Lab Tools
Molecular Weight Calculator
Calculate molecular weight from a chemical formula. Supports elements, subscripts, parentheses, and hydrate notation (CuSO4·5H2O). Click any drug in the reference table to fill the formula.
Enter Chemical Formula
Examples:
Molecular Weight
— g/mol
Atom Breakdown
| Element | Symbol | Count | Atomic Weight | Contribution (g/mol) | % of MW |
|---|
Pharmaceutical API Reference Table
Click any row to fill the formula into the calculator.
| Drug | Formula | MW (g/mol) | Notes |
|---|---|---|---|
| Aspirin | C9H8O4 | 180.16 | Acetylsalicylic acid |
| Ibuprofen | C13H18O2 | 206.28 | NSAID |
| Paracetamol | C8H9NO2 | 151.16 | Acetaminophen |
| Metformin HCl | C4H12ClN5 | 165.62 | Biguanide antidiabetic |
| Atorvastatin | C33H35FNO5 | 558.64 | Statin (free acid) |
| Omeprazole | C17H19N3O3S | 345.42 | Proton pump inhibitor |
| Amoxicillin | C16H19N3O5S | 365.40 | β-lactam antibiotic |
| Ciprofloxacin | C17H18FN3O3 | 331.35 | Fluoroquinolone |
| Metoprolol | C15H25NO3 | 267.37 | Beta blocker (free base) |
| Insulin (human) | C257H383N65O77S6 | 5807.65 | Protein drug |
| Glucose | C6H12O6 | 180.16 | IV fluid excipient |
| Mannitol | C6H14O6 | 182.17 | Excipient / osmotic agent |
| Sodium Chloride | NaCl | 58.44 | Normal saline component |
| Caffeine | C8H10N4O2 | 194.19 | Reference standard |
| Vancomycin | C66H75Cl2N9O24 | 1449.25 | Glycopeptide antibiotic |
| Heparin sodium | complex | ~15,000 | Polysaccharide — not calculable from single formula |
How Molecular Weight Is Used in Pharma
Calculating Molarity
To convert a weight-based concentration to molar: M = (g/L) ÷ MW. For example, 9 g/L NaCl (MW 58.44) = 0.154 mol/L = 154 mM — normal saline.
Weight ↔ Mole Conversions
Moles = Mass (g) / MW. Mass = Moles × MW. This is used when preparing exact molar concentrations for buffers, assay standards, and synthesis reactions.
Salt Form Calculations
When a drug is supplied as a salt, the label concentration refers to the salt form. To calculate free base equivalent: free base mass = salt mass × (MW free base / MW salt). This is critical for comparing doses across formulations.
Synthesis Stoichiometry
In API synthesis, molar equivalents are calculated from MW to ensure the correct ratio of reactants. For example, if a reaction calls for 1.2 equivalents of a reagent, the mass needed = 1.2 × (moles of starting material) × MW(reagent).
Frequently Asked Questions
Molecular weight (MW) is the sum of atomic weights of all atoms in a molecule, expressed in atomic mass units (amu) or equivalently as grams per mole (g/mol). It is a fundamental property used in chemistry to convert between amount (moles) and mass (grams). For example, aspirin (C9H8O4) has MW 180.16 g/mol — meaning one mole of aspirin weighs 180.16 grams.
MW is used to: (1) convert grams to moles and vice versa; (2) prepare molar solutions (M = g/L ÷ MW); (3) calculate stoichiometry in synthesis; (4) determine equivalent doses between salt and free base forms; and (5) compare potency of drugs with different molecular sizes. In clinical pharmacokinetics, MW influences absorption and membrane permeability.
Molecular weight is technically dimensionless (a ratio relative to 1/12 of carbon-12 mass). Molar mass has units of g/mol. However, their numerical values are identical, and in pharmaceutical practice the two terms are used interchangeably. Both equal the sum of atomic weights for all atoms in the formula. For most practical purposes, if MW = 180.16, the molar mass is also 180.16 g/mol.
Enter the full formula of the salt including the counter-ion. For metformin hydrochloride, the formula is C4H12ClN5 (metformin C4H11N5 plus HCl). The MW 165.62 includes the chloride. If a drug is available as multiple salt forms (tartrate, maleate, mesylate), each will have a different MW and a different free-base conversion factor. This calculator handles any formula you enter, including salts with halogens, sodium, potassium, or calcium counter-ions.
Protein drugs have very large molecular weights. Human insulin is 5808 g/mol (C257H383N65O77S6). Monoclonal antibodies are typically 145,000–150,000 Da (145–150 kDa). Biosimilar proteins are characterised by average MW because glycosylation varies between batches. Complex polysaccharides like heparin are reported as average MW ranges (standard heparin ~15,000 Da; low molecular weight heparin 3,000–7,000 Da) because they are polydisperse mixtures rather than single compounds.