Weight & Mass Converter

Mass measures the amount of matter in an object, expressed in kilograms (SI) or pounds (US customary), and remains constant regardless of location — your mass is the same whether you're on Earth, on the Moon, in orbit, or in deep space. Weight is the gravitational force acting on that mass, expressed in newtons (SI) or pounds-force (US customary), and varies with the local gravitational field. On the Moon, you would weigh one-sixth of your Earth weight because lunar gravity is ~1.62 m/s² versus Earth's 9.81 m/s², but your mass would be exactly unchanged. In orbit you would weigh essentially zero (microgravity) while retaining full mass.

In everyday language the terms are used interchangeably, and bathroom scales colloquially show "weight" in pounds or kilograms — technically inconsistent but practically useful. Scientific and engineering contexts require the distinction: accelerating a 100 kg object in space requires the same force as on Earth (F = ma), independent of weight, while the force required to lift that object against gravity depends very much on the local gravitational field. Rocket science, astronomy, and physics classes always use mass (kg) for calculations because it's the invariant property; weight appears as a derived quantity F = m × g.

Until May 20, 2019, the kilogram was the last SI unit still defined by a physical artifact — the International Prototype Kilogram (IPK), a platinum-iridium cylinder stored in a climate-controlled vault at the International Bureau of Weights and Measures (BIPM) in Sèvres, France, under three nested glass bell jars. The IPK was cast in 1879, and for 140 years, "one kilogram" meant "the mass of this specific object." Scientists periodically compared the IPK to six official copies, and mysteriously the copies seemed to be drifting apart in mass by tens of micrograms over decades — either the IPK was losing mass or the copies were gaining, and there was no way to know which.

The kilogram was redefined on World Metrology Day 2019 using the Planck constant, fixing its value at exactly 6.62607015 × 10⁻³⁴ joule-seconds and deriving the kilogram from this fundamental physical constant. The new definition uses a Kibble balance — an instrument that equates mechanical and electromagnetic power — to realize the kilogram from electrical measurements, meaning the kilogram can be reproduced anywhere in the universe using only Planck's constant and fundamental electromagnetic physics. The old IPK artifact is preserved for historical purposes but no longer defines the unit. This redefinition ensures the kilogram is permanent and cannot drift over time.

The troy weight system predates the imperial system and originated in medieval trade, likely named after the French city of Troyes which hosted major commercial fairs. Today the troy system survives almost exclusively for precious metals (gold, silver, platinum, palladium) and gemstones, while the avoirdupois system (French for "goods of weight") handles everything else — groceries, packages, body weight, industrial materials. A troy ounce weighs 31.1035 grams, about 10% heavier than the avoirdupois ounce (28.3495 grams). The troy pound (12 troy ounces = 373.24 g) is actually lighter than the avoirdupois pound (16 avoirdupois ounces = 453.59 g) despite the heavier ounces, because troy uses 12 ounces per pound while avoirdupois uses 16.

Gold prices quoted per "ounce" always mean troy ounce — an important distinction when comparing spot prices to physical weight. If you buy "1 ounce" of gold at the current spot price of $2,300 (as of 2024-2025 market), you're getting 31.1 grams of gold, not 28.3 grams. Jewelry industries further complicate matters by using pennyweights (1 dwt = 1.555 g, 20 dwt per troy ounce) for fine work. Always verify which ounce system is being quoted when dealing with precious metals to avoid overpaying or shortchanging yourself on a transaction, and use this calculator to convert cleanly between troy, avoirdupois, and metric when needed.