while 2.2 is a rougher estimate. This tool uses standard floating-point arithmetic and displays results rounded to six significant figures, with trailing zeros omitted. Switching the input unit writes that rounded result into the input field. Repeated switches can therefore introduce small differences; enter the original measurement when precision matters.
Worked example: body weight
A gym scale reads 68 kg, and a US fitness app wants pounds. Converting: 68 × (1,000 ÷ 453.59237) = 149.914 lb, which rounds to 149.9 lb at one decimal place. Going the other way, a US doctor's chart listing 150 lb converts to 150 × 0.45359237 = 68.0389 kg, close to but not exactly the 68 kg the scale first showed. The calculator rounds the converted result to six significant figures.
Worked example: a newborn's weight
Suppose a birth weight is 8 lb 6 oz and needs to be expressed in metric units. Converting: 8 lb 6 oz is 134 oz total (8 × 16 + 6), and 134 × 28.349523125 = 3,798.84 g, or 3.799 kg. Rounded to one decimal place, this is 3.8 kg. Parents comparing weight against age in weeks versus months can switch between those units with the time converter.
Worked example: a medication dose
For example, a prescription may state a dose as 200 mcg while another reference gives the amount in milligrams. Since 1 mg = 1,000 mcg exactly, 200 mcg = 0.2 mg. That distinction matters clinically: typing "200" into a field expecting milligrams when the real value is in micrograms would be a 1,000-fold dosing error. Always convert dosing units explicitly; never assume the number carries over unchanged.
The stone unit
The Weights and Measures Act 1985 (Schedule 1, Part VI) defines a stone as exactly 14 avoirdupois pounds. At 0.45359237 kg per pound, that is 6.35029318 kg.
Worked example: converting a UK body weight
A person reports a body weight of 11 st 3 lb and needs the figure in kilograms. First convert to a single unit: 11 st 3 lb = (11 × 14) + 3 = 157 lb. Then apply the exact pound: 157 × 0.45359237 = 71.21400209 kg, which this converter displays as 71.214 kg.
Going the other way, a 60 kg reading converts to 60 ÷ 6.35029318 = 9.4484 stone. Since stone readings are conventionally given as whole stone plus remaining pounds rather than a decimal, that splits into 9 st plus 0.4484 st × 14 lb/st = 6.28 lb, commonly rounded to "9 stone 6."