# Molecular Weight Calculator URL: https://speedycalculator.com/calculators/chemistry/molecular-weight/ Last updated: 2026-10-30 > Enter a molecular formula to get its molecular weight in daltons (u) and kDa, the absolute mass of a single molecule in grams and kilograms, how many molecules are in one gram, and – for organic formulas – the monoisotopic and nominal masses used in mass spectrometry. ## Inputs - Molecular formula — Examples: C8H10N4O2 (caffeine), C9H8O4 (aspirin), CH3COOH, C2H5OH ## Results - Molecular weight - Molecular weight in kDa - Mass of one molecule - Mass of one molecule (SI) - Molecules in 1 g - Monoisotopic mass (most abundant isotopes) - Nominal mass - Molecular formula (Hill notation) - Atoms per molecule - Molecular weight breakdown ## 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) ``` ## Explanation 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 QuantityRefers toUnitWaterMolecular weight (molecular mass)One moleculeu = Da18.015 uRelative molecular mass MrRatio to 1/12 of ¹²Cnone18.015Molar massOne mole (6.022 × 10²³ molecules)g/mol18.015 g/molMass of one moleculeOne moleculeg2.99 × 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). ## 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. ## FAQ **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). ## Sources - [NIST – Atomic weights and isotopic compositions](https://www.nist.gov/pml/atomic-weights-and-isotopic-compositions-relative-atomic-masses) - [IUPAC Commission on Isotopic Abundances and Atomic Weights – Standard atomic weights](https://www.ciaaw.org/atomic-weights.htm) - [NIST – Fundamental physical constants (CODATA)](https://physics.nist.gov/cuu/Constants/)