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Mole Calculator

Ready to calculate
n = m / M.
5 mole units.
Avogadro 6.022×10²³.
100% Free.
No Data Stored.

How it Works

01Weigh the Substance

Mass on an analytical balance. Accept g, mg, µg, ng, kg, lb, oz — the calculator converts internally.

02Look Up the Molecular Weight

From PubChem, CRC Handbook, NIST WebBook, or supplier datasheet. In g/mol (= Da).

03Apply n = m / M

Divide mass by molar mass. The result is moles — the SI base unit for amount of substance.

04Get All Mole Units + Molecules

Output in mol, mmol, µmol, nmol, kmol, plus the number of molecules via Avogadro Nₐ = 6.022×10²³.

What is a Mole Calculator?

The Mole Calculator implements the foundational stoichiometry identity n = mass / molar mass — the most-used formula in all of chemistry. Every limiting-reagent problem, every percent-yield calculation, every titration, every dilution, every reagent prep starts with the conversion between mass (what you weigh) and moles (what reacts). The mole is the SI base unit for amount of substance; since the 2019 SI redefinition, one mole is defined as exactly 6.02214076 × 10²³ entities (Avogadro's number Nₐ). Whether you're working with atoms, molecules, ions, formula units, electrons, or photons, the count per mole is the same fundamental constant.

Our calculator accepts mass in 7 unit systems (g, mg, µg, ng, kg, lb, oz) and molecular weight in g/mol (numerically equal to daltons / Da for molecules). Output: moles in 5 simultaneous units (mol, mmol, µmol, nmol, kmol) with smart auto-pick of the cleanest unit for hero display, plus the absolute number of molecules via n × Nₐ — useful for kinetics, single-molecule biology, mass-spec entity counting, and order-of-magnitude sanity checks. Smart warnings catch common errors: MW < 1 g/mol (unphysical — only sub-atomic masses are below 1 g/mol), MW > 1 MDa (typical only for very large biomolecules — verify), and unrealistically large mole counts (industrial-scale only).

Designed for general chemistry students learning stoichiometry, organic and inorganic chemists computing reagent moles for syntheses, biochemists preparing enzyme assays and buffers, pharmacists compounding solutions, environmental analysts working with quantitative aqueous chemistry, and anyone needing a clean unit-rich mole conversion — runs entirely in your browser, no account, no data stored.

Pro Tip: Pair this with our Molarity Calculator for solution concentration, our Grams to Moles Calculator (the same conversion in a more elaborate stoichiometry workflow), our Moles to Atoms Calculator for entity counting, or our Mass Percent Calculator for percent composition.

How to Use the Mole Calculator?

Weigh the Substance: Use an analytical balance (±0.1 mg precision) for milligram-to-gram quantities; a precision balance for kg-scale; a microbalance (±1 µg) for sub-milligram. The calculator accepts mass in g / mg / µg / ng / kg / lb / oz and converts internally to grams.
Look Up the Molecular Weight: Standard sources are PubChem (pubchem.ncbi.nlm.nih.gov) for organic compounds and most biomolecules; CRC Handbook of Chemistry and Physics for inorganic; NIST WebBook for thermophysical-grade data; supplier datasheets / Certificates of Analysis (CoA) for the EXACT lot. Critical: if the compound is hydrated (·nH₂O notation), use the hydrate MW, not the anhydrous.
Enter Mass and MW: The calculator computes n = m / M automatically. Internal arithmetic uses g and g/mol regardless of input units to avoid rounding errors.
Read Moles in 5 Unit Prefixes: mol, mmol, µmol, nmol, kmol — pick whichever is cleanest for the magnitude. The hero display auto-selects based on absolute value (mol for ≥1, mmol for 0.001-1, µmol for 10⁻⁶-10⁻³, nmol for 10⁻⁹-10⁻⁶, pmol below).
Get the Number of Molecules (or Atoms / Ions / Formula Units): The result panel shows n × Nₐ (= 6.02214076 × 10²³ × n) for the absolute entity count. Use this for kinetic calculations (collisions per second), single-molecule biology (molecules per cell), or mass-spec quantification.
Cross-Check Against Common Reference MWs: H₂O 18.02; NaCl 58.44; glucose C₆H₁₂O₆ 180.16; sucrose 342.30; Tris base 121.14; CaCO₃ 100.09; BSA serum albumin ~66,500; IgG antibody ~150,000. If your MW is wildly different from a similar known compound, double-check the formula.
For Stoichiometry Problems: compute moles of each reactant separately; identify the limiting reagent (smallest mole quantity divided by stoichiometric coefficient); apply the molar ratio from the balanced equation to find moles of product; convert back to mass using the product's MW.

How is the number of moles calculated?

The mole-mass relationship is the gateway to all of stoichiometry. Every quantitative chemistry calculation — limiting reagents, theoretical yield, titration endpoints, equilibrium concentrations — starts here. The arithmetic is trivial; the chemistry intuition (which MW to use, hydrate vs anhydrous, formula units vs ions) is what separates correct answers from common errors.

References: SI Brochure (9th ed., 2019); IUPAC Compendium of Chemical Terminology (Gold Book); CRC Handbook of Chemistry and Physics; Atkins' Physical Chemistry; Levine's Physical Chemistry.

Core Formula

Moles n = mass m / molar mass M

m in g, M in g/mol → n in mol. The mole is the SI base unit for amount of substance.

Avogadro's Number — Post-2019 SI Definition

Nₐ = 6.02214076 × 10²³ entities per mole, EXACT.

Since the 2019 SI redefinition, the mole is defined by fixing Nₐ to this exact value (no longer derived from the kilogram-of-carbon-12 reference). The number of molecules in n moles is N = n × Nₐ.

Worked Example — Glucose

5.00 g of glucose (C₆H₁₂O₆, MW 180.16 g/mol).

  • n = 5.00 / 180.16 = 0.02775 mol = 27.75 mmol.
  • Number of molecules: 0.02775 × 6.022 × 10²³ = 1.671 × 10²² molecules.
  • Equivalent: enough glucose for ~5,000 mL of 5.6 mM physiological-glucose solution.

Worked Example — Sodium Chloride

10.0 g of NaCl (formula mass 58.44 g/mol).

  • n = 10.0 / 58.44 = 0.1711 mol = 171.1 mmol (formula units of NaCl).
  • In solution: 0.1711 mol of Na⁺ + 0.1711 mol of Cl⁻ (ionic compounds dissociate to give equal moles of each ion when fully soluble).
  • Enough to make ~1 L of 0.171 M NaCl solution (close to physiological saline 0.154 M).

Worked Example — Antibody Mass to Molecules

100 µg of IgG antibody (MW ~150,000 g/mol).

  • n = 100 × 10⁻⁶ / 150,000 = 6.67 × 10⁻¹⁰ mol = 0.667 nmol.
  • Molecules: 6.67 × 10⁻¹⁰ × 6.022 × 10²³ = 4.01 × 10¹⁴ antibody molecules.
  • Per cell: in a 10 mL aliquot at 100 mL volume, this is ~4 × 10⁹ molecules per mL — useful for ELISA / Western-blot quantification.

Common Molecular Weights You Should Know (g/mol)

  • Water H₂O: 18.015. Reference for solvent calculations.
  • Carbon dioxide CO₂: 44.01.
  • Methane CH₄: 16.04.
  • Sodium chloride NaCl: 58.44.
  • Calcium carbonate CaCO₃: 100.09.
  • Sodium bicarbonate NaHCO₃: 84.01.
  • Glucose C₆H₁₂O₆: 180.16.
  • Sucrose C₁₂H₂₂O₁₁: 342.30.
  • Ethanol C₂H₆O: 46.07.
  • Acetic acid CH₃COOH: 60.05.
  • Tris base (THAM): 121.14.
  • EDTA disodium dihydrate: 372.24.
  • HEPES: 238.30.
  • Concentrated H₂SO₄: 98.08.
  • Concentrated HCl: 36.46.
  • Concentrated HNO₃: 63.01.
  • Bovine Serum Albumin (BSA): ~66,500.
  • IgG antibody: ~150,000.

Hydrate Forms — The Most Common Mistake

  • CuSO₄·5H₂O (the blue copper sulfate): MW = 249.69. Anhydrous CuSO₄: 159.61.
  • MgSO₄·7H₂O (Epsom salt): MW = 246.47. Anhydrous: 120.37.
  • FeSO₄·7H₂O: 278.01. Anhydrous: 151.91.
  • Na₂CO₃·10H₂O (washing soda): 286.14. Anhydrous: 105.99.
  • Na₂B₄O₇·10H₂O (borax): 381.37. Anhydrous: 201.22.
  • EDTA disodium dihydrate (most common form): 372.24. Anhydrous Na₂EDTA: 336.21.

Using the wrong form gives 30-100% concentration / mole errors — always check the bottle label.

Real-World Example

Worked Example — Stoichiometry From Mass to Moles to Product

Reaction: 2 H₂(g) + O₂(g) → 2 H₂O(l). MW(H₂) = 2.016; MW(O₂) = 32.00; MW(H₂O) = 18.015.

Question: If you have 4.0 g of H₂ and 16.0 g of O₂, how many grams of water can you make?

Step 1 — Convert masses to moles using n = m / M.

  • n(H₂) = 4.0 / 2.016 = 1.984 mol.
  • n(O₂) = 16.0 / 32.00 = 0.500 mol.

Step 2 — Identify the limiting reagent.

  • From the balanced equation, the H₂:O₂ stoichiometric ratio is 2:1.
  • Available H₂:O₂ ratio = 1.984 / 0.500 = 3.97.
  • 3.97 > 2 → H₂ is in excess; O₂ is the limiting reagent.

Step 3 — Apply the molar ratio to find moles of water.

  • From balanced equation: 1 mol O₂ → 2 mol H₂O.
  • n(H₂O) = 0.500 × 2 = 1.000 mol.

Step 4 — Convert moles back to mass using m = n × M.

  • m(H₂O) = 1.000 × 18.015 = 18.015 g of water.

Step 5 — Sanity check.

  • Mass conservation: 4.0 g H₂ (1.984 mol used: 1.000 mol; excess 0.984 mol = 1.984 g) + 16.0 g O₂ used = 16.0 g + 1.000 × 2 × 1.008 = 16.0 + 2.016 g consumed = 18.016 g of reactants used → 18.015 g water ✓.
  • Number of water molecules formed: 1.000 × 6.022 × 10²³ = 6.022 × 10²³ molecules.

Who Should Use the Mole Calculator?

1
The mole concept is foundational — every limiting-reagent, percent-yield, gas-law, and titration problem starts with n = m / M.
2
Compute moles to weigh out from molar mass; combine with target volume and molarity for solution prep. The first half of every buffer-prep workflow.
3
Convert reactant masses to moles to identify limiting reagent and theoretical yield. The calculator handles the m → n step; balance the equation manually.
4
Convert tablet weight to moles of active ingredient for dose calculations; verify lot-to-lot consistency from supplier CoA.
5
Convert mg of protein (BSA, IgG, enzyme) to moles or molecules per assay. Critical for stoichiometric binding studies and enzyme kinetics.
6
Convert ng-µg quantities to absolute molecule counts via Avogadro's number — essential for fluorescence-correlation spectroscopy, super-resolution microscopy, and digital PCR quantification.
7
Convert ppb/ppm contaminant concentrations to molar concentrations for chemical-equilibrium and kinetic modeling.

Technical Reference

SI Definition (Post-2019). Per the 2019 SI redefinition, the mole is defined by fixing Avogadro's number to exactly Nₐ = 6.02214076 × 10²³ mol⁻¹. The mole is the SI base unit for amount of substance. Previously (1971-2019), the mole was defined as the amount of substance containing as many entities as there are atoms in 12 g of carbon-12; the new definition decouples the mole from the kilogram entirely, making both more precisely realizable.

Molar Mass Units and Daltons. Molar mass M has units g/mol; numerically M (g/mol) = average mass of one entity in unified atomic mass units (Da or amu). Example: water H₂O has 1 unit (Da) average molecule mass = 18.015 Da, and molar mass = 18.015 g/mol. The Da is typically used in mass spectrometry and protein chemistry; g/mol in classical chemistry. Both are interchangeable for normal-isotope substances.

Atomic Weights — IUPAC Conventional Values. Standard atomic weights are tabulated by IUPAC CIAAW (Commission on Isotopic Abundances and Atomic Weights) and updated periodically. For elements with naturally varying isotopic compositions (H, Li, B, C, N, O, Mg, Si, S, Cl, Br, Tl), CIAAW reports an interval rather than a single value; the conventional midpoint is used in textbooks and most calculations. Variation across natural samples is < 0.01% for most common elements.

Number-Average vs Weight-Average MW (Polymers). Polymers are polydisperse — molecules of different chain lengths coexist. Number-average MW (Mn) = total mass / total number of molecules; sensitive to small molecules. Weight-average MW (Mw) = Σ(N_i M_i²) / Σ(N_i M_i); sensitive to large molecules. Polydispersity index (PDI) = Mw / Mn; PDI = 1.0 is monodisperse, PDI = 2.0 is typical for free-radical polymerization. For mole calculations on polymers, specify which average you're using; Mw is the most commonly reported value on supplier datasheets.

Formula Unit Mole vs Molecule Mole. For molecular compounds (water, glucose, methane), 1 mole = Nₐ molecules. For ionic compounds (NaCl, CaCl₂, MgSO₄), 1 mole = Nₐ formula units, but in solution this gives Nₐ × (number of ions per formula unit) of dissolved ions. NaCl: 1 mol formula units → 1 mol Na⁺ + 1 mol Cl⁻ in solution. CaCl₂: 1 mol formula units → 1 mol Ca²⁺ + 2 mol Cl⁻. This matters for colligative-property calculations (van't Hoff factor i = total moles of dissolved species / moles of solute formula units; i = 1 for non-electrolyte, i = 2 for NaCl, i = 3 for CaCl₂).

Significant Figures and Precision. The precision of n = m / M is limited by the less-precise input. Analytical balance ±0.1 mg on a 100 mg sample gives 0.1% uncertainty in mass. Standard MW values are typically known to 4-5 significant figures (limited by atomic weight intervals). Net precision in moles is usually 0.1-1% — keep 4 significant figures in calculations to avoid round-off propagation; report final answer with appropriate sig figs based on inputs.

Beyond n = m / M — When the Simple Formula Doesn't Apply. (1) Mixtures: no single MW; calculate per-component if composition is known, or use weighted-average MW for crude approximation. (2) Solutions: n = M × V (molarity × volume) is more direct than mass-based for known-concentration solutions. (3) Gases: at STP, n = V / 22.414 L/mol (ideal gas); under non-ideal conditions use PV = nRT or compressibility-corrected forms. (4) Quantum / electromagnetic: for photons or electrons, use n = total energy / (Nₐ × hν) for photon-mole; for electrochemistry, n = Q / (z × F) (charge / valence × Faraday). References: SI Brochure (9th ed., 2019); IUPAC Compendium of Chemical Terminology; Atkins' Physical Chemistry; Levine's Physical Chemistry; CRC Handbook of Chemistry and Physics.

Conclusion

The mole calculation is the universal stoichiometry currency of chemistry. The arithmetic is simple — n = m / M — but the implications are profound: this single conversion lets you bridge the macroscopic world (grams on a balance) and the molecular world (Avogadro's 6.022 × 10²³ entities per mole). Master this, and every other quantitative chemistry calculation falls into place: limiting reagents, percent yield, equilibrium constants, gas laws, titrations, dilutions, kinetics — all start from moles.

Two pitfalls dominate real-world errors: (1) Hydrate vs anhydrous mismatch — using anhydrous MW for a hydrated salt (or vice versa) gives 30-100% errors. CuSO₄·5H₂O 249.69 vs CuSO₄ 159.61. Always check the bottle. (2) Ions vs formula units — moles of NaCl formula units ≠ moles of Na⁺ ions for stoichiometry; for ionic precipitation NaCl gives 1 mol Na⁺ AND 1 mol Cl⁻ per mol formula unit. Specify which you mean. The calculator handles the math; the chemistry intuition (which MW to use, which entity to count) remains the user's responsibility — and is what every general-chemistry course is fundamentally testing.

Frequently Asked Questions

What is the Mole Calculator?
It implements the foundational stoichiometry identity n = mass / molar mass, the most-used formula in chemistry. Mass inputs accept g / mg / µg / ng / kg / lb / oz; molecular weight in g/mol. Output: moles in 5 unit prefixes (mol, mmol, µmol, nmol, kmol) plus the absolute number of molecules computed via Avogadro's number Nₐ = 6.02214076 × 10²³ (exact, post-2019 SI redefinition).

Pro Tip: Pair this with our Molarity Calculator for solution chemistry.

What is a mole?
The SI base unit for amount of substance. Defined since the 2019 SI redefinition as exactly 6.02214076 × 10²³ entities (Avogadro's number Nₐ). 1 mole of any substance contains exactly Nₐ entities — atoms, molecules, ions, formula units, electrons, photons. The mole concept lets chemists bridge between macroscopic mass (grams) and microscopic entity counts (molecules) using the simple identity n = m / M.
What is the formula for moles?
Moles n = mass m / molar mass M, with m in g and M in g/mol. Algebraic check: g / (g/mol) = mol. Inverse: mass = moles × molar mass = n × M (when you need to find how much to weigh out for a target moles). For molecule count: N = n × Nₐ = (m × Nₐ) / M, giving the absolute number of entities.
What is Avogadro's number?
Nₐ = 6.02214076 × 10²³ entities per mole, exact by definition since the 2019 SI redefinition. Named after Italian scientist Amedeo Avogadro (1776-1856) who first proposed (in 1811) that equal volumes of gases at the same T and P contain equal numbers of molecules. Originally derived experimentally; now defined exactly to anchor the mole as an SI base unit independent of the kilogram. Mnemonic: 6.022 × 10²³ — about 600 billion trillion entities per mole.
How do I convert grams to moles?
n = grams / molar mass. Step by step: (1) weigh the substance to get mass in grams. (2) look up the molar mass on PubChem, CRC Handbook, or supplier datasheet. (3) divide. Example: 5.00 g of NaCl (MW 58.44 g/mol) → n = 5.00 / 58.44 = 0.0856 mol = 85.6 mmol. Always verify the form (anhydrous vs hydrate) — using the wrong form gives 30-100% errors.
How do I convert moles to molecules?
Number of molecules = moles × Avogadro's number. N = n × 6.02214076 × 10²³. Example: 0.5 mol of glucose contains 0.5 × 6.022 × 10²³ = 3.011 × 10²³ glucose molecules. Useful for kinetic calculations (collision frequency), single-molecule biology (molecules per cell), and mass-spec entity quantification.
What units does molecular weight use?
g/mol (grams per mole) is the standard chemistry unit; numerically equal to Da (daltons) or u (unified atomic mass units) per molecule. Examples: water 18.015 g/mol = 18.015 Da; glucose 180.16 g/mol = 180.16 Da; BSA serum albumin ~66,500 g/mol = 66.5 kDa; IgG antibody ~150,000 g/mol = 150 kDa. Da/kDa notation is preferred in protein chemistry and mass spec; g/mol in classical chemistry. Both are interchangeable for normal-isotope substances.
What is the molecular weight of water?
18.015 g/mol. Math: 2 × 1.008 (H atomic weight) + 1 × 15.999 (O) = 2.016 + 15.999 = 18.015. Mole reference: 18 g of water = 1 mole = 6.022 × 10²³ water molecules. Useful for computing the molarity of water in pure water (1000 g/L ÷ 18.015 g/mol = 55.5 M); for verifying limiting-reagent problems with water as a reactant; and as a reality check on other MW values (water is the smallest stable molecule routinely encountered in chemistry).
Why does my mole calculation seem wrong?
Six common causes. (1) Hydrate vs anhydrous mismatch — CuSO₄·5H₂O 249.69 vs CuSO₄ 159.61. Always check bottle label for ·nH₂O notation. (2) Wrong molecular formula — confusing C₆H₁₂O₆ glucose (180.16) with C₁₂H₂₂O₁₁ sucrose (342.30). (3) Mixed units — entering mass in mg but expecting g; the calculator handles unit conversion if you select the right unit. (4) Salt purity — "≥ 99%" means up to 1% impurity; high-purity work needs ≥ 99.9%. (5) Hygroscopic salts — NaCl, LiCl, CaCl₂ absorb atmospheric moisture; weighed mass overestimates anhydrous compound mass. Dry in oven before weighing. (6) Polymer polydispersity — for polymers, Mn vs Mw differs; verify which average your supplier reports.
How are moles used in stoichiometry?
The mole is the universal currency of stoichiometry. Workflow: (1) balance the chemical equation. (2) convert each reactant's mass to moles via n = m / M. (3) identify the limiting reagent (smallest moles ÷ stoichiometric coefficient). (4) apply the molar ratio from the balanced equation to find moles of product. (5) convert product moles back to mass via m = n × M. Example: 4 g H₂ + 16 g O₂ → ? g water. n(H₂) = 1.98 mol, n(O₂) = 0.50 mol; ratio 3.97:1 vs required 2:1, so O₂ is limiting; n(H₂O) = 0.50 × 2 = 1.00 mol → m(H₂O) = 18 g.
What's the difference between a mole and a molecule?
A molecule is one entity — a single H₂O, glucose, or NaCl unit. A mole is a counting unit equal to 6.022 × 10²³ entities — like a dozen (12), a gross (144), or a score (20), just much bigger. Relation: 1 mole of water = 6.022 × 10²³ water molecules; their combined mass is 18.015 g. Why the mole? A scaling factor lets chemists work with macroscopic masses (1 g, 1 kg) while preserving the molecular-scale meaning. Without it, every calculation would involve numbers like 10²³, which is unwieldy.

Author Spotlight

The ToolsACE Team - ToolsACE.io Team

The ToolsACE Team

Our ToolsACE chemistry team built this calculator to handle the foundational stoichiometry identity <strong>n = mass / molar mass</strong> in a clean, fast, multi-unit form. The mole is the SI base unit for amount of substance; since the 2019 SI redefinition, 1 mole is defined as exactly 6.02214076 × 10²³ entities (Avogadro's number). Mass inputs accept g / mg / µg / ng / kg / lb / oz; molecular weight in g/mol (= Da). Output: moles in 5 unit prefixes (mol, mmol, µmol, nmol, kmol) auto-selected for cleanest hero display, plus the absolute number of molecules computed via Avogadro's number — useful for converting mole-based stoichiometry into entity-count for kinetics, single-molecule biology, or mass-spec quantification.

SI 2019 Redefinition (Nₐ = 6.02214076 × 10²³ exact)IUPAC Compendium of Chemical Terminology (Gold Book)CRC Handbook of Chemistry and Physics

Disclaimer

The mole calculation n = m / M assumes you know the EXACT form of substance being weighed. Hydrate forms are the most common error source: CuSO₄·5H₂O 249.69 g/mol vs anhydrous CuSO₄ 159.61. Polymers and biopolymers are typically polydisperse — the reported MW depends on the average chosen (Mn, Mw, Mz). Mixtures have no single molar mass; calculate per-component. The mole as a unit applies to any specific entity (atoms, molecules, ions, formula units) — specify which you mean. Avogadro's number Nₐ = 6.02214076 × 10²³ is exact (post-2019 SI redefinition). References: SI Brochure (9th ed., 2019); IUPAC Compendium of Chemical Terminology (Gold Book); CRC Handbook of Chemistry and Physics; Atkins' Physical Chemistry; Levine's Physical Chemistry.