Weight & Mass Converters — kg, lbs, stones, grams, ounces, tonnes
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Weight and mass conversions are the most frequent unit translations in everyday life because almost every country except the United States uses kilograms while the US still uses pounds for body weight, food packaging, freight, and shipping. The technical distinction between mass and weight matters in physics — mass is the amount of matter in an object, measured in kilograms or pounds, while weight is the gravitational force on that mass, measured in newtons or pounds-force — but in everyday usage the two terms are interchangeable, and a "weight" reported on a bathroom scale or a food label is really a measurement of mass. The category covers seven core units: the kilogram (SI base unit, used worldwide except in the US), the pound (avoirdupois, used in the US for almost all everyday weight), the gram (metric, used for small quantities and food packaging), the ounce (avoirdupois, US small-quantity unit), the stone (UK informal body-weight unit), the metric tonne (1000 kg, used for freight and bulk commodities), and the US short ton (2000 lbs, used for North American freight and scrap metal). Choosing the right unit depends on the audience, the regulator, and the precision the calculation demands. A US shipper sending a parcel internationally needs kilograms on the customs declaration; a UK doctor reporting a patient's weight conversationally uses stones-and-pounds even though the clinical record stays metric; a North American grain merchant invoices in short tons while the export contract crosses to metric tonnes at the port.
Units in this category
Pounds (lbs)
One avoirdupois pound (lb) is exactly 0.45359237 kilograms — a value fixed by the 1959 International Yard and Pound Agreement and unchanged since. The pound divides into 16 ounces of 437.5 grains apiece, with the grain itself defined as exactly 64.79891 milligrams; the apparently arbitrary factor exists because there are exactly 7,000 grains in a pound, and seven was a convenient divisor for the gunpowder, apothecary and assay measurements that drove early standardisation. In engineering and physics texts, "pound" without qualification can mean either pound-mass (lbm), a unit of mass, or pound-force (lbf), the gravitational force on one pound-mass at standard gravity (9.80665 m/s²).
Kilograms (kg)
Since 20 May 2019 the kilogram (kg) is defined by fixing the numerical value of the Planck constant h at exactly 6.62607015 × 10⁻³⁴ when expressed in J·s, which is equivalent to kg·m²·s⁻¹. Because the metre and second appearing in that expression are themselves anchored to the speed of light c and the caesium-133 hyperfine transition frequency Δν_Cs, the kilogram ultimately rides on three fixed constants of nature and can be realised in any sufficiently equipped laboratory without reference to a physical artefact. National metrology institutes do so by one of two routes: a Kibble balance (renamed in 2016 in honour of the late NPL physicist Bryan Kibble, having previously been called the watt balance), which equates electrical and mechanical power to relate mass to the Planck constant via a precisely-measured electromagnetic force; or the X-ray crystal density method, which counts the atoms in a near-perfect spherical single crystal of silicon-28 enriched to roughly 99.995% purity.
Grams (g)
The gram (g) is one one-thousandth of the kilogram. By international convention adopted at the 11th CGPM in 1960 and unchanged since, decimal multiples and submultiples of mass are formed from the root "gram" rather than from the base unit "kilogram" — so 1,000 grams is one kilogram, one million grams is a megagram (Mg, equivalently a metric tonne), and one-thousandth of a gram is a milligram (mg), not a "microkilogram". The SI symbol for the gram is the lowercase "g", standardised by ISO 80000-1 and the BIPM SI Brochure (9th edition, 2019).
Stone (st)
One stone (st) is exactly 14 avoirdupois pounds. Through the 1959 International Yard and Pound Agreement's fixing of the avoirdupois pound at 0.45359237 kilograms, the stone has held a precise SI-equivalent value of 6.35029318 kg since 1 July 1959 — a relationship that runs through the pound rather than from any independent definition of the stone itself. The conventional symbol is "st" (no period), and body-weight measurements are typically written as a compound figure: "11 st 4 lb", or its short form "11 stone 4", denotes eleven stone four pounds, equal to 158 lb or 71.7 kg.
Ounces (oz)
The English word "ounce" refers to four distinct units in 2026, three of mass and one of volume. The avoirdupois ounce (oz) is 1/16 of the avoirdupois pound, equal to exactly 28.349523125 g via the 1959 International Yard and Pound Agreement. This is the ounce of US food packaging, postal rates and everyday goods.
Tonnes (t)
The tonne (t) is exactly 1,000 kilograms — equivalently 1 megagram (Mg) in coherent SI notation, though the megagram form is rarely seen outside metrology textbooks. The official symbol is the lowercase "t" per the SI Brochure (9th edition, 2019); the older "T" was sometimes used in mid-twentieth-century engineering literature but conflicts with the SI symbol for tesla and is no longer recognised. Decimal multiples are formed with standard SI prefixes: a kilotonne (kt) is 10⁶ kg, a megatonne (Mt) is 10⁹ kg (a billion kilograms), and a gigatonne (Gt) is 10¹² kg, with the megatonne and gigatonne in routine use for industrial-commodity statistics, atmospheric carbon-emission accounting and yield estimates for nuclear weapons.
Short Tons (short ton)
The short ton is exactly 2,000 avoirdupois pounds, equal to 907.18474 kilograms through the 1959 International Yard and Pound Agreement's fixing of the avoirdupois pound at 0.45359237 kg. There is no single internationally-recognised symbol for it: "ton" or "tn" appears in informal US usage and an uppercase "T" in older mining and freight ledgers, but the BIPM-recognised lowercase "t" denotes the metric tonne (1,000 kg) and is therefore reserved against use for the short ton. The short ton subdivides into twenty short hundredweights (cental, abbreviated cwt in US usage) of 100 lb each, the structure inherited from early-American customs accounting that gave the unit its name.
Milligrams (mg)
The milligram (mg) is exactly one one-thousandth of a gram, or one one-millionth of a kilogram, by SI prefix definition. Since the SI redefinition adopted at the 26th CGPM in 2018 and entered into force on 20 May 2019, the kilogram is anchored to a fixed numerical value of the Planck constant (h = 6.62607015 × 10⁻³⁴ J·s exactly), and the milligram inherits that anchoring as a derived submultiple. The recognised SI symbol is the lowercase "mg", standardised by ISO 80000-1 and the BIPM SI Brochure (9th edition).
Micrograms (mcg)
The microgram (μg, or mcg in clinical writing) is exactly one one-millionth of a gram, or one one-billionth of a kilogram, by SI prefix definition. Since the SI redefinition adopted at the 26th CGPM in 2018 and entered into force on 20 May 2019, the kilogram is anchored to the Planck constant and the microgram inherits that anchoring as a derived submultiple. The recognised SI symbol is the lowercase Greek "μg" (Unicode U+03BC), standardised by ISO 80000-1 and the BIPM SI Brochure.
History of weight & mass measurement
Modern mass measurement traces back to the Roman libra of about 328.9 grams, which gave the pound its abbreviation "lb" and seeded the medieval European tradition of regional weight standards that varied by city, trade, and commodity. England consolidated the avoirdupois pound (~454 g) for general goods by the 14th century and codified it in the Weights and Measures Act of 1855, while France had already replaced its dozens of regional units with the metric system in 1795, defining the kilogram as the mass of a litre of water at 4°C and depositing a platinum prototype in the Archives. The Metre Convention of 1875 made the kilogram an international standard, and the International Yard and Pound Agreement of 1959 fixed the avoirdupois pound at exactly 0.45359237 kg, ending decades of small national variations that mattered in scientific and industrial work. The most recent major revision was the 2019 SI redefinition, which replaced the platinum kilogram prototype with a definition based on a fixed value of the Planck constant, finally severing the kilogram from any physical artefact.
Where weight & mass conversions matter
Mass conversions appear in every major regulated industry that crosses national borders. International freight and customs declarations require kilograms because nearly every country outside the United States operates customs and tariff schedules in metric, so US-origin shipments convert at the airway-bill or customs entry, and the kg figure determines the duty band the parcel falls into. Healthcare worldwide depends on weight-based medication dosing in mg per kg of body weight, and an American patient treated outside the US has their pounds converted to kilograms before any prescription is written, with paediatric and critical-care dosing especially sensitive to the conversion accuracy. The aviation industry uses pounds globally for aircraft weight calculations even in fully metric countries because of American manufacturer dominance, while sports federations (boxing, MMA, weightlifting, powerlifting) standardise on kilograms for weight classes worldwide except in domestic US leagues. US food packaging carries dual-unit net-weight statements under FTC and FDA labelling rules, with retail-audit checks comparing the lb/oz figure against the gram conversion for compliance. Freight contracts distinguish carefully between metric tonnes (1000 kg), US short tons (907.185 kg), and UK long tons (1016.047 kg), and global commodity markets quote bulk grain, ore, and scrap metal in tonnes while local handling units stay in kilograms or pounds. Carbon-emissions reporting under the GHG Protocol and EU CSRD aggregates kg of CO2-equivalent into tonnes for ESG disclosure, with the conversion sitting at the boundary between instrument-level evidence and the published sustainability figure.
How to convert weight & mass units
The foundational conversion is the international pound: exactly 0.45359237 kg, fixed by treaty since 1959. Every avoirdupois mass conversion ultimately traces back to this single constant — pounds to kilograms multiplies by 0.45359237, and the inverse divides by the same figure (or multiplies by 2.20462262 lbs per kg). For mental math the "divide by 2.2" or "multiply by 2.2" shortcuts run within 0.3% of the precise answer and are fine for body weight, packaging, and casual cooking but inadequate for medication dosing, customs declarations crossing tariff thresholds, and any scientific work. The avoirdupois ounce is exactly 1/16 of the pound (28.3495 g), distinct from the troy ounce used for precious metals (31.1035 g). The "ton" is the most ambiguous unit in the category: a US short ton is 907.185 kg, a UK long ton is 1016.047 kg, and a metric tonne is exactly 1000 kg, so any contract or calculation involving "tons" needs explicit qualification. Mixed-unit reporting (lb-and-oz for newborn weights, st-and-lb for UK body weight) converts by aggregating to the smaller unit before scaling to the metric equivalent.
All weight & mass conversions
Frequently asked questions
What is the difference between weight and mass?
Mass is the amount of matter in an object, while weight is the gravitational force exerted on that mass. A 70 kg person on Earth weighs 686 newtons (or 154 pounds-force), but the same person on the Moon would still have 70 kg of mass while weighing only 114 newtons because lunar gravity is about one-sixth of Earth's. In everyday usage and in this category's converters, "weight" almost always means mass — bathroom scales, food labels, and freight documents all report mass values rather than true weight in newtons.
Why do the US and UK still use pounds when most countries use kilograms?
Britain and its former colonies adopted metric measurement officially during the 20th century, but the United Kingdom retained pounds informally for body weight and groceries, and the United States has never legally required metric conversion for everyday commerce. The 1975 Metric Conversion Act made metrication voluntary in the US, and consumer resistance combined with industry investment in imperial-unit infrastructure (scales, packaging machinery, freight networks) preserved the pound as the everyday unit. Aviation and engineering use pounds worldwide because of American manufacturer dominance, regardless of the host country's official measurement system.
Is a metric tonne the same as a US ton?
No, the three "ton" units differ significantly. A metric tonne equals exactly 1000 kg by SI definition, a US short ton equals 2000 lbs (907.185 kg), and a UK long ton equals 2240 lbs (1016.047 kg). The gap matters in commodity contracts and freight pricing — a 100-tonne metric contract delivers about 9.3% more material than a 100-short-ton US contract at the same numeric figure. International trade documents always specify which "ton" is meant, often using "MT" or "metric tonne" to disambiguate.
What is a stone in UK body weight?
A stone is a UK imperial unit equal to 14 pounds, used informally for body weight in the UK and Ireland. A person reporting "11 stone 4 pounds" weighs 158 lbs total, equivalent to 71.7 kg. The stone is no longer a legal unit for trade in the UK (metric has been required since 2000) but remains the dominant conversational unit for personal weight on bathroom scales and in everyday speech. UK driver's licences, medical records, and fitness apps typically display stones-and-pounds alongside the metric kilogram figure.
Did the 2019 SI redefinition change everyday weight measurements?
No, the 2019 redefinition of the kilogram against the Planck constant did not change the value of any everyday mass measurement. Before 2019, the kilogram was defined as the mass of a specific platinum-iridium cylinder kept in Paris (the International Prototype of the Kilogram), and the new definition was carefully calibrated so that the new kilogram has the same mass as the old one to within the precision of the best laboratory measurements. The change affects only the foundational metrology infrastructure, not consumer scales or commercial weighing.
Why is "pound" abbreviated "lb"?
The abbreviation "lb" comes from the Roman libra, the ancient Roman unit of mass that gave the pound both its name (via Latin pondus, "weight") and its symbol. The libra was about 328.9 grams, smaller than the modern avoirdupois pound, but the abbreviation persisted through medieval European weight systems and into modern English. The plural "lbs" follows the standard English pluralisation rule applied to the Latin-derived abbreviation, even though "lb" itself is unchanged in singular and plural in formal Latin usage.
What is the difference between an avoirdupois ounce and a troy ounce?
An avoirdupois ounce is 28.3495 grams and is used for everyday solids — food, mail, body weight when expressed in pounds-and-ounces. A troy ounce is 31.1035 grams, about 9.7% heavier, and is used exclusively for precious metals (gold, silver, platinum, palladium) and historically for gunpowder. The two units share the name "ounce" but represent different masses, which is why a "1 oz" gold coin weighs 31.1 g rather than the 28.35 g of a "1 oz" packet of crackers. Confusing the two undervalues precious-metal calculations by nearly 10%.