Weight Unit Converter

A weight unit converter lets you quickly convert between different weight units. It supports 14 common weight units including metric units (metric tons, kilograms, grams, milligrams, micrograms), imperial units (pounds, ounces, stone, long tons, short tons), and Chinese traditional units (jin, liang, qian, dan). Whether you're measuring body weight, weighing cargo, or doing everyday conversions, this tool gives you accurate results instantly. All calculations happen locally in your browser - no data uploads, protecting your privacy. Weight and mass are often spoken of interchangeably in daily life, but technical contexts may treat them differently. Grams, kilograms, tonnes, pounds, ounces, and stones appear in recipes, shipping, fitness, trade, and engineering. The converter helps compare values quickly, yet rounding, local standards, and scale tolerance can matter. For lab work, medication, customs, freight, or contracts, the unit, precision, and measurement method should be stated explicitly.

Since the 2019 SI redefinition, the kilogram is no longer defined by the International Prototype Kilogram artefact in Sèvres. It is now fixed by setting the Planck constant `h = 6.62607015 × 10⁻³⁴ J·s` exactly, which through `E = mc²` and the second/metre definitions gives every laboratory access to a kilogram via a Kibble balance rather than a physical lump of platinum-iridium. All other metric weight units are decimal multiples: 1 kg = 1000 g = 10⁶ mg = 10⁹ μg; 1 metric tonne (t) = 1000 kg. Imperial weights are defined by exact conversion to the kilogram rather than the other way around. The international avoirdupois pound has been fixed at exactly `1 lb = 0.45359237 kg` since the 1959 international yard-and-pound agreement, which makes pound-based and kilogram-based calculations agree to every decimal place. Derived from this: 1 oz = 1/16 lb = 0.028349523125 kg, 1 stone (st) = 14 lb = 6.35029318 kg, 1 long ton = 2240 lb = 1016.0469088 kg, and 1 short ton (US) = 2000 lb = 907.18474 kg. The three "ton" variants differ by about 12% between long and short, which is the most common source of freight billing errors. Specialty and regional units have their own exact constants. The metric carat used for gemstones is fixed at `1 ct = 0.2 g exactly` (since 1907). Mainland China's market jin is standardised at 1 市斤 = 500 g, divided into 10 liang = 50 g each; the Taiwanese 台斤 is 600 g and the Hong Kong 司馬斤 is 604.78982 g, so the same character maps to three different masses depending on jurisdiction. Conversion in the tool uses `result = value × factor_from / factor_to` against a kilogram base, with JavaScript IEEE-754 double precision providing ~15-17 significant digits — enough that rounding artefacts only appear well below the precision of any practical scale.

• Since 2019 the kilogram is defined by fixing the Planck constant `h = 6.62607015 × 10⁻³⁴ J·s` exactly (SI base unit redefinition).
• Avoirdupois pound is fixed at exactly `1 lb = 0.45359237 kg` (1959 international agreement), so kg ↔ lb conversions are exact, not approximate.
• Three tons: 1 metric tonne = 1000 kg, 1 long ton (UK) = 2240 lb = 1016.0469088 kg, 1 short ton (US) = 2000 lb = 907.18474 kg.
• Carat = 0.2 g exactly (1907 metric carat); ounce = 1/16 lb; stone = 14 lb; troy ounce (precious metals) = 31.1034768 g, distinct from avoirdupois.
• Chinese 市斤 = 500 g (mainland, 1959 standard), but 台斤 = 600 g and 香港司馬斤 = 604.78982 g — same character, different mass by region.
• Conversion goes through kg as a base: `target = (value × factor_from_to_kg) / factor_to_to_kg`, using IEEE-754 doubles (~15 significant digits).
• Weight (force) and mass differ in physics: 1 kg of mass weighs ~9.807 N on Earth but only ~1.62 N on the Moon; everyday "weight" units are mass units.