The molar mass of water, H2O, is 18.015 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 18.015 g, and a gram of it is 3.343e+22 formula units. The calculator turns any weighed mass into that count, and reports the amount in moles, millimoles and the mass of a single formula unit alongside it.
The figure comes from the formula. Water is 2 × 1.008 (H) + 15.999 (O), which is 18.015 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Oxygen makes up the largest share of the mass: 1 of the 3 atoms in a formula unit is oxygen, and they account for 15.999 g of the 18.015 g, or 88.81% by mass. The derivation table below breaks the whole molecule down element by element.
The numbers a bench actually uses sit at the millimole scale. One millimole of water is 18.015 mg and one micromole is 18.015 µg, so a balance reading to a milligram places you within 55.51 µmol. A single formula unit weighs 2.9915e-23 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
The reference substance for almost everything. One mole is 18.015 g, which is 18.0 mL at room temperature — the only compound where those numbers coincide. At 18.015 g/mol it is heavier than 2 of the 11 gases and small molecules covered here, and that ranking matters more than it looks: ammonia has a molar mass of 17.031 g/mol, so a gram of it contains 5.78% more formula units than a gram of water. Weigh by mass and you are not weighing equal amounts of substance.
Because this is a gas, volume is often the easier measure. One mole of any ideal gas occupies 22.414 L at 0 °C and 1 atm and 24.4655 L at 25 °C, so 18.015 g of water — one mole — fills about 24.47 litres at room temperature. Real gases depart from that by a percent or so, more near their boiling point, but for ordinary work the molar volume is the quickest route from grams to litres.
The formula
m- The mass of water in grams
18.015- The molar mass of water in g/mol, derived from its formula
N- The number of formula units that mass contains
How it works, step by step
- Enter a mass of water in grams.
- It is divided by the molar mass, 18.015 g/mol, giving the amount in moles.
- That amount is multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole, to give the number of formula units.
- The panel also reports the amount in moles and millimoles and the mass of one formula unit.
Worked examples
0.5 g of water as a molecule count
0.5 g divided by 18.015 g/mol is 0.0277546 mol, and multiplying by the Avogadro constant gives 1.671e+22 formula units of water. The same mass is 27.755 mmol, which is the figure a reaction is actually planned in.
5 g of water as a molecule count
5 g divided by 18.015 g/mol is 0.277546 mol, and multiplying by the Avogadro constant gives 1.671e+23 formula units of water. The same mass is 277.55 mmol, which is the figure a reaction is actually planned in.
50 g of water as a molecule count
50 g divided by 18.015 g/mol is 2.77546 mol, and multiplying by the Avogadro constant gives 1.671e+24 formula units of water. The same mass is 2775.5 mmol, which is the figure a reaction is actually planned in.
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Frequently asked questions
What is the molar mass of water (H2O)?
It is 18.015 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 2 × 1.008 (H) + 15.999 (O). One mole of water therefore weighs 18.015 g, and a gram of it is 55.509 mmol.
What percentage of water is oxygen?
88.81% by mass. Each formula unit contains 1 oxygen atom contributing 15.999 g of the 18.015 g total, so 100 g of water contains 88.81 g of oxygen and a kilogram contains 888.09 g of it.
How many molecules are in one gram of water?
About 3.343e+22. One gram is 0.0555093 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 18.015.
How much does a single molecule of water weigh?
2.9915e-23 g, which is 18.015 daltons. It is the molar mass divided by the Avogadro constant, and the number in daltons is numerically the same as the molar mass in g/mol — that equivalence is what makes the mole convenient.
Is molar mass the same as molecular weight?
They are the same number in ordinary use but not the same quantity. Molecular weight, properly relative molecular mass, is a ratio and has no unit: for water it is 18.015. Molar mass carries grams per mole: 18.015 g/mol. Because the mole is defined so those agree, you can read one off the other.
Why is the molar mass not a whole number?
Because standard atomic weights are averages over the isotopes found in nature. Oxygen is quoted as 15.999 rather than a whole number for that reason, and the same applies to the other elements here, which is why water comes out at 18.015 g/mol rather than 18.
Molar mass, composition and mole equivalents of water
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 1 | 15.999 | 15.999 | 88.81% |
| Hydrogen (H) | 2 | 1.008 | 2.016 | 11.19% |
| Total — one mole of water | 18.015 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 18.015 g/mol.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000018 | 0.018015 | 6.022e+17 |
| 10 µmol | 0.00018015 | 0.18015 | 6.022e+18 |
| 100 µmol | 0.0018015 | 1.8015 | 6.022e+19 |
| 1 mmol | 0.018015 | 18.015 | 6.022e+20 |
| 10 mmol | 0.18015 | 180.15 | 6.022e+21 |
| 50 mmol | 0.90075 | 900.75 | 3.011e+22 |
| 100 mmol | 1.8015 | 1801.5 | 6.022e+22 |
| 250 mmol | 4.50375 | 4503.75 | 1.506e+23 |
| 500 mmol | 9.0075 | 9007.5 | 3.011e+23 |
| 750 mmol | 13.5113 | 13,511.2 | 4.517e+23 |
| 1 mol | 18.015 | 18,015 | 6.022e+23 |
| 2 mol | 36.03 | 36,030 | 1.204e+24 |
| 5 mol | 90.075 | 90,075 | 3.011e+24 |
| 10 mol | 180.15 | 180,150 | 6.022e+24 |
Mass is the amount multiplied by 18.015 g/mol. The last column is that amount multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole.
| Compound | Formula | Molar mass (g/mol) | Millimoles in 1 g |
|---|---|---|---|
| Methane | CH4 | 16.043 | 62.332 |
| Ammonia | NH3 | 17.031 | 58.716 |
| Water (this page) | H2O | 18.015 | 55.509 |
| Carbon monoxide | CO | 28.01 | 35.702 |
| Nitrogen | N2 | 28.014 | 35.696 |
| Oxygen | O2 | 31.998 | 31.252 |
| Hydrogen peroxide | H2O2 | 34.014 | 29.4 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.