The letter g hangs from a spring and holds a paper clip by its tail, and the clip weighs about a gram. The spring stretches, bounces a few times and settles, and the pointer attached to it travels along the scale on the right to the 1 g mark. On the left, drops fall from a pipette and a one-cubic-centimetre cube slowly fills with water, gathering the very gram the unit started from; on the right a feather sways, and a grain of sugar flashes in the air.
Quantity data sheet
SI · quantity: mass sheet 1/1 · rev. 2026-09Gram g
SI submultiple unit · quantity: mass · one thousandth of a kilogram
Why bakers don’t trust cups
A recipe calls for a cup of flour, and two people on opposite sides of the ocean fill it differently: one scoops straight from the bag and packs it down, the other spoons it in and levels the top with a knife, so their doughs end up almost a fifth apart. The dispute is settled by a scale, which reads one hundred twenty-five grams in one case and one hundred fifty in the other, and from that moment the recipe no longer depends on the hands that make it.
The gram is the most everyday unit of mass: it gives the weight of food on the package, doses of medicines and vitamins, the weight of a letter at the post office and the protein on your plate. It is handy because it matches the size of things we pick up, and because from the very beginning it was tied to water, so that a cubic centimetre of water still weighs almost exactly a gram today.
But that coincidence holds only for water, and all the confusion in the kitchen comes from the fact that volume and mass measure different things. Flour is fluffy, honey is dense, salt is heavier than sugar, and the same spoon carries two grams one time and seven the next, while a gram stays a gram whatever the container and whatever planet it is weighed on.
The gram is written with a lowercase Latin g, with no full stop and with a space before it: 250 g. A capital G is the prefix giga, and the abbreviations “gr” and “gm” are not recognised by the standards: the first is confused with the grain, the second survives in English-language recipes out of habit but does not exist in the SI.
The gram measures mass, not weight: on the Moon a 250 g packet stays 250 g, but it presses on a spring six times more weakly. Nor is a gram equal to a millilitre: that coincidence holds only for water, while a cup of flour weighs almost half as much as a cup of water, and a cup of honey almost one and a half times more.
01 · Definition
fraction of a kilogram · mass and volume · mass and weightThe gram is an SI submultiple unit of mass equal to one thousandth of a kilogram; its symbol is g. Since 2019 the kilogram has been defined by a fixed value of the Planck constant, and the gram inherits that definition without a standard of its own, while historically the kilogram itself was conceived as a thousand grams, so the base unit got its name from the submultiple.
Mass is related to volume through density, and only for water is that ratio close to one: a cubic centimetre of water at four degrees weighs 0.99997 g, and at room temperature 0.998 g. For every other substance the gram and the millilitre part ways, the more so the fluffier the substance, because in a granular product the volume is taken up by the grains and by the air between them.
Mass does not depend on place, whereas weight — the force with which a body presses on its support — is proportional to the acceleration of free fall. On Earth a gram presses with a force of about 9.8 millinewtons, on the Moon about 1.6, and so spring scales calibrated in one city may be off by tenths of a percent in another, while beam balances, which compare masses directly, are never off at all.
How many grams fit in a cup
On the left, a measuring vessel stands on a kitchen scale: the product pours or flows in up to the brim, the display counts up the grams, then the vessel empties and it all starts again. On the right are eight products in the same measure: the length of the bar is the mass with ordinary filling, the thin whiskers are the spread from sifted to packed, and the blue dashed line is water, for which a millilitre equals a gram.
Half a cup of flour is air
Flour particles are smaller than a tenth of a millimetre and cling to one another, so a lot of air stays between them, and how much depends on how the flour got into the cup. Sifted flour settles fluffily, flour scooped from the bag is compacted by the pressure, and one and the same cup takes anywhere from one hundred twenty to one hundred sixty grams. Sugar grains are large and smooth and settle almost the same way however they are handled, so their spread is several times smaller, and liquids have none at all, apart from what is left on the walls.
02 · Conversion
units of mass · volume of a substance · weightGrams, ounces and millilitres
Enter a mass in grams or in any other measure, and the sheet will convert it into the rest. The second tab shows what volume the same mass takes up for different substances, from water to mercury and from flour to gold, and the third shows the force with which that mass presses on its support on Earth, on the Moon, on Mars and on Jupiter.
A rule to remember: an ounce is about twenty-eight and a third grams, a pound about four hundred fifty-four, and a litre of water is a kilogram.
There is the ordinary ounce and the troy ounce, the troy one being a tenth heavier, and the fluid ounce is a measure of volume altogether; in cookbooks, pharmacy prescriptions and on the metals market three different numbers stand behind one word. The densities of granular products in the table are typical bulk values, and for a particular flour or salt they may differ by a tenth.
| Quantity | Value | Note |
|---|---|---|
| {k} | {v} | {note} |
{text}
04 · Measuring instruments
spring · load cell · compensation · quartzFour ways to feel a gram
Scales that actually measure force
A spring balance stretches in proportion to force, not mass, and its scale is marked in grams only because on the Earth’s surface one is almost uniquely tied to the other. At the equator it will read half a percent less than at the pole; for a market that is unnoticeable, for a laboratory it is fatal — which is why spring scales have almost vanished from laboratories.
An aluminium beam instead of a spring
Inside kitchen and postal scales there is a metal beam with thin foil grids bonded to it, and under a load the beam bends by fractions of a millimetre, so the resistance of the grids changes by tenths of a percent. A bridge circuit turns that change into a voltage and a processor into grams, which is why scales zero themselves when switched on: they remember not zero but their own beam without a load.
Scales that bend nothing
In an analytical balance the load does not move the lever: a position sensor notices the first deflection, and a coil in the field of a permanent magnet immediately pulls the lever back. The mass is read from the current that took, and so the balance tells apart a tenth of a milligram on a two-hundred-gram load — one two-millionth of the load.
A nanogram heard in the frequency
Where a billionth of a gram matters, weighing is done without a pan: a thin quartz plate vibrates at a few megahertz, and every nanogram of film that settles on it lowers the frequency by a noticeable fraction of a hertz. That is how the thickness of coatings is monitored during deposition and how much matter has settled out of the air, and weighing turns into counting vibrations.
05 · Writing rules
g with a space · mass, not volumeOne letter written a thousand ways
The gram appears in writing more often than any other unit of mass, and so it has the most wrong forms: home-made abbreviations, a capital letter, a full stop, a number glued to the symbol and confusion with volume.
The first three lines on the right are home-made abbreviations: “gr” reads as grain, G as giga, and mcg and mgr live on in recipes and packaging out of habit, though in the SI a microgram is written µg. The fourth equates volume with mass by the rule for water, which for flour is off by half; the fifth puts a division sign where words are needed; and the sixth confuses mass with weight: on the Moon the mass of the packet does not change, only the force with which it presses on the scale.
06 · Neighbouring units
kilogram · tonne · industry sheetThe gram is surrounded by its multiples and submultiples, and its two nearest neighbours are the kilogram, from which it is counted, and the tonne, used for what is awkward to write in grams. For the jewellery trade the gram has a separate industry sheet covering the weight of the piece, of the metal and of the pure metal.
The SI base unit, the only one with a prefix in its name. The gram is defined through it, and it in turn through the Planck constant, the metre and the second.
A non-SI unit accepted for use with the SI — a million grams, or a cubic metre of water. The megagram equals it, but you will hardly ever meet it in real life.
Weight of the piece, of the metal without settings and of pure metal by fineness, conversion of troy ounces, tolas and taels, fire assay and weighing in water.
07 · Historical section
archive · from the gravet to the Planck constantHow a small unit named a big one
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Same mass, different spring
Four identical springs each hold a gram, but they hang on the Moon, on Mars, on Earth and on Jupiter, and each stretches in proportion to gravity. A red marker above them visits the springs in turn, and the labels below name the force with which a gram pulls on the spring.
A beam balance would show the same on all four bodies, because gravity acts equally on both pans, and that is exactly why the gram measures the amount of matter, not how hard it presses.
A gram that weighs differently
In everyday speech “weight” and “mass” mean the same, and there is no harm in that as long as you weigh on one planet. But nearly all modern scales measure force — the bending of a beam or the current in a coil — and get grams by dividing it by the local acceleration of free fall, which on Earth varies from 9.78 m/s² at the equator to 9.83 at the poles.
A difference of half a percent means nothing in the kitchen but is huge for precision scales, and so they are calibrated at the place of installation with weights that already account for local gravity. Scales calibrated at a factory in one country and taken to another are off by grams per kilogram without such an adjustment.
In 1901 the General Conference on Weights and Measures, by a separate resolution, distinguished mass from weight and fixed the standard acceleration of 9.806 65 m/s², so that old units of force such as the gram-force would have an unambiguous value. That resolution is still in force, and the number in the corner of the plate rests on it: at standard acceleration a gram presses with a force of 9.806 65 millinewtons.
Catalogue of quantities
mass: from an atom to a tonneSI base units
the kilogram, and the metre and second through which it is definedHow many grams in familiar measures
ordinary filling, calculated from density| Product | g/ml | Teaspoon | Tablespoon | 250 ml glass | US cup |
|---|---|---|---|---|---|
| {n} | {d} | {tsp} | {tbsp} | {glass} | {cup} |
The US cup is five percent smaller than the metric 250 ml glass, and for liquids the difference usually drowns in the error of the hand, but for flour it adds to the spread of filling, so converting an American recipe through a glass without a scale means erring twice at once.