In short: the molar mass of ordinary water (H₂O) is 18.015 g/mol — two hydrogen atoms at 1.008 each, plus one oxygen atom at 15.999. Heavy water (D₂O) comes in around 20.027 g/mol, about 11% heavier, because deuterium carries an extra neutron. Use the calculator above for any isotope combination.
These figures use IUPAC standard atomic weights and are suitable for classroom and general laboratory use. For isotope-ratio mass spectrometry or regulatory reporting, consult the primary CIAAW/NIST tables.
How to calculate the molar mass of water
Water's molar mass is nothing more than the sum of the atomic masses of the atoms in one molecule. Every water molecule has the same skeleton — two hydrogen-site atoms bonded to one oxygen atom — so the whole calculation comes down to which isotopes sit in those three positions.
This calculator reads the atomic masses for your selected isotope combination, adds them together, and reports the total in grams per mole, kilograms per mole, daltons and more.
Worked example. For ordinary water, the IUPAC standard atomic weight of hydrogen is 1.008 and of oxygen is 15.999. Two hydrogens plus one oxygen: 1.008 + 1.008 + 15.999 = 18.015 g/mol. That single addition is the entire calculation — the work is in knowing which atomic masses to add.
Molar mass vs. molecular weight, in one line. Molecular weight is the mass of a single molecule, typically expressed in daltons (u). Molar mass is the mass of a mole (6.022 × 10²³ molecules) of that substance, in grams per mole. The mole is defined so the two numbers are numerically identical — 18.015 u per molecule and 18.015 g per mole — which is why chemists often use the terms interchangeably.
Why isotopes change the answer
Every hydrogen atom has one proton, and every oxygen atom has eight — that's what makes them hydrogen and oxygen. But atoms of the same element can carry different numbers of neutrons, and neutrons add mass without changing chemical identity. Those variants are called isotopes.
Deuterium (D, or ²H) is hydrogen with one extra neutron, roughly doubling its atomic mass to 2.014. Tritium (T, or ³H) adds two neutrons, reaching about 3.016, and is radioactive. On the oxygen side, oxygen-18 carries two extra neutrons versus the most common oxygen-16, pushing its atomic mass from about 15.995 up to 17.999.
Swap any of water's three atoms for a heavier isotope and the molecule — an isotopologue — gets measurably heavier, even though it is still chemically water. Heavy water (D₂O) is used as a neutron moderator in some nuclear reactors and as a solvent in NMR spectroscopy; oxygen-18 water is a common tracer in climate and medical research, since its extra mass is easy to track without radioactivity. Read more about how we source these constants on our methodology page.
Molar mass of water and its isotopologues
The atomic masses this calculator uses, and the resulting molar mass for each common combination.
| Water type | Formula | Atomic masses used | Molar mass |
|---|---|---|---|
| Ordinary water (default) | H₂O | 2 × 1.008 H + 15.999 O | 18.015 g/mol |
| Semi-heavy water | HDO | 1.008 H + 2.014 D + 15.999 O | 19.021 g/mol |
| Heavy water | D₂O | 2 × 2.014 D + 15.999 O | 20.027 g/mol |
| Oxygen-18 water | H₂¹⁸O | 2 × 1.008 H + 17.999 ¹⁸O | 20.015 g/mol |
| Tritiated water | T₂O | 2 × 3.016 T + 15.999 O | 22.031 g/mol |
Worked examples
One mole of ordinary water
18.015 g
By definition, one mole of any substance weighs its molar mass in grams. One mole of H₂O — 6.022 × 10²³ molecules — weighs 18.015 g, roughly one tablespoon.
A 100 mL bottle of pure D₂O
110.6 g
Heavy water is denser than ordinary water (about 1.1056 g/mL). At a molar mass of 20.027 g/mol, 100 mL contains roughly 5.52 moles of D₂O.
Comparing H₂O and D₂O by mass
+11.2%
Gram for gram of molecules, D₂O outweighs H₂O by (20.027 − 18.015) / 18.015 ≈ 11.2%, entirely from the two extra neutrons in deuterium.
What else affects the result
Natural water is already a mixture. Ordinary tap or lab water is overwhelmingly H₂O, but it also contains trace amounts of HDO and H₂¹⁸O in the proportions found in nature (defined by the VSMOW reference standard). The "standard atomic weight" values already bake this natural mixture in, which is why hydrogen's weight (1.008) sits slightly above the mass of a pure protium atom (1.00783).
Pressure and temperature don't matter here. Molar mass is a property of the atoms themselves, not of the sample's physical state. A mole of steam and a mole of ice have the same molar mass — only their volume and density change.
Converting to a real sample. Molar mass is the bridge between a mass you can weigh and a mole count used in chemistry. If you have a mass of water and want its mole count, our Moles of Water Calculator handles that conversion directly.
Frequently asked questions
What is the molar mass of water?
The molar mass of ordinary water (H₂O) is 18.015 grams per mole. It comes from adding the standard atomic weights of its atoms: two hydrogen atoms at 1.008 each, plus one oxygen atom at 15.999.
Why is water's molar mass 18.015 and not exactly 18?
Hydrogen and oxygen aren't whole numbers on the periodic table because their standard atomic weights average every naturally occurring isotope, including trace deuterium and oxygen-17/18. That averaging pushes the total from a round 18 to 18.015.
What is the molar mass of heavy water (D₂O)?
Heavy water, D₂O, has a molar mass of about 20.027 g/mol. It replaces both hydrogen atoms with deuterium, an isotope with an extra neutron and an atomic mass of about 2.014 — roughly double that of ordinary hydrogen.
How do I convert grams of water to moles?
Divide the mass in grams by the molar mass in g/mol. For ordinary water, divide by 18.015. Our Moles of Water Calculator does this conversion directly if you have a mass to work from.
Does the molar mass of water change with temperature?
No. Molar mass depends only on which atoms make up the molecule, not on temperature or pressure. Temperature changes water's density and volume, but a mole of water always has the same mass for a given isotope composition.
What's the difference between molar mass and molecular weight?
They describe the same underlying quantity from two angles. Molecular weight (or molecular mass) is the mass of one molecule, usually in daltons (u). Molar mass is the mass of one mole, in grams per mole. Numerically the two figures match — 18.015 u per molecule and 18.015 g per mole — because the mole is defined so that they do.
Sources & references
- IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW). Standard Atomic Weights 2021.
- National Institute of Standards and Technology (NIST). Atomic Weights and Isotopic Compositions.
- International Bureau of Weights and Measures (BIPM). The International System of Units (SI) — definition of the mole and the Avogadro constant.
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)