Mole Calculator — Grams, Moles and Molar Mass

Background.

Balances read grams; chemical equations count particles. Hydrochloric acid and sodium hydroxide neutralise each other one for one, yet mixing 1 g of each does not give a neutral solution. One gram of HCl (36.458 g/mol) is 0.0274 mol, one gram of NaOH (39.997 g/mol) is 0.0250 mol, and close to 9% of the acid is left unreacted. To compare two weighings chemically you have to convert both to moles, and the conversion factor for each substance is its molar mass.

Three questions come up on that bridge. Grams to moles: you have weighed a sample and want to know how much substance it is. Moles to grams: a method calls for 0.1 mol and you need a figure to put on the balance. And the direction textbooks tend to skip: you know the mass and, from a separate measurement such as a titration, the amount, so the molar mass follows — which is how an unknown compound is narrowed down or a supplier's purity claim tested. Whichever you ask, the other quantities come back with it, including the amount in millimoles, the scale most bench work runs at.

The relation n = m / M is exact. What can be off is the molar mass you type in. Standard atomic weights are averages over the isotope mixtures found in ordinary terrestrial material, and for chlorine, lithium, hydrogen, carbon, oxygen and several other elements that mixture varies enough between sources that IUPAC publishes a range instead of a single number. For a reagent taken off the shelf, the conventional values are good to about four significant figures. For material enriched in one isotope — heavy water, a carbon-13 labelled internal standard — they are simply the wrong number, and the molar mass has to be built from the isotopes actually present.

No temperature, pressure or physical state enters the calculation, because mass and amount are both properties of the sample itself. That sets it apart from molarity, which divides by a volume that grows when the solution warms, and from gas calculations, which need a stated pressure and temperature.

What is mole calculator?

Amount of substance, symbol n, is one of the seven base quantities of the International System of Units, and its unit is the mole. Since the SI revision that took effect on 20 May 2019, one mole is defined as exactly 6.02214076 × 10²³ specified elementary entities: atoms, molecules, ions, formula units or electrons, whichever you name. The older definition tied the mole to the number of atoms in 12 g of carbon-12; the current one fixes the count and does not mention carbon at all.

Molar mass, M, is the mass of a sample divided by the amount of substance in it, M = m / n. Its coherent SI unit is kg/mol, but chemistry works in g/mol because the number then matches the relative molecular mass printed in tables: sodium chloride's relative formula mass of 58.44 becomes 58.44 g/mol. Strictly, that match stopped being an identity in 2019. Carbon-12 now has a measured molar mass of 12.0000000126 g/mol (CODATA 2022), heavier than 12 by about one part in a billion — far below anything a balance can detect.

The entity always has to be stated. A mole of oxygen could mean O atoms or O₂ molecules, and the two differ in mass by a factor of two. For an ionic solid such as sodium chloride there are no discrete molecules, so the entity is the formula unit NaCl.

How to use this calculator.

1. 01Choose the question under 'What do you want to find?': Moles if you have already weighed the sample, Mass if you need to know how much to weigh, Molar mass if you know both the mass and the amount from separate measurements.
2. 02Enter the molar mass in g/mol for the exact form of the substance on your balance. Bottle labels and certificates of analysis print it beside the formula. Check whether that formula carries water of crystallisation (·5H₂O, ·10H₂O): if you are weighing the hydrate, the hydrate's molar mass is the one you need. With only a formula to go on, the molar mass calculator will add up the atomic weights for you.
3. 03Enter the mass in grams or the amount in moles, as the chosen question requires. The field a mode does not read is ignored, so it can keep whatever value it has.
4. 04Read the amount in mol and in mmol. A quick sense check: a heavier substance should give fewer moles than a lighter one from the same mass. If yours went the other way, the mass and molar mass have probably been entered in each other's boxes.
5. 05Report only the precision you measured. A three-place balance and a molar mass good to four significant figures support three or four figures in the answer, however many the display shows.

The formula.

The IUPAC Green Book (section 2.10) defines molar mass as M = m / n. Each mode solves that one relation for a different letter:

n = m / M gives the amount from a weighed mass. m = n × M gives the mass to weigh for a wanted amount. M = m / n gives the molar mass from a mass and an independently measured amount.

The units cancel cleanly: grams divided by grams per mole leaves moles. Because a value in g/mol is numerically the same in mg/mmol and kg/kmol, any consistent scaling gives the same digits. Twenty-five grams of glucose at 180.156 g/mol is 0.1388 mol; 25 mg of it is 0.1388 mmol. If you type a milligram mass into the gram field, read the mole result as millimoles.

Sensitivity is simple because the relation is a plain ratio: an error of a given percentage in M produces the same percentage error in n. Using ordinary water's molar mass for heavy water puts every amount 11% out. By contrast, the stated uncertainty in chlorine's conventional atomic weight, ±0.01 in 35.45, moves the molar mass of sodium chloride by less than 0.02%.

Some entries have no physical answer and are refused instead of producing infinity: a molar mass of zero when solving for moles or mass, zero moles when solving for molar mass (M = m / n has no value at n = 0), and zero mass with a non-zero amount, which would describe weightless matter. Negative masses and amounts are refused rather than quietly made positive. One zero is accepted — a mass of 0 g when solving for moles returns 0 mol, the correct answer for an empty weighing boat.

Tiny amounts keep their significant figures instead of collapsing to zeros. One milligram of a 66,000 g/mol protein, roughly the size of serum albumin, comes out as 1.515 × 10⁻⁸ mol, or about 15.15 nanomoles.

A worked example.

Frequently asked questions.

References& sources.

1. [1]BIPM, The International System of Units (SI), 9th edition (2019) — definition of the mole, in force since 20 May 2019 following CGPM Resolution 1 (2018): 'The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.022 140 76 x 10^23 elementary entities.' Open access, HTML; retrieved and verified 2026-07-29.
2. [2]IUPAC, Quantities, Units and Symbols in Physical Chemistry ('Green Book'), 3rd edition, 2nd printing 2012, section 2.10 'General chemistry', p. 47 — defines molar mass M_B = m / n_B (SI unit kg mol^-1), amount of substance n_B = N_B / L, and relative atomic mass A_r = m_a / m_u; p. 53 requires that the elementary entity always be specified. Open-access searchable PDF; text-verified 2026-07-29.
3. [3]IUPAC CIAAW, Abridged Standard Atomic Weights, 2024 revision (the 2021 technical report as revised for Gd, Lu and Zr in 2024) — the conventional single values used in this page's worked examples: Na 22.990(1), Cl 35.45(1), H 1.0080(2), O 15.999(1), C 12.011(2). Open access; retrieved and verified 2026-07-29.
4. [4]IUPAC CIAAW, Standard Atomic Weights, 2024 revision — the interval-valued table for elements whose terrestrial isotopic composition varies: H [1.00784, 1.00811], Li [6.938, 6.997], C [12.0096, 12.0116], O [15.99903, 15.99977], Cl [35.446, 35.457]. Sodium is a point value, 22.98976928(2). Open access; retrieved and verified 2026-07-29.
5. [5]NIST, CODATA 2022 recommended values of the fundamental physical constants — molar mass constant M_u = 1.000 000 001 05(31) x 10^-3 kg mol^-1 (relative standard uncertainty 3.1 x 10^-10), from which M(carbon-12) = 12 x M_u = 12.000 000 0126(36) g/mol; and the Avogadro constant N_A = 6.022 140 76 x 10^23 mol^-1, listed as exact. Open access; retrieved and verified 2026-07-29.
6. [6]NIST Standard Reference Database 144 — 'Atomic Weights and Isotopic Compositions with Relative Atomic Masses'. Source of the nuclide masses quoted on this page: H-1 1.00782503223(9), H-2 (deuterium) 2.01410177812(12), from which heavy water's molar mass of 20.027 g/mol is derived. Open access; retrieved and verified 2026-07-29.
7. [7]United States Pharmacopeia-National Formulary, General Notices and Requirements, section 8.40 'Atomic Weights': 'Atomic weights used in computing molecular weights and the factors in the assays and elsewhere are those established by the IUPAC Commission on Atomic Weights and Isotopic Abundances.' Consulted as the independent second authority for this page; the USP Sodium Chloride monograph header 'NaCl 58.44' agrees with the CIAAW-derived 22.990 + 35.45 = 58.440 g/mol used in the worked example. Open access PDF; text-verified 2026-07-29.