Weight Converter

Kilograms, grams, tonnes, pounds, ounces and stone, any pair in either direction. The result panel shows the other units alongside your answer and prints the conversion factor it used, so you can check the arithmetic rather than trust it.

One defined number sits under all six units

The pound is not measured against the kilogram. Since the international yard and pound agreement of 1959, it is *defined* as exactly 0.45359237 kilograms — a legal definition adopted by the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa, with no uncertainty attached and no experiment that could revise it.

Everything customary here descends from that one line:

• 1 ounce = 0.45359237 ÷ 16 = 0.028349523125 kg exactly, or 28.349523125 grams
• 1 stone = 0.45359237 × 14 = 6.35029318 kg exactly
• 1 kilogram = 2.2046226218… pounds, and this one never terminates

Notice which direction is exact. Pounds to kilograms is a finite decimal; kilograms to pounds is a reciprocal that runs forever. The familiar 2.2 is a shortcut, and on a 1,000 kg pallet it is off by 4.6 pounds.

A conversion worked through

Enter 70 in kilograms with pounds selected as the target. The tool computes 70 ÷ 0.45359237 = 154.3235835 lb, and prints the factor 2.2046226218 lb per kg beside it. The other rows fill in at the same time: 70,000 g, 0.07 t, 2,469.18 oz, 11.02 st.

Going the other way, 12 stone is 12 × 6.35029318 = 76.20351816 kg, and 500 g is 17.6369810 oz.

Rounding is where a round trip goes astray

The conversion itself is computed at full precision, and the decimal-places control only affects display — an earlier build rounded inside each function before the control ever saw the number, so asking for ten decimals still showed four. That is fixed.

The trap that remains is copying the rounded figure. Convert 1 kg to pounds at four decimal places, read 2.2046, and convert that back: 2.2046 × 0.45359237 = 0.99998974 kg. You are ten milligrams short of where you started, purely because the number you carried across was truncated. When a value has to survive a return journey, either raise the precision or keep the original.

Mass, weight, and what the scale is doing

The units on this page measure mass, and the page name is the everyday word rather than the physical one. A kilogram is the same kilogram in orbit; a *weight* — the force gravity exerts on it — is not. For shipping, cooking and body measurements the distinction never bites. For structural or aerospace work it does, and kilograms-force and newtons are not offered here.

Two things this deliberately leaves out. The ounce is avoirdupois, the everyday commercial ounce of 28.349523125 g. Precious metals are quoted in troy ounces of exactly 31.1034768 g — nearly 10% heavier — and that unit is not in the list, so do not price gold with it. And the fluid ounce shares a name with neither: it measures volume, and no converter can turn it into mass without knowing what the liquid is.

Precision beyond the scale is decoration

An eight-decimal answer is arithmetically correct and often practically meaningless. A bathroom scale reads to 100 g at best and a luggage scale is worse. If the input was 70 kg because the display said 70, the honest output is "about 154 pounds", not 154.3235835. Carry the extra digits only when the measurement genuinely justifies them.

Your unit selection and precision setting are remembered between visits. The value itself is not — reload and the box is empty.

Other converters here

• Length Converter — metres, feet, inches and miles from one defined inch.
• Temperature Converter — Celsius, Fahrenheit, Kelvin and Rankine.
• BMI Calculator — takes height and weight in either system directly.