Molecular Weight Calculator
Enter a chemical formula to calculate molar mass, expand its atom counts, and review each element's mass contribution.
Formula
Element breakdown (0)
Calculation trace
How to use the molecular weight calculator
Enter a chemical formula using standard element symbols and subscripts, then calculate. The result breaks the formula into element counts, shows each element's atomic-weight contribution, and reports its percentage of the total mass. Check the parsed formula before using a result in later work: capitalization and grouping change the substance.
Molecular weight, relative molecular mass, and molar mass
IUPAC defines relative molecular mass as the ratio of the mass of a molecule to the unified atomic mass unit, so it has no unit. “Molecular weight” is a common name for this dimensionless quantity. Molar mass describes the mass of one mole and is expressed here in grams per mole (g/mol). When both use the same formula and atomic-weight values, their numerical values correspond even though the quantities and units are different.
For substances that do not consist of discrete molecules, such as many ionic compounds, “relative formula mass” is the more precise term. The same element-count calculation still gives the corresponding molar-mass value for the entered formula unit.
Parentheses, nested brackets, and hydrates
Multipliers outside parentheses or brackets apply to every element in that group. Hydrate notation uses a middle dot to separate formula parts; the coefficient after the dot multiplies the entire following part. These examples make the expanded counts explicit:
| Formula | Feature | Expanded atom counts |
|---|---|---|
| H2O | Simple subscripts | H: 2, O: 1 |
| Ca(OH)2 | Parentheses | Ca: 1, O: 2, H: 2 |
| CuSO4·5H2O | Hydrate | Cu: 1, S: 1, O: 9, H: 10 |
| K4[ON(SO3)2]2 | Nested brackets | K: 4, O: 14, N: 2, S: 4 |
What the CIAAW 2024 values mean
This calculator uses the CIAAW 2024 abridged standard atomic weights, which are suitable for routine calculations with ordinary terrestrial materials. Standard atomic weights reflect natural isotopic composition; they are measured values, not exact constants. The full CIAAW table gives intervals or uncertainties where applicable, while the abridged table provides compact representative values for calculations.
Some synthetic or radioactive elements have no standard atomic weight in the CIAAW table. A formula containing one of those elements is rejected rather than being assigned a selected isotope mass without your knowledge.
Standard atomic weight is not monoisotopic mass
Monoisotopic mass uses the mass of a specific isotope for each element and is common in mass-spectrometry workflows. This calculator does not calculate monoisotopic mass or isotope-resolved distributions. Its standard-weight result may therefore differ from an exact-mass database or instrument result. It also accepts chemical formulas only; protein and DNA sequences require separate residue, terminal-group, and modification rules.
Frequently asked questions
How do I calculate molecular weight from a formula?
Enter element symbols with their subscripts, such as H2O or Ca(OH)2. The calculator expands supported groups and hydrate parts, multiplies each element count by its CIAAW 2024 abridged standard atomic weight, and adds the contributions. Review the element table rather than treating the displayed decimal as an exact constant.
Are molecular weight and molar mass the same?
They are closely related but not the same quantity. Relative molecular mass, often called molecular weight, is dimensionless. Molar mass is mass per mole and is reported here in g/mol. Their numerical values correspond when the same formula and atomic-weight values are used.
Can I enter parentheses, nested brackets, or a hydrate?
Yes, for supported formulas. Examples include Ca(OH)2, K4[ON(SO3)2]2, and CuSO4·5H2O. A multiplier applies to the group or hydrate part next to it. If a delimiter is unmatched or a multiplier is misplaced, correct the reported input position before using the result.
Does this calculate monoisotopic mass?
No. It uses CIAAW 2024 abridged standard atomic weights for ordinary materials. Monoisotopic and exact-isotope masses answer a different question and can differ from this result, especially for elements with several naturally occurring isotopes.
Why is an element symbol rejected?
Element symbols are case-sensitive: Co is cobalt, while CO represents carbon and oxygen. A made-up symbol such as Xx is invalid. The calculator also rejects elements for which the CIAAW table does not provide a standard atomic weight instead of silently substituting a selected isotope mass.
Can I use this for proteins or DNA sequences?
No. This tool parses chemical formulas, not amino-acid or nucleotide sequences. Sequence molecular weight depends on residue conventions, terminal groups, modifications, and other choices that are outside this calculator's scope.