About the Weight Converter

The weight converter translates between eight units of mass: milligram, gram, kilogram, metric tonne, ounce, pound, stone, and US short ton. All conversions are expressed in grams, the cgs unit of mass, with the kilogram being the SI base unit since the 2019 redefinition of the SI base units.

The kilogram was redefined in November 2018 (effective May 2019) in terms of the Planck constant, fixing its value at exactly 6.62607015 * 10^-34 kg m^2 / s. This replaced the previous definition based on the International Prototype Kilogram, a platinum-iridium cylinder kept at the BIPM in Sevres, France, whose mass had drifted by about 50 micrograms over a century. Per the SI Brochure 9th edition §2.3.1, the gram is then exactly 10^-3 kg.

The imperial units are defined exactly in terms of SI: the international pound is exactly 0.45359237 kg per the 1959 International Yard and Pound Agreement, which makes the ounce exactly 28.349523125 g (one sixteenth of a pound). The stone, used in the UK and Ireland for body weight, is exactly 14 pounds or 6.35029318 kg. The US short ton is 2000 pounds or 907.18474 kg; the long ton used in the UK is 2240 pounds.

The converter handles any real input, including negatives (meaningful in accounting for net weights) and zero. Conversions are exact within the limits of double-precision arithmetic.

How It Works

The converter uses the two-step method through grams:

base_in_grams = value * factor[from]
result = base_in_grams / factor[to]

The factor table is:

mg  = 0.001 g         g = 1 g            kg = 1000 g
t   = 1,000,000 g      oz = 28.3495 g     lb = 453.592 g
st  = 6,350.29 g       ton (US short) = 907,185 g

The metric units (mg, g, kg, t) are exact powers of ten. The imperial units (oz, lb, st, ton) are defined exactly in terms of the kilogram per the 1959 agreement: 1 lb = 0.45359237 kg, which is approximately 453.592 grams. The displayed value 453.592 is a rounded form of the exact factor; the actual computation uses the full-precision value.

The two-step method through grams avoids the need for a pairwise table of 64 entries. It also makes the conversion transparent: you can verify any result by converting both units to grams and dividing.

Note that the US short ton (2000 lb) is not the same as the long ton (2240 lb) used in the UK, nor the metric tonne (1000 kg). The converter uses the US short ton, which is the most common meaning of "ton" in American English.

Worked Examples

Converting 1 kilogram to pounds: the input in grams is 1000 g, and the result is 1000 / 453.59237 = 2.20462 lb. This is the origin of the popular approximation "1 kg is about 2.2 lb." The exact factor is 2.20462262, accurate to eight decimal places.

Converting 1 stone to kilograms: the input in grams is 6350.29318 g, and the result is 6.35029318 kg. A 10-stone person therefore weighs about 63.5 kg or 140 pounds. The stone is still used in the UK and Ireland for body weight, even though other measurements are metric.

Converting 1 US short ton to metric tonnes: the input in grams is 907,185 g, and the result is 0.907185 metric tonnes. A US short ton is therefore about 9.3% lighter than a metric tonne. This explains why shipping weights quoted in "tons" need clarification: the US short ton, the UK long ton, and the metric tonne differ by about 7 percent between consecutive values.

When to Use This Tool

Use the weight converter when you need to:

Convert body weight between stones, pounds, and kilograms for medical or fitness tracking.
Translate cooking measurements between metric and imperial recipes.
Compute shipping weights for international freight quotes.
Convert supplement dosages between milligrams and ounces.
Translate produce prices quoted per pound into per-kilogram equivalents.
Convert bulk commodity weights between metric tonnes and US short tons.
Verify that a labelled weight matches the actual measured weight across unit systems.

Limitations & Disclaimer

This converter uses exact conversion factors defined in SI and the 1959 International Yard and Pound Agreement. It does not handle troy ounces, carats, grains, or long tons. The "ton" option refers to the US short ton (2000 lb), not the UK long ton or the metric tonne. Results are subject to IEEE 754 floating-point precision for very large or very small magnitudes. See our disclaimer for full details.

Frequently Asked Questions

Why was the kilogram redefined in 2019?

Because the previous definition, based on the International Prototype Kilogram (a platinum-iridium cylinder kept at the BIPM in Sevres), had drifted by about 50 micrograms over a century due to surface contamination and cleaning. The 2019 redefinition fixes the Planck constant at exactly 6.62607015 * 10^-34 kg m^2 / s, which makes the kilogram stable and reproducible from fundamental constants.

What is the difference between a US short ton, a UK long ton, and a metric tonne?

The US short ton is 2000 pounds (907.185 kg). The UK long ton is 2240 pounds (1016.05 kg). The metric tonne is 1000 kg (2204.62 lb). The three differ by about 7 percent between consecutive values. This converter uses the US short ton, the most common meaning of 'ton' in American English.

Why is the stone still used for body weight?

The stone (14 pounds or 6.35 kg) is still used in the UK and Ireland for body weight, even though other measurements have been metricated. Scales in those countries often display both stones and kilograms. The stone is rarely used for anything other than body weight.

What is the difference between mass and weight?

Mass is the amount of matter in an object, measured in kilograms. Weight is the gravitational force on that mass, measured in newtons. On the surface of the Earth, a 1 kg mass weighs about 9.81 N. The converter works on mass units; converting weights measured on the Moon or Mars would require adjusting for local gravity.

Are the conversion factors exact?

The metric-to-metric conversions are exact. The imperial-to-metric conversions are also exact, since the 1959 International Yard and Pound Agreement defined the international pound as exactly 0.45359237 kg. The displayed factors (such as 28.3495 for the ounce) are rounded for readability; the actual computation uses the full-precision value.

Does the converter handle troy ounces?

No. Troy ounces (about 31.1035 g) are used for precious metals and differ from avoirdupois ounces (about 28.35 g). This converter uses avoirdupois ounces, the common everyday ounce. For precious-metal conversions, use a dedicated troy-weight converter.

Last updated: September 9, 2026 · Author: HT99 Tools Editorial Team