Molecular weight – more precisely relative molecular mass or molecular mass – is the mass of one molecule expressed in unified atomic mass units (u), also called daltons (Da). One dalton is one twelfth of the mass of a carbon-12 atom, about 1.6605 × 10⁻²⁴ g. Caffeine, C₈H₁₀N₄O₂, has a molecular weight of 194.19 Da, so a single caffeine molecule weighs about 3.22 × 10⁻²² g. Biochemists quote proteins in kilodaltons: haemoglobin is roughly 64.5 kDa.
Numerically, molecular weight equals molar mass in g/mol – the difference is only the scale: one molecule versus one mole. This page focuses on the single-molecule view: it gives the weight in Da and kDa, the real mass of one molecule and the number of molecules in a gram, and for formulas made of common elements (C, H, N, O, F, Na, Si, P, S, Cl, K, Br, I) the monoisotopic mass – the exact mass of the molecule built only from the most abundant isotope of each element – and the nominal (integer) mass.
Mass spectrometers resolve individual isotopes, so the main peak of a small organic molecule appears at the monoisotopic mass (194.0804 for caffeine), not at the average molecular weight (194.19). Knowing both avoids a common source of confusion when reading spectra.
How to use the Molecular Weight Calculator
- Molecular formula — Examples: C8H10N4O2 (caffeine), C9H8O4 (aspirin), CH3COOH, C2H5OH
Everything is calculated in your browser, so results appear instantly and nothing you type is sent to a server.
How it works
The formula is expanded into atom counts; the average molecular weight uses IUPAC standard atomic weights, the monoisotopic mass uses the exact masses of ¹H, ¹²C, ¹⁴N, ¹⁶O and the other most abundant isotopes, and the mass of one molecule divides the molecular weight by the Avogadro constant 6.02214076 × 10²³ mol⁻¹.
Worked example
Caffeine, C₈H₁₀N₄O₂: average molecular weight = 8 × 12.011 + 10 × 1.008 + 4 × 14.007 + 2 × 15.999 = 96.088 + 10.080 + 56.028 + 31.998 = 194.194 u (0.194 kDa). One molecule weighs 194.194 ÷ 6.02214076 × 10²³ = 3.2247 × 10⁻²² g, and one gram contains 3.10 × 10²¹ molecules. Using the isotope masses ¹²C = 12, ¹H = 1.007825, ¹⁴N = 14.003074 and ¹⁶O = 15.994915 gives a monoisotopic mass of 194.0804 u and a nominal mass of 194.
Molecular weight vs molar mass
| Quantity | Refers to | Unit | Water |
|---|---|---|---|
| Molecular weight (molecular mass) | One molecule | u = Da | 18.015 u |
| Relative molecular mass Mr | Ratio to 1/12 of ¹²C | none | 18.015 |
| Molar mass | One mole (6.022 × 10²³ molecules) | g/mol | 18.015 g/mol |
| Mass of one molecule | One molecule | g | 2.99 × 10⁻²³ g |
Tips
- Always start from a balanced equation – coefficients give the mole ratio.
- Convert everything to moles first, then use the ratio, then convert back.
- Every field has its own unit menu – g, mg, mol, mmol, L and mL all work.
- Everything is calculated in your browser.
Common mistakes to avoid
- Using mass ratios instead of mole ratios.
- Forgetting water of crystallisation in the molar mass of hydrates.
- Confusing molarity (per litre of solution) with molality (per kilogram of solvent).
Formula
MW = Σ (nᵢ × Aᵢ) in u (Da) · mass of one molecule = MW ÷ Nᴀ (g) · molecules per gram = Nᴀ ÷ MW · monoisotopic mass = Σ (nᵢ × mass of the most abundant isotope)Worked example
Inputs — Molecular formula: C8H10N4O2.
Results — Molecular weight = 194.194 u (Da); Molecular weight in kDa = 0.194194 kDa; Mass of one molecule = 3.2247 × 10⁻²² g; Mass of one molecule (SI) = 3.2247 × 10⁻²⁵ kg; Molecules in 1 g = 3.1011 × 10²¹ molecules; Monoisotopic mass (most abundant isotopes) = 194.0804 u.
Key terms
- Dalton (Da) / unified atomic mass unit (u)
- 1/12 of the mass of a carbon-12 atom, 1.66053907 × 10⁻²⁷ kg.
- Relative molecular mass (Mr)
- Molecular mass divided by 1 u – a pure number.
- Monoisotopic mass
- Mass of a molecule made only of the most abundant isotope of each element.
- Nominal mass
- Sum of the integer mass numbers of the most abundant isotopes.
Frequently asked questions
What is the molecular weight of water?
2 × 1.008 + 15.999 = 18.015 u, so one water molecule weighs 18.015 × 1.6605 × 10⁻²⁴ g = 2.99 × 10⁻²³ g.
What is the molecular weight of caffeine?
C₈H₁₀N₄O₂: 8 × 12.011 + 10 × 1.008 + 4 × 14.007 + 2 × 15.999 = 194.19 u. Its monoisotopic mass is 194.0804 u.
What is the difference between molecular weight and molar mass?
Same number, different meaning: molecular weight is the mass of one molecule in u (Da); molar mass is the mass of one mole in g/mol.
What is a dalton?
1 Da = 1 u = 1/12 of the mass of a carbon-12 atom = 1.66053907 × 10⁻²⁴ g. 1 kDa = 1000 Da.
Why does mass spectrometry show a different mass?
Spectrometers separate isotopes, so the main peak sits at the monoisotopic mass (all ¹²C, ¹H, ¹⁶O …), which is lower than the average molecular weight that includes heavier isotopes.
What is the molecular weight of aspirin?
C₉H₈O₄ = 9 × 12.011 + 8 × 1.008 + 4 × 15.999 = 180.159 u.
Is "molecular weight" a correct term?
Strictly it is a mass, and IUPAC prefers "relative molecular mass" (Mr, dimensionless) or "molecular mass" (in Da). "Molecular weight" is still universal in labs and catalogues.
What formula does the Molecular Weight Calculator use?
MW = Σ (nᵢ × Aᵢ) in u (Da) · mass of one molecule = MW ÷ Nᴀ (g) · molecules per gram = Nᴀ ÷ MW · monoisotopic mass = Σ (nᵢ × mass of the most abundant isotope).