Weight Conversion

Supported units

Every unit below is defined against the gram. To convert between any two, divide their factors.

The imperial units are exact by definition, not by measurement: the international avoirdupois pound has been exactly 453.59237 g since the 1959 agreement, and the ounce, stone and tons follow from it. The kilogram they are defined against has itself been fixed by the Planck constant since 2019, replacing the last physical artefact in the SI.

Metric System

Quectogram (qg)

One nonillionth of a gram, written 10^-30 g. The quecto- prefix was added to the SI in 2022 and gives the smallest mass unit the system can express.

Particle physics

Masses far below anything a laboratory balance can register.

Cosmology

Bounds on the mass of hypothetical ultra-light particles.

Rontogram (rg)

One octillionth of a gram, written 10^-27 g. Like quecto-, the ronto- prefix joined the SI in 2022 to name quantities that had previously needed exponents.

Particle physics

Masses of subatomic particles and their decay products.

Quantum measurement

The sensitivity limit of nanomechanical resonators.

Yoctogram (yg)

One septillionth of a gram, written 10^-24 g. A single proton weighs roughly 1.67 yoctograms, which makes this the natural unit for individual particles.

Atomic physics

The mass of a proton is about 1.67 yg.

Nanomechanics

Resonator sensors that can weigh single molecules.

Zeptogram (zg)

One sextillionth of a gram, written 10^-21 g. This is the scale of large biomolecules: a typical protein weighs a few zeptograms.

Molecular biology

Masses of proteins and other large biomolecules.

Nanotechnology

Quantities deposited in atomic-layer processes.

Attogram (ag)

One quintillionth of a gram, written 10^-18 g. Attogram-resolution balances exist as microfabricated cantilevers rather than as conventional scales.

Nanotechnology

Mass of nanoparticles and engineered nanostructures.

Biotechnology

Detection limits for single-virus and single-cell assays.

Femtogram (fg)

One quadrillionth of a gram, written 10^-15 g. A single bacterium weighs on the order of a few hundred femtograms.

Microbiology

Dry mass of individual bacterial cells.

Genetics

The DNA content of a single cell nucleus.

Chemical analysis

Detection limits in mass spectrometry.

Picogram (pg)

One trillionth of a gram, written 10^-12 g. Picograms are routine in the laboratory: a human cell weighs a few hundred picograms.

Biotechnology

DNA and RNA quantities in sequencing and PCR workflows.

Environmental science

Trace contaminants such as dioxins in soil and water.

Medicine

Hormone concentrations, usually as picograms per millilitre.

Nanogram (ng)

One billionth of a gram, written 10^-9 g. It is the working unit for trace analysis, where results are usually reported as nanograms per millilitre.

Pharmaceuticals

Drug concentrations in blood plasma.

Environmental science

Pollutant levels in air and water samples.

Biochemistry

Protein and nucleic acid quantities on a gel.

Microgram (μg)

One millionth of a gram, written 10^-6 g. Vitamin and mineral doses are given in micrograms, often abbreviated mcg on labels to avoid confusion with mg.

Medicine

Doses of potent drugs and of vitamins such as B12 and D.

Nutrition

Micronutrient content on food labels.

Air quality

Particulate concentrations in micrograms per cubic metre.

Milligram (mg)

One thousandth of a gram, written 10^-3 g. The milligram is the standard unit for medication strength and for most nutrients on a food label.

Pharmaceuticals

Tablet strengths and prescribed dosages.

Nutrition

Sodium, calcium and other nutrient amounts.

Chemistry

Reagent quantities weighed on an analytical balance.

Centigram (cg)

One hundredth of a gram, written 10^-2 g. Rarely used in practice, since quantities at this scale are normally expressed as milligrams.

Pharmacy

Compounding small quantities of active ingredients.

Laboratory work

The readability of a common two-decimal balance.

Decigram (dg)

One tenth of a gram, written 10^-1 g. Like the centigram, it is a valid SI multiple that everyday practice mostly skips over.

Chemistry

Small reagent quantities in teaching laboratories.

Food science

Precise ingredient amounts in recipe development.

Gram (g)

One thousandth of a kilogram, and the practical everyday unit of the metric system. The SI base unit is the kilogram, so the gram is formally defined from it rather than the other way round.

Cooking

The standard unit for weighing ingredients.

Retail

Net weights on food and consumer packaging.

Science

Sample masses in most laboratory work.

Decagram (dag)

Ten grams. It survives in Central and Eastern European delicatessens, where cheese and cold cuts are still ordered by the deka.

Food retail

Deli portions in Austria, Poland and neighbouring countries.

Cooking

Ingredient quantities in older regional recipes.

Hectogram (hg)

One hundred grams. Italian shops price food per etto, which is a hectogram, and nutrition tables almost universally use a 100 g reference amount.

Food retail

Pricing by the etto in Italy.

Nutrition

Nutrient content is quoted per 100 g.

Agriculture

Yields and sample weights for produce.

Kilogram (kg)

The SI base unit of mass, equal to one thousand grams. Since 2019 it has been defined by fixing the Planck constant at exactly 6.626 070 15 x 10^-34 J s, retiring the platinum-iridium cylinder that had served as the standard since 1889.

Everyday measurement

Body weight, groceries and luggage allowances.

Science and engineering

The reference mass from which all other SI masses derive.

Commerce

Bulk goods sold and shipped by the kilogram.

Tonne /megagram/ (Mg)

One thousand kilograms, also called the metric ton or megagram. It is accepted for use with the SI and is distinct from both the short ton and the long ton.

Shipping and freight

Cargo weights and vehicle load limits.

Agriculture

Harvest yields and bulk commodity trading.

Environmental reporting

Carbon dioxide emissions, usually as tonnes per year.

Gigagram (Gg)

One billion grams, or one thousand tonnes, written 10^9 g. National emissions inventories often report in gigagrams rather than kilotonnes.

Environmental science

Greenhouse gas inventories submitted under climate treaties.

Industry

Annual production volumes of bulk materials.

Teragram (Tg)

One trillion grams, or one million tonnes, written 10^12 g. Global material flows — plastics produced, nitrogen fixed, methane emitted — are usually tallied in teragrams.

Climate science

Annual global emissions of methane and other trace gases.

Biogeochemistry

Carbon and nitrogen fluxes between ecosystems.

Petagram (Pg)

One quadrillion grams, or one billion tonnes, written 10^15 g. One petagram of carbon is the standard unit of the global carbon budget, where it is often written as a gigatonne.

Climate science

The global carbon budget, in petagrams of carbon per year.

Geology

Sediment transported by the world’s rivers.

Exagram (Eg)

One quintillion grams, or one trillion tonnes, written 10^18 g. The oceans hold roughly 1.4 million exagrams of water, which gives a sense of the scale.

Planetary science

Masses of ice sheets and other planetary reservoirs.

Geophysics

Large-scale mass redistribution measured by satellite gravimetry.

Zettagram (Zg)

One sextillion grams, written 10^21 g. Earth’s atmosphere weighs about 5.1 zettagrams, so this is the unit at which whole planetary envelopes are counted.

Atmospheric science

The total mass of Earth’s atmosphere is about 5.1 Zg.

Planetary science

Masses of small moons and large asteroids.

Yottagram (Yg)

One septillion grams, written 10^24 g. Earth itself weighs about 5 972 yottagrams, which makes this the smallest prefix useful for whole planets.

Planetary science

The mass of Earth is roughly 5 972 Yg.

Astronomy

Masses of planets and large moons in the solar system.

Ronnagram (Rg)

One octillion grams, written 10^27 g. The ronna- prefix was adopted in 2022 partly so that planetary and stellar masses could be written without exponents: Jupiter weighs about 1 898 ronnagrams.

Astronomy

Masses of gas giants; Jupiter is about 1 898 Rg.

Astrophysics

Mass budgets for protoplanetary discs.

Quettagram (Qg)

One nonillion grams, written 10^30 g, and the largest SI multiple defined. The Sun weighs about 1 989 quettagrams.

Astrophysics

Stellar masses; the Sun is about 1 989 Qg.

Cosmology

Mass estimates for galaxies and larger structures.

Imperial System

Ounce (oz)

One sixteenth of an avoirdupois pound, or exactly 28.349 523 125 grams. It is distinct from the troy ounce used for precious metals, which is heavier.

Food

Portion sizes and package weights in the United States.

Postage

Letter and small-parcel rates are banded by the ounce.

Small goods

Jewellery, cosmetics and other lightweight items.

Pound (lb)

Sixteen ounces, defined since 1959 as exactly 0.453 592 37 kilograms. The abbreviation lb comes from the Roman libra.

Body weight

The everyday unit in the United States.

Groceries

Produce and meat sold by the pound.

Commerce

Shipping weights and freight rates in the US.

Stone (st)

Fourteen pounds, or about 6.35 kilograms. It is used almost exclusively for body weight in the UK and Ireland, quoted as stones and pounds — 10 st 7 lb.

Body weight

The customary unit in the UK and Ireland.

Sport

Weight classes and jockey weights in horse racing.

Ton (short ton)

Two thousand pounds, or about 907.18 kilograms. Called the short ton to distinguish it from the long ton and the metric tonne, both of which are heavier.

Freight

Cargo and truck load weights in the United States.

Construction

Bulk materials such as gravel, sand and steel.

Mining

Ore and coal production figures.

Long ton

Two thousand two hundred and forty pounds, or about 1 016 kilograms. Also called the imperial ton, it is close enough to the metric tonne that the two are easily confused in shipping documents.

Shipping

Vessel displacement and cargo capacity.

Heavy industry

Steel and raw material tonnages in the UK.

Mass and weight are not the same thing

Strictly, these units measure mass — the amount of matter in an object, which does not change no matter where the object is. Weight is the force gravity exerts on that mass, so it varies with location: the same object weighs about a sixth as much on the Moon while its mass is unchanged.

In everyday use the words are interchangeable, and scales are calibrated so that reading a mass in kilograms works perfectly well on Earth. The distinction matters in physics and engineering, where force is properly measured in newtons, but for cooking, shipping, and shopping the conversions here are exactly what you want.

The metric prefixes

Every metric unit on this page is the gram scaled by a power of ten, which is why converting within the system never requires a conversion factor — only a shift of the decimal point.

• Everyday range— milligram, gram, kilogram, tonne. These cover nearly all practical use.
• Small scales— microgram and nanogram appear in pharmacology and analytical chemistry, where doses and trace quantities are routinely this small.
• Very small scales— picogram down to quectogram belong to molecular and particle physics. The quectogram, 10⁻³⁰ grams, was added to the SI in 2022 along with the ronto-, ronna-, and quetta- prefixes.
• Large scales— gigagram and above are used in earth science and climate work, where emissions and ice masses are quoted in teragrams or petagrams.

A note on the tonne: it is 1,000 kilograms and is also called the megagram. It is not the same as either imperial ton, which is the source of a good deal of confusion in shipping.

The three different tons

This is the single most common source of error in mass conversion, because all three are called a "ton" and they differ by enough to matter commercially.

• Tonne (metric ton)— 1,000 kg, about 2,204.6 lb. The international standard.
• Short ton— 2,000 lb, about 907.2 kg. Standard in the United States, and what "ton" usually means there.
• Long ton— 2,240 lb, about 1,016 kg. The British imperial ton, still used in some shipping contexts.

The spread between the smallest and largest is about 12%. On a single shipping container that is a substantial quantity of cargo, so it is worth confirming which ton a document means.

Worked examples

• A 12 stone person— 168 pounds, or about 76.2 kilograms.
• A recipe calling for 8 ounces of flour— about 227 grams.
• A 5 kg parcel— about 11 pounds, or 11 lb 0.4 oz.
• A 500 mg tablet— half a gram, or 0.0176 ounces.
• A 1.5 short ton vehicle— 3,000 pounds, about 1,361 kilograms, or 1.36 tonnes.

Common uses

• Cooking and baking— converting recipes between grams and ounces, where baking in particular rewards precision.
• Shipping and postage— matching parcel weights to carrier brackets stated in the other system.
• Health and fitness— moving between kilograms, pounds, and stones.
• Science and medicine— dosages and reagent quantities in milligrams and micrograms.
• Freight and logistics— reconciling tonnes with short and long tons.
• Trade and manufacturing— reading material specifications written for another market.