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Molar Mass Calculator

Ready to calculate
M = Σ (count × atomic mass).
70 Elements (IUPAC 2021).
Mass % per Element.
100% Free.
No Data Stored.

How it Works

01Pick Each Atom

70 elements supported — Hydrogen through Uranium with IUPAC 2021 atomic weights

02Set the Quantity

1–50 per element, side-by-side with the atom dropdown. Add up to 10 elements

03Sum Atomic Masses

M = Σ (count × atomic mass) — closed-form stoichiometric arithmetic

04Get Formula + Mass %

Empirical formula in subscript notation, total g/mol, and percent composition

What is a Molar Mass Calculator?

Molar mass is the bridge between mass — what you can weigh on a balance — and moles, which is what chemical reactions actually count. Without it, you can't prepare a 1 M solution, calculate a limiting reagent, convert ppm to molarity, or do any quantitative chemistry. Our Molar Mass Calculator builds the molar mass of any compound atom-by-atom by letting you select each element and how many of it appears, then summing the products of count × atomic mass. 70 elements supported from Hydrogen through Uranium, up to 10 element slots per compound, with the side-by-side Atom + Quantity dropdowns making fast entry painless.

Every atomic mass uses the IUPAC 2021 standard atomic weight — the natural-abundance average for each element. Pick "Hydrogen" and the calculator uses 1.008 g/mol; pick "Carbon" it uses 12.011, etc. The output gives you the molar mass in g/mol (and kg/mol, plus grams per single molecule for context); the empirical formula in proper subscript notation; the percent composition by mass for each element with visual progress gauges; and — when you build a well-known compound — a "common compound match" callout for water, glucose, NaCl, caffeine, aspirin, and more.

The terms molar mass, molecular weight, molecular mass, and relative molecular mass all refer to essentially the same number with different naming conventions. The IUPAC-preferred term is "molar mass" with units of g/mol — what this calculator outputs.

Pro Tip: Pair this with our Molarity Calculator for solution-prep math, or our PPM to Molarity Calculator for environmental work where molar mass bridges mass and moles.

How to Use the Molar Mass Calculator?

Pick the First Atom and Quantity: The "1st Element" card has two side-by-side dropdowns — Atom (70 elements from Hydrogen through Uranium) and Quantity (1 through 50). Choose your first atom and how many appear in the formula.
Pick the Second Atom and Quantity: Same in the "2nd Element" card. For water (H₂O), that's Hydrogen × 2 + Oxygen × 1.
(Optional) Add More Elements: Click "Add another element" — up to 10 slots are supported. Each slot has its own Atom + Quantity dropdowns. The "−" button on each card removes that slot.
Press Calculate: The tool sums (count × atomic mass) across all filled slots using the IUPAC 2021 atomic weights — accurate to 3–5 significant figures depending on the element's isotopic-abundance variability.
Read the Results: Empirical formula in subscript notation (H₂O, C₆H₁₂O₆, CH₃COOH); molar mass in g/mol; per-element mass percent with visual progress bars; calculation breakdown showing every multiplication; common-compound match if your formula matches a well-known one.

How do I calculate molar mass?

Molar mass is the most basic stoichiometric calculation in chemistry — sum the atomic masses of every atom in the compound. Here's the complete breakdown:


Think of a compound's molar mass like the weight of a recipe: each ingredient (element) contributes its individual weight (atomic mass) times how much of it you use (number of atoms). Total it up and you have the weight of one "serving" — one mole — of the compound.

The Core Formula


M = Σᵢ (countᵢ × atomic_massᵢ)


Sum over all elements i in the compound. Each element contributes (count × atomic mass). Result has units of g/mol. For water (H₂O): M = 2 × 1.008 + 1 × 15.999 = 18.015 g/mol.

Standard Atomic Weights (IUPAC 2021)


The atomic mass used is the natural-abundance average — weighted by each isotope's abundance on Earth. So carbon's atomic weight is 12.011 (not exactly 12), reflecting the ~98.9% ¹²C and ~1.1% ¹³C natural mixture. Some elements have wider isotopic variability, leading to bracketed values which are conventionally rounded for routine use.


For radioactive elements with no stable isotopes (Tc, Pm, Po, At, Rn, Fr, Ra, Ac, U), the atomic weight reported is the mass number of the longest-lived isotope.

Percent Composition by Mass


%mass_i = (count_i × atomic_mass_i) ÷ M × 100%


For glucose C₆H₁₂O₆ (M = 180.156): C = 6 × 12.011 / 180.156 = 40.0% · H = 12 × 1.008 / 180.156 = 6.71% · O = 6 × 15.999 / 180.156 = 53.3%. Mass percentages always sum to 100%.

Mass per Single Molecule


m_one = M / N_A


where N_A = 6.022 × 10²³ /mol is Avogadro's number. So one water molecule weighs 18.015 / 6.022×10²³ ≈ 2.99 × 10⁻²³ g. Tiny — but with Avogadro's number of them, you get one mole = 18 grams.

Real-World Example

Molar Mass Calculator – Compound Mass In Practice

Consider caffeine, C₈H₁₀N₄O₂ — the most-consumed psychoactive compound on Earth:
  • Step 1: Identify the elements and counts. Carbon × 8, Hydrogen × 10, Nitrogen × 4, Oxygen × 2. Four element slots needed.
  • Step 2: Look up atomic masses. C = 12.011, H = 1.008, N = 14.007, O = 15.999 g/mol (IUPAC 2021).
  • Step 3: Compute each contribution. C: 8 × 12.011 = 96.088. H: 10 × 1.008 = 10.080. N: 4 × 14.007 = 56.028. O: 2 × 15.999 = 31.998.
  • Step 4: Sum. M = 96.088 + 10.080 + 56.028 + 31.998 = 194.194 g/mol.
  • Step 5: Percent composition. C: 49.5%. H: 5.2%. N: 28.9%. O: 16.5%. Sum: 100.1% (rounding) ✓.
  • Step 6: Compare to literature. Reported caffeine molar mass: 194.19 g/mol — exact match. The calculator's "common compound" callout would flag this as Caffeine immediately.

Now consider glucose (C₆H₁₂O₆): M = 6(12.011) + 12(1.008) + 6(15.999) = 72.066 + 12.096 + 95.994 = 180.156 g/mol. The "fingerprint mass" of all hexose sugars (they're isomers).

For an inorganic example: sodium chloride NaCl: M = 22.990 + 35.45 = 58.44 g/mol. One mole of table salt weighs 58.44 grams — about a tablespoon. This is also why a "1 M" NaCl solution dissolves 58.44 g per liter.

Who Should Use the Molar Mass Calculator?

1
Chemistry Students: Solve stoichiometry problems, find limiting reagents, compute theoretical yields, prepare lab solutions of known molarity.
2
Organic Chemists: Calculate reagent masses for synthesis. To run a 10 mmol reaction with a reagent of M = 180, you weigh 10 × 0.180 = 1.80 g.
3
Biochemists & Molecular Biologists: Convert between mass and moles for proteins, nucleic acids, lipids, and small-molecule cofactors.
4
Pharmaceutical Scientists: Drug formulation — tablet strength specs, salt-form conversions, IV dosing in mg/kg ↔ mmol calculations.
5
Analytical Chemists: Calibration curve preparation, internal standard concentrations, response-factor calculations.
6
Environmental Scientists: Convert between concentration units (ppm ↔ molarity) — molar mass is the conversion factor for solutes.

Technical Reference

Standard Atomic Weights. The atomic masses used are the IUPAC 2021 standard atomic weights — the natural isotopic-abundance-weighted average for each element. Updated periodically by the IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW).

Why the values aren't exact integers. Most elements have multiple stable isotopes. Carbon's atomic weight is 12.011 because natural carbon is ~98.9% ¹²C and ~1.1% ¹³C. Hydrogen's 1.008 reflects ~99.985% ¹H and trace ²H (deuterium).

Reference Molar Masses for Common Compounds (g/mol):

  • Water (H₂O): 18.015
  • Carbon dioxide (CO₂): 44.009
  • Sodium chloride (NaCl): 58.44
  • Sodium hydroxide (NaOH): 39.997
  • Sulfuric acid (H₂SO₄): 98.079
  • Hydrochloric acid (HCl): 36.458
  • Nitric acid (HNO₃): 63.013
  • Glucose (C₆H₁₂O₆): 180.156
  • Sucrose (C₁₂H₂₂O₁₁): 342.297
  • Ethanol (C₂H₅OH): 46.069
  • Acetic acid (CH₃COOH): 60.052
  • Aspirin (C₉H₈O₄): 180.159
  • Caffeine (C₈H₁₀N₄O₂): 194.194
  • Calcium carbonate (CaCO₃): 100.087
  • Ammonium nitrate (NH₄NO₃): 80.043

Naming Conventions. Molar mass (IUPAC-preferred), molecular weight (older common use), molecular mass (sometimes reserved for mass per molecule), relative molecular mass — all refer to the same number with units of g/mol. The calculator outputs in g/mol and labels it "molar mass (M)".

Hill System Ordering. The standard formula-writing convention: carbon first, then hydrogen, then other elements in alphabetical order. Glucose is C₆H₁₂O₆ (Hill order). Inorganic compounds without carbon usually go alphabetically. The calculator displays the formula in input order — re-arrange your input to match Hill convention if your application requires it.

Key Takeaways

Molar mass is the bridge between what your balance can measure (grams) and what reactions actually count (moles). Without it, no solution can be prepared correctly, no limiting reagent can be identified, no ppm-to-molarity conversion can be made. The ToolsACE Molar Mass Calculator builds the answer atom-by-atom from IUPAC 2021 standard atomic weights, supports 70 elements and up to 10 element slots, and gives you the empirical formula, percent composition by mass, and per-element contribution breakdown alongside the molar mass. Bookmark it as your everyday stoichiometry utility.

Frequently Asked Questions

What is the Molar Mass Calculator?
Molar mass (M) is the mass of one mole of a compound, in grams per mole (g/mol). Our calculator builds it by letting you select each element and how many atoms of it appear in the compound, then summing the products of count × atomic mass. 70 elements supported from Hydrogen through Uranium, up to 10 element slots, with side-by-side Atom + Quantity dropdowns for fast entry. Uses IUPAC 2021 standard atomic weights.

Output includes the empirical formula in subscript notation, molar mass in g/mol with conversions to kg/mol and grams per single molecule, percent composition by mass for each element, per-element contribution breakdown, and a common-compound match for well-known formulas (water, glucose, NaCl, caffeine, aspirin, etc.). Designed for chemistry homework, organic synthesis, biochemistry, pharmaceutical work — runs entirely in your browser.

Pro Tip: For more chemistry tools, try our Molarity Calculator.

What's the difference between molar mass, molecular weight, and molecular mass?
All three refer to the same number, with subtle convention differences. Molar mass is the IUPAC-preferred term, with explicit units of g/mol. Molecular weight is the older, somewhat inaccurate term still common in everyday chemistry use — it's actually a mass, not a weight. Molecular mass is sometimes reserved for the mass of a single molecule (in atomic mass units, u or Da). For most stoichiometric purposes, treat them as interchangeable.
Why isn't carbon's atomic weight exactly 12?
Because natural carbon is a mixture of isotopes. About 98.9% is ¹²C (mass 12 by definition) and 1.1% is ¹³C (mass 13.003). The natural-abundance average is 0.989 × 12 + 0.011 × 13.003 = 12.011 g/mol. Other elements have similar isotopic mixtures.
Where do the atomic weights come from?
IUPAC 2021 standard atomic weights — the most recent recommendations from the Commission on Isotopic Abundances and Atomic Weights (CIAAW). Based on extensive measurements of natural isotopic abundances and atomic masses. Updated every few years; the calculator uses the 2021 values.
How do I include a hydrate?
Add it as additional element slots. For copper(II) sulfate pentahydrate CuSO₄·5H₂O: Cu × 1, S × 1, O × 4 (from SO₄) + 5 × O (from 5H₂O) = O × 9 total, H × 10 (from 5H₂O). Total: Cu × 1, S × 1, O × 9, H × 10. M = 63.546 + 32.06 + 9(15.999) + 10(1.008) = 249.685 g/mol — matches the literature value for the pentahydrate.
What about monoisotopic mass for mass spectrometry?
The calculator uses natural-abundance average atomic weights, which are appropriate for stoichiometry, solution preparation, and most analytical chemistry. For high-resolution mass spectrometry, the monoisotopic mass is needed — calculated using only the most abundant stable isotope of each element. For glucose: monoisotopic 180.0634 vs average 180.156 — a difference of ~0.09 Da, important for HR-MS.
Why is the maximum quantity 50 atoms per element?
Practical convenience for the dropdown. 50 covers nearly all small molecules and common drug compounds. For larger structures (proteins with hundreds of carbons, polymer chains), build from monomer units: a polyethylene chain (CH₂CH₂)ₙ at n = 1000 weighs 1000 × 28.05 = 28,050 g/mol. Compute the monomer weight, multiply by n.
Can I use this for ionic compounds?
Yes. The 'molar mass' of an ionic compound (NaCl, K₂SO₄) is technically called the formula weight because they don't form discrete molecules — they're crystal lattices of ions. But the arithmetic is identical: sum atomic weights × counts. NaCl formula weight = 22.99 + 35.45 = 58.44 g/mol.
Why does my calculated molar mass differ slightly from a textbook?
Three reasons: (1) atomic-weight values get updated periodically — IUPAC 2021 values differ slightly from 2007 values for some elements. (2) Different rounding conventions (1.008 vs 1.0079 for H). (3) Some textbooks use older or non-IUPAC values. Differences are usually well under 0.1% — irrelevant for stoichiometry.
What's the relationship between molar mass and density?
Density ρ = mass/volume; molar mass M = mass/moles. They're connected via molar volume: V_m = M / ρ. For water at 25°C: V_m = 18.015 g/mol / 0.997 g/mL = 18.07 mL/mol. For ideal gases: V_m ≈ 24.5 L/mol at 25°C and 1 atm (regardless of identity).
Can I share or export the result?
Yes. The result panel has Copy buttons on the empirical formula and molar mass. The footer has Share and Download Report buttons. The downloadable PDF includes the composition table, total molar mass, percent composition, and any matched common compound name.

Author Spotlight

The ToolsACE Team - ToolsACE.io Team

The ToolsACE Team

Our chemistry tools team uses IUPAC 2021 standard atomic weights — natural-abundance averages for each element. The calculator supports 70 elements (Hydrogen through Uranium), up to 10 element slots per compound, and 1–50 atoms per element, with side-by-side Atom + Quantity dropdowns for fast entry. Output includes the empirical formula in subscript notation, total molar mass in g/mol, percent composition by mass with visual gauges, and a common-compound match for well-known formulas (water, glucose, NaCl, caffeine, aspirin, etc.).

StoichiometryIUPAC 2021 Standard Atomic WeightsSoftware Engineering Team

Disclaimer

Atomic weights use IUPAC 2021 standard values based on natural isotopic abundance. For isotope-labeled compounds, use exact monoisotopic masses instead. The empirical formula is built in input order — apply Hill ordering manually if your application requires it.