Weight Converter

What this calculates

Enter a weight in milligrams, grams, kilograms, tonnes, ounces, pounds or stone, and this shows it in all seven at once, with each conversion worked through. It also places the weight on a scale running from a tenth of a milligram to a thousand tonnes, so an ounce and a tonne can be read against each other.

Every figure comes from exact definitions — nothing is converted with a rounded shortcut factor.

The formula

Every conversion goes through kilograms: into kilograms with one unit's size, then out again with another's.

The metric units are powers of ten: a tonne is 1,000 kg, a kilogram is 1,000 g or 1,000,000 mg. The imperial units are exact too, and each one builds on the pound.

Because every factor is exact, a conversion is exact until it is rounded for display — and the calculator rounds each result once, from the exact figure.

Worked example

Not every conversion comes out round. 10 kg is 10 ÷ 0.45359237 = 22.04623 lb and 352.7396 oz, each shown to seven significant figures, because a kilogram is not a whole number of pounds.

Going from an imperial unit into a metric one is exact: 1 lb is 1 × 0.45359237 = 0.45359237 kg in the working, and 16 oz comes back as exactly 1 lb.

When to use it, and the mistakes to avoid

Use it when a recipe, a product listing or a set of instructions gives a weight in a unit you do not think in, when checking a body weight or a parcel weight, and when a figure is quoted in kilograms and you think in pounds or stone, or the other way round.

The mistakes that cost the most:

• Treating kilograms and pounds as the same. A kilogram is about 2.2 pounds — exactly 1 ÷ 0.45359237 — so 10 kg is 22.04623 lb, not 10. The quick "times 2.2" rule gives 220 lb for 100 kg against the true 220.4623 lb: close for a rough sense of weight, but not an answer.
• Reading stone and pounds as a decimal. 11 st 4 lb means 11 stone and 4 pounds, which is 158 lb or 71.66759 kg. It is not 11.4 st: 11.4 st is 159.6 lb, or 72.39334 kg. Convert the stone to pounds and add the pounds before doing anything else.
• Confusing ounces with fluid ounces. An ounce here is a weight — a sixteenth of a pound. A fluid ounce measures volume, and how much a fluid ounce weighs depends on what is in it. This calculator converts weight only.
• Assuming a "ton" is a tonne. A tonne is 1,000 kg, which is 2,204.623 lb. The word "ton" is also used for non-metric units of a different size, so check which one a figure means before converting; this calculator uses the metric tonne only.
• Expecting every digit of every result. Results are shown to seven significant figures — 1 lb shows as 453.5924 g, not 453.59237 g — while the working keeps the exact kilogram figure. A weight far too small for a unit, such as 0.0000000001 mg in pounds, reads as 0, and a note under the result says so rather than leaving a silent zero.
• Entering a negative weight. A weight cannot be less than zero, so the calculator refuses one with a message instead of converting it.

FAQ