Weight Conversion Calculator

Weight conversion expresses the same physical mass using a different unit, based on a fixed relationship between the two. A kilogram and a pound both measure mass, but the metric and imperial systems size their units differently. The metric system builds on base-ten relationships, so a kilogram is exactly 1,000 grams. The imperial system uses historical conventions instead, such as 16 ounces to a pound or 14 pounds to a stone.

Nearly every country outside the US, Liberia, and Myanmar uses the metric system officially. That's why international shipping labels and most food packaging list weight in kilograms or grams. A metric to standard conversion, in other words, is often just as necessary as the reverse. The US, and to a lesser extent the UK, still rely on pounds, ounces, and stones for body weight, cooking, and retail. Because both systems stay in active use worldwide, converting between them is a routine need. The factors themselves are fixed by international standards, not estimates.

Every conversion relies on a fixed factor: how many units of the target measurement equal one unit of the source. To convert kilograms to pounds, multiply by 2.20462, since one kilogram equals 2.20462 pounds. To go the other way, divide by 2.20462, or multiply by its inverse, 0.453592. The same multiply-or-divide logic applies to every other unit pair, from grams to ounces to stones to tons.

The direction determines whether you multiply or divide, and mixing this up is the most common source of errors. A quick sanity check helps: converting to a smaller unit should produce a larger number, and vice versa. This calculator handles the direction automatically based on the units you pick. For manual checks, or to embed the logic in your own code, the exact formulas are below.

The formulas below use the standard avoirdupois pound, the definition used for body weight, food, and most US/UK commerce. Kilograms to pounds: multiply by 2.20462262. Pounds to kilograms: multiply by 0.45359237. Grams to ounces: multiply by 0.03527396. Ounces to grams: multiply by 28.349523125. These constants are exact by international definition, so any error comes from display rounding, not the formula.

For developers who want to reproduce these conversions programmatically, here is the core function in JavaScript and Python.

function kgToLb(kg) {
  return kg * 2.20462262;
}

function lbToKg(lb) {
  return lb * 0.45359237;
}

def kg_to_lb(kg):
    return kg * 2.20462262

def lb_to_kg(lb):
    return lb * 0.45359237

These two constants are exact inverses to eight decimal places, precise enough for scientific, commercial, and everyday use. Rounding to fewer decimals, such as the commonly cited 2.2, works fine for mental math but drifts over repeated conversions or large values. This calculator uses full precision internally and only rounds the final displayed result, so accuracy holds even across multi-step conversions.

Kilograms (kg) and grams (g) are the primary metric units for everyday weight measurements. One kilogram equals 1,000 grams, and one gram equals 1,000 milligrams (mg), the unit used for very small masses like medication doses. Pounds (lb) and ounces (oz) are the primary imperial units, with one pound equal to 16 ounces. Stones (st) are used mainly in the UK and Ireland for body weight. Each stone equals 14 pounds, so someone weighing 154 pounds is 11 stone.

Three units cause frequent confusion because their names overlap without meaning the same thing. A metric ton (tonne) equals 1,000 kilograms. A long ton (the UK imperial ton) equals 2,240 pounds. A short ton (the US ton) equals 2,000 pounds, so the three are not interchangeable. Troy ounces, used for gold and silver, equal 31.1034768 grams, about 10% heavier than a standard ounce. Carats, used for gemstones, equal exactly 200 milligrams by international definition.

The table below shows how one unit of each common weight measurement converts into every other major unit. It works as a measurement conversion chart for standard values, rounded for readability. For a custom or unusual value, use the interactive converter above instead of scaling these figures by hand. Manual scaling compounds rounding error.

This chart covers the eight most-searched units. The interactive converter above also supports troy ounces and carats for jewelry and precious metals. For a value not shown directly, like stones to grams, convert to kilograms first as an intermediate step. Every row here uses kilograms as its base.

Body weight, kg to lb. Someone weighing 70 kg wants their weight for a US gym scale. 70 × 2.20462262 = 154.32 lb. It's the most common single-unit conversion, and it only takes one multiplication since both units describe the same scale.

Newborn weight, kg to lb and oz. A baby born at 3.2 kg needs the lb-oz format used on US birth certificates. First, total pounds: 3.2 × 2.20462262 = 7.0548 lb. The whole number, 7, is the pound figure. The remainder, 0.0548, times 16 gives the ounces: 0.9 oz. The baby's weight converts to 7 lb 1 oz.

Body weight, kg to stone and lb. An adult weighing 68 kg wants the UK stone-and-pound format used on British medical forms. Total pounds first: 68 × 2.20462262 = 149.91 lb. Divide by 14 for whole stones: 10 stone, with a remainder. That remainder, 149.91 minus 140, is 9.9 lb. The result is 10 stone 9.9 lb, usually rounded to 10 stone 10 lb.

Cooking, grams to ounces. A UK recipe calls for 250 g of butter, but a US kitchen scale reads ounces. 250 × 0.03527396 = 8.82 oz, or just under two standard 4 oz butter sticks.

Shipping, grams to kg and lb with rounding. A parcel weighs 25,000 g on a digital scale. That's a direct decimal shift to kilograms: 25 kg. In pounds for a US customs form: 25 × 2.20462262 = 55.12 lb. Many carriers round declared weight up for billing, which would make this parcel's billable weight 56 lb rather than the true 55.12 lb. Always check your carrier's rounding policy before declaring a weight.

This calculator uses full-precision, internationally defined constants rather than the rounded figures common in casual references. The pound is defined as exactly 0.45359237 kilograms under the 1959 International Yard and Pound Agreement. The US, UK, Canada, and Australia adopted it to standardize imperial units against the metric system. Every other imperial unit here, including the ounce, stone, and both ton definitions, derives directly from that one fixed constant.

Results display at a practical number of decimal places, not full internal precision. Eight decimals on a body-weight result adds noise, not useful accuracy. Internally, though, nothing rounds until the final display step. Chaining conversions, kg to lb to stone for example, doesn't compound error the way manual step-by-step math can. For legal trade, medical dosing, or scientific reporting, confirm the required precision standard for your context. This tool targets everyday accuracy, not calibrated laboratory use.

Most conversion mistakes come from a handful of repeatable traps rather than random arithmetic slips. The table below covers the errors most likely to affect a real conversion, from unit-name confusion to rounding order. Two entries deserve extra attention: the three different "tons," and the gap between a decimal stone and a stone-pound pair. Both produce a plausible-looking wrong answer rather than an obviously broken one.

None of these errors come from the conversion factor itself being wrong. They come from applying the right factor to the wrong assumption, whether that's which ton, which ounce, or which rounding step comes first. Checking the unit definition before running the number catches most of them before they reach a form, invoice, or recipe.

The table below lines up this converter against the general pattern across major weight-conversion tools. It focuses on the gaps found during this page's build. Those gaps center on live conversion, unit coverage, developer access to the formulas, and static reference content beyond the interactive tool.

The two features most worth checking for yourself, regardless of which tool you use, are unit coverage and whether conversion happens live. A tool that only handles the common units will send you elsewhere the moment you need a troy ounce or a long ton. A tool that requires a manual click adds friction you won't notice until you're converting several values in a row.