Grams to Moles Calculator (n = m ÷ MW)

Use the formula n = m ÷ MW, where n is moles, m is mass in grams, and MW is the molecular weight (molar mass) in g/mol. Example: 36 g of water (MW = 18.015 g/mol) → n = 36 ÷ 18.015 = 1.999 mol ≈ 2 mol = 1.204 × 10²⁴ molecules. To convert moles back to grams, rearrange: m = n × MW.

A mole is the SI unit for amount of substance, defined as exactly 6.02214076 × 10²³ elementary entities (atoms, molecules, ions, electrons, or other particles). This number is Avogadro's constant, redefined with its exact integer value in the 2019 SI revision. One mole of any substance contains the same number of particles, regardless of identity. The mole is essential because chemists work with macroscopic masses (grams, kilograms) but reactions occur at the molecular level — the mole bridges those two scales.

Sum the standard atomic weights of every atom in the molecular formula. For water (H₂O): 2 × 1.008 (H) + 1 × 15.999 (O) = 18.015 g/mol. For glucose (C₆H₁₂O₆): 6 × 12.011 + 12 × 1.008 + 6 × 15.999 = 180.156 g/mol. Standard atomic weights are published by IUPAC's Commission on Isotopic Abundances and Atomic Weights (CIAAW) and reflect natural isotopic composition. The NIST Chemistry WebBook at webbook.nist.gov provides authoritative values for tens of thousands of compounds. Most periodic tables include atomic weights below each element symbol — use those values directly for educational and most laboratory calculations.

Molecular weight (MW) is technically dimensionless — it expresses how heavy a molecule is compared to 1/12 the mass of carbon-12. Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol). Numerically, both are equal: water has MW = 18.015 and molar mass = 18.015 g/mol. In daily laboratory and educational practice, the terms are used interchangeably, and you can substitute one for the other without changing any calculation result. The distinction matters only in rigorous scientific writing or thermodynamic equations where dimensional consistency is enforced.

Because atoms of different elements have different masses. One mole always contains ~6.022 × 10²³ particles by definition, but heavier atoms weigh more per particle than lighter ones. Hydrogen (the lightest element) has an atomic weight of 1.008 g/mol, so 1 mol of H atoms weighs only 1.008 g. Iron has atomic weight 55.845 g/mol, so 1 mol of Fe weighs 55.845 g. Uranium (one of the heaviest naturally occurring elements) has atomic weight 238.03 g/mol, so 1 mol of U weighs 238.03 g. The same logic applies to molecules: water (small, light atoms) has MW 18.015 g/mol; sucrose (a large sugar) has MW 342.30 g/mol.

Using n = 1 / 18.015 = 0.05551 mol, then N = 0.05551 × 6.02214076 × 10²³ = 3.343 × 10²² molecules. That is 33.43 sextillion water molecules in a single gram — a number so large it dwarfs all human counting capacity. To put it in perspective: if you tried to count one molecule per second, counting them all would take more than 10¹⁵ years (over 70,000 times the current age of the universe). This is exactly why the mole concept is essential: it lets chemists work with manageable numbers (1, 2, 0.5 mol) instead of unwieldy molecular counts (10²², 10²³, 10²⁴).

Yes. For ionic compounds, you calculate formula units rather than discrete molecules, but the mathematics is identical: n = m / MW. NaCl has MW = 58.44 g/mol (sum of Na = 22.99 and Cl = 35.45), so 29.22 g of NaCl = 0.5 mol = 3.011 × 10²³ formula units. Each formula unit consists of one Na⁺ cation and one Cl⁻ anion. The same approach works for other ionic salts: KNO₃ (MW = 101.10 g/mol), CaCl₂ (MW = 110.98 g/mol), CuSO₄·5H₂O (MW = 249.69 g/mol including waters of hydration). For ionic species the term "formula unit" is technically more accurate than "molecule", though they are commonly used interchangeably.

Avogadro's number (Nₐ = 6.02214076 × 10²³ mol⁻¹) was fixed at this exact value during the 2019 SI redefinition. Before 2019, the mole was defined as the number of atoms in 12 g of carbon-12, and Nₐ was measured experimentally with increasing precision over decades (the best measurements were accurate to 9 significant figures). Since 2019, this relationship reversed: Nₐ is now an exact defined constant, and 1 mole of carbon-12 weighs approximately 12 g (with negligible uncertainty). This change improved metrological consistency across all SI base units. NIST maintains the exact value at physics.nist.gov/cgi-bin/cuu/Value?na.

Rearrange the formula: m = n × MW. For example, 2.5 mol of CO₂ (MW = 44.010 g/mol) = 2.5 × 44.010 = 110.025 g. This reverse calculation is essential when preparing reagents at a specified molar quantity in the laboratory. Other common reverse examples: 0.1 mol of NaOH (MW = 39.997 g/mol) = 3.9997 g (needed for a 0.1 M solution in 1 L); 0.025 mol of glucose (MW = 180.16 g/mol) = 4.504 g; 1 mmol of caffeine (MW = 194.19 g/mol) = 0.19419 g = 194.19 mg. Note that 1 mmol = 0.001 mol, so when working in millimoles divide masses in milligrams by MW.

Molarity is the most common concentration unit in chemistry: M = mol of solute / liters of solution. The mole is the amount; molarity is the concentration. For example: dissolving 1 mol of NaCl (58.44 g) in enough water to make 1 L of solution gives a 1 M (or 1 molar) NaCl solution. Dissolving the same 58.44 g in 2 L gives a 0.5 M solution — same amount of NaCl, lower concentration. Other concentration units derived from moles include molality (mol/kg of solvent), mole fraction (mol of one component / total mol), and normality (equivalents/L). All of these require first calculating the number of moles, which is the role of this calculator.

Because elements in nature occur as mixtures of isotopes with slightly different masses. Carbon, for example, is 98.93% ¹²C (mass 12 exactly, by definition) and 1.07% ¹³C (mass 13.003). The natural average — weighted by abundance — comes to 12.011. This is the standard atomic weight reported by IUPAC. For an isotopically pure sample of ¹²C, you would use mass 12.000; for natural carbon, use 12.011. The same principle applies to chlorine (35.45, mostly ³⁵Cl with some ³⁷Cl), bromine (79.90), and most elements. In analytical or nuclear chemistry, isotope-specific molar masses are sometimes used; in general laboratory work, the standard atomic weights are sufficient.

They are closely related but not identical concepts. A mole is a unit of measurement for amount of substance (similar to how "dozen" is a unit meaning 12). Avogadro's number (Nₐ) is the count of particles in one mole: 6.02214076 × 10²³. You wouldn't say "I have 12 of a dozen apples" — you would say "I have one dozen apples" or "I have 12 apples". Likewise: "I have 1 mole of water" is correct, and that mole contains 6.022 × 10²³ water molecules. The number Nₐ is the conversion factor between moles and particle count, just as 12 is the conversion factor between dozens and individual items.