Grams to Moles Calculator

Moles = mass in grams ÷ molar mass · Millimoles = moles × 1000

moles

How many moles of the substance your sample holds — this page reports it to five decimal places

mass

The mass of your sample, in grams. A milligram is 0.001 grams, and the balance reading is the only measurement this page needs from you

molar mass

Grams per mole of the substance — computed from the chemical formula, or typed in from the label on the bottle

1000

Millimoles in one mole. A millimole is a thousandth of a mole, so the same answer in millimoles is a thousand times larger

Use it whenever a recipe, a protocol or a homework question is written in moles and your balance reads in grams — this is the step that converts one into the other. It is also the page for the reverse direction of a mole calculation: if you need to know what to weigh out for a given number of moles, multiply instead — or use the molarity page, which does exactly that for solutions. It is not the right page for a solution's concentration: moles on their own say nothing about volume.

1. One hundred grams of water

   1. Molar mass of H2O: 2 × 1.008 + 15.999 = 18.015 grams per mole
   2. Divide the mass by it: 100 ÷ 18.015 = 5.55093 moles
   3. In millimoles: 5.55093 × 1000 = 5550.93
   4. One hundred millilitres of water, then — a hundred grams and a hundred millilitres are the same thing here

   Water is the one substance where the gram and the millilitre coincide, which makes it the easiest case to sanity-check by eye: a litre bottle holds about 55.5 moles. Nothing else in the kitchen behaves this way.

2. A mole of table salt, weighed out

   1. Molar mass of NaCl: 22.99 + 35.45 = 58.44 grams per mole
   2. Weigh out exactly 58.44 grams on the balance
   3. Divide: 58.44 ÷ 58.44 = 1 mole
   4. Which is 1000 millimoles

   This is where the definition of the mole becomes concrete: one mole of a substance is its molar mass in grams. The 58.44 that a reagent bottle prints as the molar mass is also, in grams, the amount that makes one mole — the same number in two different roles.

The molar mass used here is the mass of one mole of the pure substance, so the answer is only as good as the assumption that your sample is that substance. A hydrated salt that has dried out, a reagent that has absorbed water from the air, or a sample that is only 90% pure all shift the answer, and none of them is visible to the arithmetic: 100 grams of wet salt contains fewer moles of sodium chloride than 100 grams of dry salt does. When you type both a formula and a molar mass, the two are checked against each other and the calculation stops if they differ by more than 1%, because that is a real contradiction rather than a rounding difference — but the check is deliberately loose, since figures printed on a bottle are quoted to three or four significant figures. For an ionic solid such as sodium chloride there are no separate molecules to count: a mole of it is a mole of formula units, which is why the molar mass of NaCl is a meaningful quantity even though a NaCl molecule is not really a thing. Finally, this page says nothing about solutions. Moles per litre is a concentration, and it needs the volume as well as the mass — that is the molarity page, and the two are not interchangeable.

How many moles are in 100 grams of water?

5.55093 moles. The molar mass of water is 18.015 grams per mole, and 100 ÷ 18.015 = 5.55093. In millimoles the same quantity is 5550.93. The figure is the same for any substance only when the molar masses match, which is why 100 grams of table salt comes out at 1.71116 moles instead — a mole of salt weighs 58.44 grams, more than three times what a mole of water weighs.

What is the difference between moles and millimoles?

A factor of a thousand, nothing else. One millimole is 0.001 moles, so 5.55093 moles is 5550.93 millimoles; both lines report the same quantity of substance and neither is more precise than the other. Millimoles are simply more convenient to say out loud in a laboratory where the amounts are small — 5.55093 moles of a reagent is a bucket, while 5.55 millimoles is a spatula.

Do I have to enter the chemical formula?

No — the formula and the molar mass are alternatives, and you need one of the two. Typing the formula is the quicker route when you know what the substance is, because the molar mass is computed for you. Entering a molar mass instead is for the case where you have a number from a certificate of analysis, a label or a problem sheet and no formula to go with it. Filling in both is allowed: if they agree the figure you typed is the one used and shown, and if they differ by more than 1% the calculation refuses to guess.

Why does it say my molar mass does not match the formula?

Because the two disagree by more than 1%. That normally means one of them belongs to a different substance: 58.44 is table salt and 180.156 is glucose, so entering one as the formula and the other as the molar mass is a mix-up worth stopping for. The 1% allowance exists because printed molar masses are rounded — water is printed as 18.02 against a computed 18.015, a difference of 0.03% — so a copied figure will not match exactly and should not have to. If you want the typed number taken at face value, clear the formula field.

How small an amount can this page handle?

One milligram, which comes out as 0.00006 moles of water rather than 0 — the moles line carries five decimal places and the millimoles line two, and both bottom out at the same 10⁻⁵ mol, so neither collapses to a bare zero on the smallest mass the mass field accepts. At the other end a tonne is 55509.29781 moles of water. The millimoles line is the more readable of the two below about a tenth of a mole, which is what it is there for.

• SI Units — Amount of Substance: the mole — National Institute of Standards and Technology (NIST)
• Standard Atomic Weights — abridged to four significant figures — Commission on Isotopic Abundances and Atomic Weights (CIAAW), IUPAC