Molar Mass Calculator

About the Molar Mass Calculator

Type a chemical formula — or a common name such as caffeine or baking soda — and get its molar mass in g/mol at once, with every element’s share: atoms, atomic weight, mass and mass percent. The calculator reads brackets of every kind (Ca₃(PO₄)₂, K₄[Fe(CN)₆]), hydrates (CuSO₄·5H₂O, written with ·, * or a full stop), charges (SO₄²⁻ as SO4^2-, Fe³⁺ as Fe3+), isotopes (D₂O, ¹³CO₂ as ^13CO2) and decimal subscripts (Fe0.95O), and shows how it read your input so nothing is guessed silently.

Atomic weights are the IUPAC CIAAW Standard Atomic Weights 2024, and isotope masses come from NIST. Next to the molar mass you also get the Hill formula, the mass of one formula unit, the water of crystallisation of a hydrate, the monoisotopic and nominal mass used in mass spectrometry (with m/z for ions), and the degree of unsaturation of organic formulas — each with the working shown.

Common uses

• Chemistry homework: molar masses for mole and stoichiometry problems, and percent composition.
• Lab work: the molar mass of the exact hydrate on the reagent bottle, before weighing out a solution.
• Checking a molecular formula from elemental analysis against its mass percentages.
• Mass spectrometry: monoisotopic and nominal mass, and m/z of an ion.
• Organic chemistry: the degree of unsaturation (rings plus π bonds) from a molecular formula.

How molar mass is calculated

The molar mass M of a compound is the sum, over its elements, of the number of atoms in the formula times the element’s atomic weight: M = Σ nᵢ × Aᵣ,ᵢ g/mol. For CuSO₄·5H₂O: 1 × 63.546 (Cu) + 1 × 32.06 (S) + 9 × 15.999 (O) + 10 × 1.0080 (H) = 249.677 g/mol. Each element’s mass percent is its share of that sum, and its atom percent is its share of the atoms.

Molar mass (g/mol), relative molecular mass Mr (no unit) and the molecular mass of one molecule in daltons are the same number. For an ionic compound such as NaCl there are no molecules, and the same sum is called the formula mass (OpenStax Chemistry 2e §3.1). Dividing M by the Avogadro constant, 6.022 140 76 × 10²³ mol⁻¹ (exact), gives the mass of one formula unit in grams.

Which atomic weights are used

• Elements with one standard atomic weight use it in full, for example Na 22.989 769 28(2) and Fe 55.845(2).
• Fourteen elements — H, Li, B, C, N, O, Mg, Si, S, Cl, Ar, Br, Tl and Pb — are given by CIAAW as an interval because their isotopic mix varies in nature (carbon: [12.0096, 12.0116]). For these the CIAAW abridged value is used: H 1.0080, C 12.011, N 14.007, O 15.999, S 32.06, Cl 35.45 and so on.
• Radioactive elements without a standard atomic weight (technetium, promethium, polonium to actinium, and neptunium onwards) use the NIST mass of their longest-lived isotope as listed by CIAAW, such as ⁹⁷Tc; label another isotope with ^99Tc if you need it.
• Isotope labels (D, T, ^13C, ¹⁸O, [15N]) use NIST relative atomic masses.

The ± under the result adds up every atom’s CIAAW uncertainty: the molar mass of any normal sample lies within it, which tells you how many decimals are meaningful (58.44 ± 0.010 g/mol for NaCl, limited by chlorine). Older tables used values such as H 1.007 94 and S 32.065, so results can differ from them in the second or third decimal.

How to type formulas

• Brackets: ( ), [ ] and { } nest freely: Ca3(PO4)2, K4[Fe(CN)6], [Co(NH3)5Cl]Cl2.
• Hydrates and adducts: CuSO4·5H2O, CuSO4*5H2O, CuSO4.5H2O or CuSO4 5H2O; halves as CaSO4·0.5H2O, CaSO4·1/2H2O or CaSO4·½H2O; a multiplier on the first part as in 2CaSO4·H2O.
• Decimal subscripts for non-stoichiometric solids: Fe0.95O, La0.7Sr0.3MnO3 (write a leading zero).
• Charges at the end: SO4^2-, SO4{2-}, SO4(2-), SO4²⁻, NH4+, Fe3+, Fe+++. One digit before the sign is read as a charge only for a lone element that has that oxidation state (Fe3+ is Fe³⁺, NH4+ is NH₄⁺); use ^ to be explicit.
• Isotopes: D and T for deuterium and tritium; ^13C, ¹³C or [13C] for any other isotope. A lone isotope can also be typed as C-14, U-235 or carbon-14.
• States at the end are allowed and ignored for the mass: (s), (l), (g), (aq).
• Oxidation states in Roman numerals are read and left out: Fe(III)Cl3 is FeCl₃, Cu(I)2O is Cu₂O.
• Names: about 175 common substances (water, glucose, sodium chloride, Tris, HEPES, EDTA …) and every element name. Each formula was checked against PubChem, and the CID is shown so you can verify it.

Monoisotopic mass and degree of unsaturation

The monoisotopic mass uses the mass of the most abundant isotope of each element instead of the average atomic weight; the nominal mass rounds each of those isotope masses to a whole number first (IUPAC Definitions of Terms Relating to Mass Spectrometry, Recommendations 2013). For molecules made of C, H, N, O and S, whose most abundant isotopes are also the lightest, it is the first peak of the isotope pattern in a high-resolution mass spectrum: 194.080 376 Da for caffeine, against an average molar mass of 194.194. For an ion the calculator also gives m/z, subtracting or adding the electrons (mₑ = 0.000 548 580 u, CODATA 2022).

The degree of unsaturation (also called double-bond equivalents) is the number of rings plus π bonds: (2C + 2 + N − H − X) ÷ 2, where halogens X count like hydrogen, oxygen and sulfur are ignored and nitrogen adds one (OpenStax Organic Chemistry §7.2). Silicon is counted like carbon and phosphorus like nitrogen. Benzene, C₆H₆, has 4: one ring and three double bonds. The formula describes one molecule, so for a hydrate or adduct such as C₆H₁₂O₆·H₂O or Tris·HCl it is worked out for the organic part alone.

Sources

• IUPAC Commission on Isotopic Abundances and Atomic Weights: Standard Atomic Weights 2024, Abridged Standard Atomic Weights, Isotopic Compositions of the Elements and Radioactive Elements.
• NIST, Atomic Weights and Isotopic Compositions with Relative Atomic Masses, version 4.1 (isotope masses from the 2012 Atomic Mass Evaluation).
• CODATA 2022 values from NIST: Avogadro constant and electron mass in u.
• IUPAC, Definitions of Terms Relating to Mass Spectrometry (Recommendations 2013), doi:10.1351/PAC-REC-06-04-06.
• OpenStax, Chemistry 2e (CC BY 4.0), §3.1 Formula Mass and the Mole Concept; OpenStax, Organic Chemistry, §7.2 Calculating the Degree of Unsaturation.
• E. A. Hill, J. Am. Chem. Soc. 22, 478–494 (1900), doi:10.1021/ja02046a005 (Hill order).
• PubChem (NCBI) for the formulas behind compound names.

Limitations

• The molar mass of a real sample depends on its isotopic composition. Results apply to normal terrestrial materials; isotopically enriched or depleted materials (for example depleted lithium or uranium) can differ by more than the ± shown.
• Lead’s atomic weight varies so much in nature (206.14–207.94) that lead compounds carry about ±1.1 g/mol per lead atom.
• Organic abbreviations such as Me, Et and Ph are not expanded: write CH3, C2H5 and C6H5.
• Names cover a curated list of common substances, not every compound. For anything else type the formula.
• The degree of unsaturation assumes the usual valences (C 4, N 3, O 2, halogens 1), so groups such as sulfones or nitro groups written with higher valences are not counted as double bonds.
• Isotopes beyond the naturally occurring ones and a few common radioactive ones load an extra data file (about 65 kB) the first time they are used; offline, that works only after one use online.

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