Molar mass of water (H₂O)
Water has a molar mass of 18.015 g/mol: two hydrogens at 1.008 plus one oxygen at 15.999. That is why a mole of water is roughly 18 millilitres, a little over a tablespoon.
Updated on October 7, 2026
Formula, molar mass and composition
H₂O
18.015 g/mol± 0.001
- Formula
- H₂O
- IUPAC
- water (oxidane)
- Name
- water
- Molar mass
- 18.015 g/mol
Composition by mass
| Symbol | Name | Atoms | Standard atomic weight | Share of mass |
|---|---|---|---|---|
| H | Hydrogen | 2 | 1.008 | 11.19 % |
| O | Oxygen | 1 | 15.999 | 88.81 % |
IUPAC publishes a range for this element because its isotopic composition varies in nature. The value shown is the conventional atomic weight, and the uncertainty is half the width of that range, not a measurement error.
What 18 g/mol means in a glass
A 200 ml glass of water weighs about 200 g, which is 11.1 moles, so roughly 6.7 × 10²⁴ molecules. The convenient coincidence that 1 ml of water weighs about 1 g is not a definition any more — it was, back when the gram was defined from water — but it is still accurate to within a fraction of a percent at room temperature.
Note that oxygen makes up 88.8% of water’s mass and hydrogen only 11.2%, even though there are twice as many hydrogen atoms. Mass share and atom count are different questions, and mixing them up is the commonest mistake in composition problems.
Source: Cálculo próprio a partir dos dados citados nesta página
How to read this page
How to read the numbers here
Every value on "Molar mass of water (H₂O)" comes from the source named beside its section, never from an estimate of our own with no origin.
Where the data comes from
Each section of "Molar mass of water (H₂O)" names its official source; the verification date sits at the top of the page.
What the number doesn’t tell you
H2O’s molar mass shifts slightly with where the sample comes from, because one of its elements has an atomic weight published as a range, not a single value.
More about “Molar mass of water (H₂O)”
H2O compared with water
At 18.015 g/mol, one molecule (or formula unit) of H2O weighs 1.0 time as much as one water molecule. A single gram of H2O holds roughly 3.3428 × 10²² units — a number that only makes sense because the mole exists.
Worth repeating, because "Molar mass of water (H₂O)" depends on it: one element in H2O has an atomic weight published as a range by the IUPAC, so the third decimal place of the molar mass shifts a little with where the sample came from.
Words on this page
Atomic weight
The average weight of an element’s atoms, weighted by how abundant each isotope is in nature, in atomic mass units. For 14 elements CIAAW publishes a range instead of a single value, because that abundance varies a little from sample to sample.
Standard definition by CIAAW — Commission on Isotopic Abundances and Atomic Weights · Atomic weight
Mole
The SI counting unit for atoms, molecules or other entities: one mole is exactly 6.02214076 × 10²³ of them, by the definition fixed in 2019. It is what ties a mass in grams to a number of particles.
Standard definition by BIPM — Bureau International des Poids et Mesures (SI) · Mole
Standard uncertainty
The margin attached to a measured value, usually written in parentheses on the last digits. A constant that is "exact by definition" has zero standard uncertainty; a measured one, like the gravitational constant G, always carries some.
Standard definition by BIPM — Bureau International des Poids et Mesures (SI) · Standard uncertainty
Isotope
Atoms of the same element (same number of protons) with a different number of neutrons, and therefore a different mass. Carbon-12 and carbon-14 are isotopes of carbon; only the second one is radioactive.
Atomic number
The number of protons in an atom’s nucleus, written Z. It is this number, not the mass, that defines which element an atom is: every atom with 8 protons is oxygen, whether it has 8 or 18 neutrons.
Standard definition by IUPAC · Atomic number