Grams to Moles Calculator

About the Grams to Moles Calculator

Convert between the four ways chemists count a substance: mass in grams, amount in moles, the number of particles (molecules, atoms, ions or formula units) and, for gases, the volume at STP or at any temperature and pressure. Type a formula or a name and the molar mass is worked out for you from IUPAC (CIAAW 2024) atomic weights — or type a molar mass yourself, in g/mol or kDa for a protein.

Whichever quantity you know, the calculator gives all the others at once, the number of atoms and the mass of each element in the sample, and the dimensional analysis behind every number, using the exact Avogadro constant 6.022 140 76 × 10²³ mol⁻¹ and molar gas constant R = 8.314 462 618… J/(mol·K).

Common uses

• Chemistry homework on the mole: grams ↔ moles ↔ particles, step by step.
• Working out how many molecules or atoms are in a weighed sample.
• Gas problems at STP or at room conditions: litres ↔ moles ↔ grams.
• Converting a protein or polymer mass to moles from its molar mass in kDa.

The formulas

• Mass and amount: n = m ÷ M, so m = n × M, where M is the molar mass in g/mol.
• Amount and particles: N = n × N_A, where N_A = 6.022 140 76 × 10²³ mol⁻¹ is the Avogadro constant. Since 2019 this number is exact: one mole contains exactly 6.022 140 76 × 10²³ elementary entities — atoms, molecules, ions, electrons or other specified particles (BIPM, SI base unit mole).
• Amount and gas volume: V = n × V_m, with the ideal-gas molar volume V_m = R·T ÷ p. R = 8.314 462 618… J/(mol·K) is exact too (R = N_A·k).

Which STP?

IUPAC defines standard conditions for gases as 273.15 K (0 °C) and 10⁵ Pa (100 kPa), and recommends that the older pressure of 1 atm (101.325 kPa) no longer be used (Gold Book). Many textbooks still use 0 °C and 1 atm, so both are offered:

• 0 °C and 100 kPa: V_m = 22.710 954 64… L/mol
• 0 °C and 1 atm: V_m = 22.413 969 54… L/mol
• 25 °C and 100 kPa: 24.790 L/mol; 25 °C and 1 atm: 24.465 L/mol

The first two are CODATA 2022 values; the others follow from V_m = R·T ÷ p. The two STPs differ by 1.3 %, so check which one your course or data source uses.

Molecules, atoms, formula units and ions

The particle count is named after the substance: atoms for an element such as Cu or He, molecules for molecular substances such as H₂O, O₂ or glucose, formula units for ionic compounds such as NaCl or NH₄Cl (which have no molecules), and ions for a charged formula. The table of elements also gives the atoms of each element: one mole of H₂O holds two moles of H atoms.

Sources

• BIPM, SI base unit: mole, and The International System of Units (SI Brochure, 9th edition, 2019).
• NIST CODATA 2022: Avogadro constant, molar gas constant, molar volume at 273.15 K and 100 kPa and at 273.15 K and 101.325 kPa.
• IUPAC Compendium of Chemical Terminology (Gold Book), standard conditions for gases.
• IUPAC CIAAW, Standard Atomic Weights 2024 (molar masses).
• OpenStax, Chemistry 2e (CC BY 4.0), §3.1 Formula Mass and the Mole Concept (worked examples).
• NIST SP 811 (2008), Appendix B.8 (pressure and mass units).

Limitations

• Gas volumes assume ideal behaviour. Real gases deviate a little, more at high pressure and low temperature, and the volume of a solid or liquid has nothing to do with these figures (use its density).
• Molar masses from a formula apply to normal isotopic composition; isotopically labelled formulas use the label’s mass.
• Results are shown to the significant figures you choose: round your final answer to the precision of your data.

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