In short: water's molar mass is 18.015 g/mol, so 1 gram of water is about 0.0555 mol, and 1 kilogram is about 55.5 mol. Starting from a volume instead of a mass takes one extra step, because the mass in a given volume depends slightly on temperature — use the calculator above to convert either one exactly.
These figures are planning and study aids based on published atomic-weight and density data. For analytical or regulatory work, confirm with a qualified chemist or lab.
How to calculate moles of water
A mole is just a very large counting unit — 6.02214076 × 10²³ particles, defined so that one mole of a substance weighs exactly its molar mass in grams. Water's molar mass is 18.015 g/mol, so converting a mass of water into moles is a single division:
This calculator does exactly that: convert your input to grams (using density if you entered a volume), then divide by 18.015 to get moles, and multiply by Avogadro's number for the molecule count. For quick chemistry questions — "how many moles are in a gram of water?" — the mass route is simplest. For lab or dosing work starting from a measured volume, the volume route folds in the correct density automatically.
Worked example. Take exactly 18.015 g of water — that is, by definition, 1.000 mol. Take 1 kg (1000 g) instead: 1000 ÷ 18.015 = 55.509 mol, the figure behind the well-known chemistry rule of thumb that "a liter of water is about 55.5 molar." A mole of water molecules is 6.02214076 × 10²³ of them — an inconceivably large number packed into about one tablespoon.
Molar mass, in one line. Water is H₂O — two hydrogens and one oxygen. Using IUPAC's standard atomic weights, 2 × 1.008 + 15.999 = 18.015 g/mol. Every mole calculation on this page rests on that one number.
Why temperature matters for volume, but not for mass
Heating water does not create or destroy molecules, so the number of moles in a fixed mass of water never changes with temperature — 1 kg of water is always 55.509 mol, whether it is ice-cold or nearly boiling. But water expands as it warms, so a fixed volume of water holds slightly less mass — and therefore slightly fewer moles — as temperature rises.
That is why converting from a volume requires knowing the temperature: 1 liter of water at 4 °C (its densest point) has a mass of about 999.97 g and holds about 55.51 mol, while the same liter at 20 °C has a mass of about 998.2 g and holds about 55.41 mol. The calculator uses the Kell equation, the standard reference formula for the density of air-free water between 0 °C and 150 °C, to make that conversion exact. Full detail is on our methodology page.
Common amounts of water, in moles
A quick-reference table for everyday and lab-scale amounts. Volume-based rows assume room temperature (20 °C / 68 °F); the mass row holds at any temperature, since mass alone determines moles.
| Amount | Mass | Moles |
|---|---|---|
| 18.015 g (exactly 1 mole) | 18.015 g | 1.000 mol |
| 1 mL (20 °C) | 0.998 g | 0.0554 mol |
| 1 US fl oz (20 °C) | 29.52 g | 1.639 mol |
| 1 US cup (20 °C) | 236.16 g | 13.11 mol |
| 1 US quart (20 °C) | 944.65 g | 52.44 mol |
| 1 liter (20 °C) | 998.20 g | 55.41 mol |
| 1 kg (any temperature) | 1,000 g | 55.51 mol |
| 1 US gallon (20 °C) | 3,778.6 g | 209.7 mol |
Worked examples
A 500 mL water bottle
27.7 mol
At room temperature, a 500 mL bottle holds about 499.1 g of water — around 27.7 mol, or roughly 1.67 × 10²⁵ molecules.
A 240 mL cup of hot tea
12.9 mol
At 90 °C, water is noticeably less dense (about 965.3 kg/m³), so this cup holds about 231.7 g of water — about 12.9 mol, slightly less than the same cup would hold cold.
An Olympic-size swimming pool
1.38 × 10⁸ mol
At about 2,500,000 liters (25 °C), a full Olympic pool holds roughly 138 million moles of water — around 8.3 × 10³¹ individual molecules.
What else affects the result
Isotopic composition. This calculator assumes ordinary water (H₂O) at natural isotopic abundance, molar mass 18.015 g/mol. Heavy water (D₂O), where both hydrogens are deuterium, has a molar mass of about 20.03 g/mol — the same mass of heavy water contains noticeably fewer moles.
Dissolved substances. If your sample is salt water, a sugar solution, or another mixture, its total mass includes the dissolved solute, not just water. For a rough answer you can still use the volume route with a custom density, but the mole count returned is moles of water only, not of the whole solution.
Pressure. Water is nearly incompressible, so pressure has a negligible effect on density — and therefore on moles per volume — outside deep-ocean or high-pressure industrial settings.
Frequently asked questions
How many moles are in a gram of water?
One gram of water is about 0.0555 mol, because water's molar mass is 18.015 g/mol (1 ÷ 18.015 = 0.05551). A single mole of water — 18.015 g — is only about a tablespoon.
How many moles of water are in a liter?
About 55.4 mol at room temperature (20 °C), where 1 liter has a mass of roughly 998.2 g. The figure shifts slightly with temperature because volume changes even though mass does not — at 4 °C, the densest point, 1 liter is closer to 55.5 mol.
What is the molar mass of water and why is it 18.015?
Water is H₂O: two hydrogen atoms (1.008 g/mol each) and one oxygen atom (15.999 g/mol), giving 2(1.008) + 15.999 = 18.015 g/mol. These are the standard atomic weights published by IUPAC.
Does temperature change the number of moles in a fixed amount of water?
No. Moles depend only on mass, and heating water does not add or remove molecules. Temperature changes water's volume and density, which is why converting from a volume — rather than a mass — requires knowing the temperature.
How many water molecules are in a mole?
Exactly Avogadro's number: 6.02214076 × 10²³ molecules. That is true for one mole of any substance, not just water — it is the defining size of the mole.
Does heavy water (D2O) have the same molar mass as regular water?
No. Heavy water replaces both hydrogen atoms with deuterium, giving a molar mass of about 20.03 g/mol instead of 18.015 g/mol. This calculator assumes ordinary water (H₂O) at natural isotopic abundance.
Sources & references
- International Union of Pure and Applied Chemistry (IUPAC). Standard Atomic Weights of the Elements.
- CODATA Internationally Recommended 2018 Values of the Fundamental Physical Constants — Avogadro constant.
- Kell, G. S. (1975). Density, thermal expansivity, and compressibility of liquid water from 0° to 150 °C. Journal of Chemical & Engineering Data, 20(1), 97–105.
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)