What is molecular weight (molar mass)?

It is the mass of one mole of a substance, reported in grams per mole (g/mol), computed from the sum of atomic weights of the constituent elements.

Which atomic weights does this calculator use?

Standard atomic weights from the Commission on Isotopic Abundances and Atomic Weights (CIAAW, 2021), with supplemental values from NIST where applicable.

Can I enter hydrates like CuSO4·5H2O?

Yes. The middle dot notation is fully supported and treated as an additive component with its multiplier.

Does the tool support parentheses and nested groups?

Yes. Parentheses (), as well as [ ] and { }, with multipliers (including nested) are supported.

Are isotope-specific masses supported?

If you prefix an element with a mass number (e.g., 13C), the calculator ignores the mass number and uses the standard atomic weight. Isotope-resolved calculations are not supported in this version.

How precise are the results?

Results use standard atomic weights and are rounded to 3–5 significant digits in the UI for readability. You can change precision using the copy/export feature of your browser if needed.

Molecular Weight Calculator

Quickly compute the molecular weight (molar mass) and elemental composition of any compound. Designed for chemists, students, and lab professionals, this tool parses chemical formulas (including parentheses and hydrates) with precision and accessibility.

Calculator

Formula

Element list

Chemical formula

Results

Normalized formula (Hill notation)

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Empirical formula

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Molar mass

0.000 g/mol

Total atoms per formula unit

Element	Count	Atomic weight (g/mol)	Mass contribution (g/mol)	% by mass
Enter a formula to see the breakdown.

Authoritative Data Source and Methodology

Primary source: Commission on Isotopic Abundances and Atomic Weights (CIAAW). “Standard Atomic Weights of the Elements 2021.” Accessed at https://ciaaw.org/atomic-weights.htm (2021). Secondary reference: NIST Chemistry WebBook / SRD, “Atomic Weights and Isotopic Compositions.” All atomic weights used are the conventional standard values suitable for general chemical calculations.
Statement: Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.

The Formula Explained

For a compound composed of elements E_i with stoichiometric coefficients n_i, the molar mass M is: $$ M = \sum_{i} n_i \, A_r(E_i) \quad [\mathrm{g\,mol^{-1}}] $$ where A_r(E_i) is the standard atomic weight of element E_i.

The mass percent of element i is: $$ w_i(\%) = \frac{n_i \, A_r(E_i)}{\sum_{j} n_j \, A_r(E_j)} \times 100 $$

Glossary of Variables

Chemical formula (input): The symbolic representation of a substance indicating element symbols and their counts, e.g., C6H12O6.
Element (input, builder mode): Individual chemical element symbol selected from the periodic table.
Count (input, builder mode): Number of atoms of the selected element in one formula unit.
Molar mass (result): The mass of one mole of the compound in grams per mole (g/mol).
Normalized formula (result): The formula written in Hill notation (C then H, followed by other elements alphabetically; or alphabetical if no carbon).
Empirical formula (result): The simplest whole-number ratio of atoms in the compound.
Elemental breakdown (result): Table listing per-element count, atomic weight, mass contribution, and mass percentage.

Worked Example

How It Works: A Step-by-Step Example (Caffeine, C8H10N4O2)

Enter the formula C8H10N4O2.
Using standard atomic weights: A_r(C)=12.011, A_r(H)=1.008, A_r(N)=14.007, A_r(O)=15.999.
Compute contributions: $$ \begin{aligned} m_C &= 8 \times 12.011 = 96.088 \\ m_H &= 10 \times 1.008 = 10.080 \\ m_N &= 4 \times 14.007 = 56.028 \\ m_O &= 2 \times 15.999 = 31.998 \end{aligned} $$
Sum to get the molar mass: $$ M = 96.088 + 10.080 + 56.028 + 31.998 = 194.194\ \mathrm{g/mol} $$
Mass percentages, e.g., for carbon: $$ w_C(\%) = \frac{96.088}{194.194} \times 100 \approx 49.48\% $$ The calculator performs the same for all elements and outputs the full breakdown.

Frequently Asked Questions (FAQ)

What’s the difference between “molecular weight” and “molar mass”?

In modern IUPAC terminology, “molar mass” (g/mol) is preferred for macroscopic amounts; “molecular weight” historically refers to a relative quantity. Many labs still use the terms interchangeably—this tool reports values in g/mol.

Can I paste formulas with subscripts (e.g., H₂SO₄)?

Yes. Unicode subscripts ₀–₉ are normalized automatically, so H₂SO₄ is treated as H2SO4.

Does the calculator support nested parentheses and brackets?

Yes. Grouping with (), [], or {} is supported, including nesting and multipliers like Al2(SO4)3.

How do I enter hydrates?

Use the middle dot: CuSO4·5H2O. You may also include a leading multiplier for any part, such as 3H2O.

What happens if I type an unknown element?

The input is validated and a clear error message appears indicating the first unrecognized symbol, so you can correct the formula.

Are charged species or electrons considered?

No. This tool computes neutral formula weights from elemental composition only. Charges do not affect molar mass at this precision.

Strumento sviluppato da Ugo Candido.
Contenuti verificati da CalcDomain Science Editorial Board.
Ultima revisione per l'accuratezza in data: September 15, 2025.