Chemical Weight Calculator
Chemical weight calculator works out Molar mass and Monoisotopic (exact) mass from Chemical formula, What to report and Sample mass. The default figures on this page give molar mass of 18.0150 g/mol; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
4 steps
- Parsed formula: H2O
- H: 2 x 1.008 = 2.016 g/mol (11.1907 % by mass)
- O: 1 x 15.999 = 15.999 g/mol (88.8093 % by mass)
- Molar mass = 18.015 g/mol
Percent composition by masstable
| symbol | element | atoms | atomicWeight | massContribution | percentByMass |
|---|---|---|---|---|---|
| H | Hydrogen | 2 | 1.008 | 2.016 | 11.190674 |
| O | Oxygen | 1 | 15.999 | 15.999 | 88.809326 |
Figures from IUPAC atomic weights, 2024 edition, checked 31 August 2026. Its 5 worked examples are re-run on every release, last on 15 September 2026. What changed · how reference data is kept current
Sources and references
4 sources
What the chemical weight calculator calculates
Computes molar mass, monoisotopic (exact) mass and percent composition from a chemical formula, including hydrates and ions, and converts between grams and moles.
How to use this chemical weight calculator
Enter Chemical formula, What to report, Sample mass and Amount of substance. Molar mass updates as you type, and “Use a worked example” loads checked figures you can adjust. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Molar mass, Monoisotopic (exact) mass and Formula as parsed.
The method follows IUPAC Commission on Isotopic Abundances and Atomic Weights - Standard Atomic Weights (abridged, five significant figures), Prohaska et al., Standard atomic weights of the elements 2021 (IUPAC Technical Report), Pure and Applied Chemistry 94(5) 573-600, NIST Atomic Weights and Isotopic Compositions with Relative Atomic Masses (AME2020 nuclide masses) and OpenStax Chemistry 2e, section 3.1 Formula Mass and the Mole Concept.
Water example
With Chemical formula: H2O, the calculator returns Molar mass: 18.015, Atoms per formula unit: 3, Net ionic charge: 0. This example is checked against an independent source in the test suite.
What you can compare
Change one input at a time to see how it affects Molar mass, Monoisotopic (exact) mass and Formula as parsed. The inputs you can vary are Chemical formula, What to report, Sample mass, Amount of substance and Report the hydrate breakdown.
Understanding Molar mass, Monoisotopic (exact) mass and Formula as parsed
Masses use the IUPAC/CIAAW abridged standard atomic weights. Check the parsed formula shown in the result before using these figures for reagent preparation, dosing or regulatory reporting.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
How to calculate the weight of a chemical?
Computes molar mass, monoisotopic (exact) mass and percent composition from a chemical formula, including hydrates and ions, and converts between grams and moles. Open “How this is worked out” beside the result to see each step for your own figures.
How to calculate chemical equivalent weight?
It needs Chemical formula, What to report, Sample mass, Amount of substance and Report the hydrate breakdown. Nothing else is required, and the figures stay in your browser.
How do I calculate the molecular weight?
The result gives Molar mass, Monoisotopic (exact) mass, Formula as parsed, Atoms per formula unit, Net ionic charge and Percent composition by mass, each labelled with its unit so the figure can be read on its own.
What is chemical weight?
Computes molar mass, monoisotopic (exact) mass and percent composition from a chemical formula, including hydrates and ions, and converts between grams and moles. On this page it is computed from Chemical formula, What to report and Sample mass and shown as Molar mass, Monoisotopic (exact) mass and Formula as parsed.
What information do I need for the chemical weight calculator?
You need Chemical formula, What to report, Sample mass, Amount of substance and Report the hydrate breakdown. The page opens with default figures that give molar mass of 18.0150 g/mol, so you can see the scale before entering your own.
What does this chemical weight calculator show?
It shows Molar mass, Monoisotopic (exact) mass, Formula as parsed, Atoms per formula unit, Net ionic charge, Percent composition by mass, Anhydrous molar mass and Water of crystallisation, with the working and any warnings for your entries.