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Molecular Weight Calculator

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

How it Works

01Pick Each Atom

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

02Set Number of Atoms

1–50 per element. Add up to 8 different elements with the + button

03Sum Atomic Masses

M = Σ (count × atomic mass) — straightforward stoichiometric arithmetic

04Get Formula + Mass %

Empirical formula, total g/mol, and percent composition for each element

What is a Molecular Weight Calculator?

Molecular weight (M, also called molar mass or molecular mass) is the foundation of stoichiometry — every solution preparation, every limiting-reagent calculation, every yield calculation in synthesis depends on getting it right. Our Molecular Weight Calculator builds the molecular weight from scratch by letting you select each element in your compound and how many atoms of it appear, then summing the products of count × atomic mass. 89 elements supported (Hydrogen through Uranium), up to 8 element slots per compound (covers nearly all small organic and inorganic molecules), and 1–50 atoms per element — wide enough for steroids, drug molecules, and most coordination complexes.

Every atomic mass is the IUPAC 2021 standard atomic weight — the natural-abundance average across each element's stable isotope distribution. Pick "Hydrogen" and the calculator uses 1.008 g/mol; pick "Carbon" it uses 12.011, etc. The output gives you the molecular weight in g/mol (and kg/mol, and grams per single molecule for context); the empirical formula in subscript notation; the percent composition by mass for each element (with a visual gauge); and — when you build a well-known compound — a "common compound match" callout (water → 18.015 g/mol, glucose → 180.156 g/mol, etc.).

Designed for general chemistry homework, organic synthesis labs, biochemistry assays, and pharmaceutical formulation, the tool runs entirely in your browser — no data is stored or transmitted.

Pro Tip: Pair this with our Molarity Calculator for solution-prep math, or our PPM to Molarity Calculator for environmental work where compound molecular weight is the bridge between mass and moles.

How to Use the Molecular Weight Calculator?

Pick the First Element: Open the "1st Element" panel, select an atom from the dropdown (89 options from Hydrogen through Uranium), and choose how many appear in your compound (1 through 50).
Add the Second Element: Repeat in the "2nd Element" panel. For water, that's Hydrogen × 2 + Oxygen × 1. For glucose, you'll need a third panel — click "Add another element" to expand.
(Optional) Add More Elements: Up to 8 element slots are supported. The "Add" button appears below the existing slots; the "−" button on each slot lets you remove it. Sufficient for almost all small-molecule chemistry; for larger biomolecules, consider building from monomer units.
Press Calculate: The tool sums (count × atomic mass) across all filled slots. The IUPAC 2021 atomic weights are used, accurate to 3–5 significant figures depending on the element's isotopic uncertainty.
Read the Results: Empirical formula in subscript notation (H₂O); molecular weight in g/mol; per-element mass percent with progress bars; calculation breakdown showing every multiplication; common-compound match if your formula matches a well-known one (water, glucose, NaCl, etc.).

How do I calculate molecular weight?

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


Think of a compound's molecular weight 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 (count_i × atomic_mass_i)


Sum over all elements i in the compound. Each element contributes (count × atomic mass). Result has units of g/mol — grams of compound per mole. For water (H₂O): M = 2 × 1.008 + 1 × 15.999 = 18.015 g/mol. The factor of 1.008 comes from hydrogen being a mixture of ¹H and trace ²H; ¹⁵.999 from oxygen being mostly ¹⁶O with a few percent ¹⁷O and ¹⁸O.

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 (e.g., sulfur is reported as [32.059, 32.076] but is rounded to 32.06 for routine use).


For radioactive elements with no stable isotopes (Tc, Pm, Po, At, Rn, Fr, Ra, Ac), the atomic weight reported is the mass number of the longest-lived isotope, which is why you'll see Tc = 98 rather than a fractional value.

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% — that's a built-in arithmetic check.

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

Molecular Weight Calculator – Compound Mass In Practice

Consider glucose, C₆H₁₂O₆ — the most fundamental sugar in biochemistry:
  • Step 1: Identify the elements and counts. Carbon × 6, Hydrogen × 12, Oxygen × 6. Three element slots needed.
  • Step 2: Look up atomic masses. C = 12.011, H = 1.008, O = 15.999 g/mol (IUPAC 2021).
  • Step 3: Compute each contribution. C: 6 × 12.011 = 72.066. H: 12 × 1.008 = 12.096. O: 6 × 15.999 = 95.994.
  • Step 4: Sum. M = 72.066 + 12.096 + 95.994 = 180.156 g/mol.
  • Step 5: Compute mass percent. C: 72.066/180.156 = 40.00%. H: 12.096/180.156 = 6.71%. O: 95.994/180.156 = 53.29%. Sum: 100.00% ✓.
  • Step 6: Verify against literature. Reported value for glucose: 180.156 g/mol — exact match. The calculator's "common compound" match would flag this as Glucose if you select C₆H₁₂O₆.

Now consider aspirin (acetylsalicylic acid), C₉H₈O₄: M = 9(12.011) + 8(1.008) + 4(15.999) = 108.099 + 8.064 + 63.996 = 180.159 g/mol. Notice that aspirin and glucose have the same molecular weight to within 0.003 g/mol — a coincidence that has fooled many a stoichiometry student. The empirical formulas (and pharmacological effects) are completely different.

For a simple 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 molar" NaCl solution dissolves 58.44 g in 1 liter of water.

Who Should Use the Molecular Weight Calculator?

1
Chemistry Students: Solve stoichiometry problems, find limiting reagents, compute theoretical yields, prepare solutions of known molarity for lab work.
2
Organic Chemists: Compute reagent masses for synthesis. To run a 10 mmol reaction with a reagent of MW 180, you weigh out 10 × 0.180 = 1.80 g.
3
Biochemists & Molecular Biologists: Convert between mass and moles for proteins, nucleic acids, lipids, and small-molecule cofactors. Quick sanity-check on commercial reagent labels.
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) — molecular weight is the conversion factor for solutes.

Technical Reference

Standard Atomic Weights. The atomic masses used are the IUPAC 2021 standard atomic weights, which represent the natural isotopic-abundance-weighted average for each element. The values are updated periodically by the IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW); current values reflect the best available data on terrestrial isotope distributions.

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 (mass 12 by definition) and ~1.1% ¹³C (mass 13.003). Hydrogen's 1.008 reflects ~99.985% ¹H and trace ²H (deuterium). Some elements show abundance variation by source — the IUPAC table reports interval values [a, b] for these (e.g., S = [32.059, 32.076]); routine work uses the conventional rounded value.

Reference Molecular Weights 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

Monoisotopic vs. Average Masses. For mass spectrometry of small molecules, the monoisotopic mass (using the lightest stable isotope of each element) is preferred — water's monoisotopic mass is 18.0106 (using ¹H and ¹⁶O exactly), while the average is 18.015. The ~5 ppm difference matters in high-resolution MS but is irrelevant for routine wet chemistry. This calculator uses average atomic weights, suitable for solution preparation, stoichiometry, and routine analytical work.

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 (NaCl, H₂SO₄). The calculator displays the formula in input order — re-arrange your input to match Hill convention if your application requires it.

Key Takeaways

Molecular weight is the bridge between mass — which you can weigh on a balance — and moles, which is what reactions actually count. Without it, you can't prepare a 1 M solution, calculate a limiting reagent, or convert ppm to molarity. The ToolsACE Molecular Weight Calculator builds the answer atom by atom from IUPAC 2021 standard atomic weights, supports 89 elements and up to 8 element slots, and gives you the empirical formula, percent composition by mass, and per-element contribution breakdown alongside the molecular weight. Bookmark it as your everyday stoichiometry utility — the calculation you'll do more times than any other in chemistry coursework or lab work.

Frequently Asked Questions

What is the Molecular Weight Calculator?
Molecular weight (M) is the mass of one mole of a compound, in grams per mole (g/mol). Our calculator builds the molecular weight by letting you select each element and the number of atoms of it in the compound, then summing the products of count × atomic mass. 89 elements supported from Hydrogen through Uranium, up to 8 element slots per compound, with IUPAC 2021 standard atomic weights.

Output includes the empirical formula in subscript notation (H₂O, C₆H₁₂O₆), the molecular weight in g/mol with conversions to kg/mol and grams per single molecule, percent composition by mass for each element, the per-element contribution breakdown, and a common-compound match for well-known formulas (water, glucose, NaCl, etc.).

Designed for general chemistry homework, organic synthesis, biochemistry, and pharmaceutical work, the tool runs entirely in your browser — no data is stored or transmitted.

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

What's the difference between molecular weight, molar mass, and molecular mass?
All three names refer to the same number, with subtle convention differences. Molecular weight is the older, somewhat inaccurate term still common in everyday chemistry use — it's actually a mass, not a weight. Molar mass is the IUPAC-preferred term, with explicit units of g/mol. Molecular mass is sometimes reserved for the mass of a single molecule (in atomic mass units, u or Da). For most stoichiometric purposes, you can treat them as interchangeable; the calculator outputs in g/mol and labels it 'molecular weight (M)' to match common usage.
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 — hydrogen 1.008, oxygen 15.999, nitrogen 14.007 — and the atomic weight reflects the natural-abundance average.
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). These values are based on extensive measurements of natural isotopic abundances and atomic masses. They're updated every few years; the calculator uses the 2021 values, which are accurate to 3–5 significant figures depending on the element's isotopic-abundance variability.
How do I include a hydrate (water of crystallization)?
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 — the mass calculated using only the most abundant stable isotope of each element (¹H = 1.00783, ¹²C = 12.000 exactly, ¹⁶O = 15.99491). For glucose: monoisotopic 180.0634 vs average 180.156 — a difference of ~0.09 Da or 500 ppm.
Why is the maximum 50 atoms per element?
Practical convenience for the dropdown UI. 50 covers nearly all small molecules and small polymers. For larger structures (proteins with hundreds of carbons, polymer chains), you'd 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.
Why is hydrogen 1.008 instead of 1.00794?
Both are correct. The calculator uses the conventional 4-significant-figure value (1.008) recommended by IUPAC for routine use. The more precise value is 1.00794, with isotopic-variability brackets [1.00784, 1.00811]. For solution preparation accurate to 1%, 1.008 is plenty. For high-precision work, switch to monoisotopic masses or use a higher-precision atomic weight table.
What's the percent composition for, exactly?
It's the mass fraction of each element in the compound. For water: H is 11.19% by mass, O is 88.81% — meaning if you have 100 g of water, 11.19 g is hydrogen and 88.81 g is oxygen. Useful for elemental analysis (CHN combustion) verification — if you synthesize a compound and the measured C, H, N percentages match your target compound's calculated values, that's strong evidence of correct structure.
Why does my calculated molecular weight 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. Trust the calculator's IUPAC 2021 value or whichever your course uses.
Can I use this for ionic compounds?
Yes. The 'molecular weight' 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. The calculator computes it the same way as for true molecular compounds.

Author Spotlight

The ToolsACE Team - ToolsACE.io Team

The ToolsACE Team

Our chemistry tools team uses the IUPAC 2021 standard atomic weights — the natural-abundance average for each element across its stable isotope distribution. The calculator supports 89 elements (Hydrogen through Uranium), up to 8 element slots per compound (covering nearly all small organic and inorganic molecules), and 1–50 atoms per element. Output includes the empirical formula, total molecular weight, percent composition by mass, and a common-compound match for well-known formulas.

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.