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Lab Tools · Analytical & Formulation

Molecular Weight Calculator: Formula → g/mol

Calculate molecular weight from any chemical formula with element breakdown. Supports salts, hydrates, and parentheses — plus a pharmaceutical API reference table. Built for molarity prep, osmolarity estimation, and salt-form dose conversion.

Quick Answer

Molecular weight (MW) is the sum of atomic weights for all atoms in a formula, expressed as g/mol — numerically equal to molar mass. Pharma teams use MW to convert mass↔moles (M = g/L ÷ MW), prepare molar buffers, compare salt vs free-base doses, and calculate osmolarity contributions. Enter any formula (salts, hydrates, parentheses) or pick from the API reference table — then chain to Molarity and Osmolarity calculators.

Enter Chemical Formula

Supports elements, subscripts, parentheses, and hydrate notation (CuSO4·5H2O). Click an example or a drug row below to fill the formula.

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
AspirinC9H8O4180.16Acetylsalicylic acid
IbuprofenC13H18O2206.28NSAID
ParacetamolC8H9NO2151.16Acetaminophen
Metformin HClC4H12ClN5165.62Biguanide antidiabetic
AtorvastatinC33H35FNO5558.64Statin (free acid)
OmeprazoleC17H19N3O3S345.42Proton pump inhibitor
AmoxicillinC16H19N3O5S365.40β-lactam antibiotic
CiprofloxacinC17H18FN3O3331.35Fluoroquinolone
MetoprololC15H25NO3267.37Beta blocker (free base)
Insulin (human)C257H383N65O77S65807.65Protein drug
GlucoseC6H12O6180.16IV fluid excipient
MannitolC6H14O6182.17Excipient / osmotic agent
Sodium ChlorideNaCl58.44Normal saline component
CaffeineC8H10N4O2194.19Reference standard
VancomycinC66H75Cl2N9O241449.25Glycopeptide antibiotic
Heparin sodiumcomplex~15,000Polysaccharide — 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).

Pharma & laboratory context

Molecular weight appears on every API certificate of analysis and drives stoichiometry in batch manufacturing records, analytical reference standard prep, and stability study reporting. Regulatory submissions cite MW for salt form identification, impurity qualification (percent w/w vs mol%), and biologics characterisation where average MW replaces discrete formula mass.

Apply calculated MW in the Molarity Calculator for solution prep, the Osmolarity Calculator for tonicity contributions, the Unit Converter for mg/mL↔mmol/L, and the Solution Preparation workflow for stepwise lab prep documentation.

Evidence & sources

Frequently Asked Questions

Molecular weight (MW) is the sum of atomic weights of all atoms in a molecule, in daltons (Da) or g/mol. Water H2O has MW 18.015 g/mol. MW converts between grams and moles: moles = mass / MW; mass = moles × MW. It is a fundamental property on every API certificate of analysis.
MW enables: (1) molarity from weight — M = (g/L) / MW; (2) osmolarity — mmol/L = (mg/mL / MW) × 1000; (3) salt-to-free-base dose equivalence; (4) stoichiometry in API synthesis; (5) bioanalytical unit conversion between ng/mL and nmol/L. Always use batch-specific MW from the COA for GMP calculations.
Molecular weight is dimensionless (ratio to carbon-12); molar mass has units g/mol. Numerically they are identical for practical pharma work — aspirin MW 180.16 g/mol means one mole weighs 180.16 grams. IUPAC recommends "molar mass" with units; "molecular weight" remains common in pharmacy literature.
Enter the full salt formula including counter-ion. Metformin HCl C4H12ClN5 has MW 165.62 vs free base C4H11N5 at 129.17. Dose conversion: free-base mass = salt mass × (MW free base / MW salt). Labels may express concentration as free base or salt — verify before compounding.
Human insulin ≈ 5808 g/mol (C257H383N65O77S6). Monoclonal antibodies ≈ 145–150 kDa. Glycoproteins vary by batch glycosylation — MW is often reported as a range. Polydisperse polymers (heparin, pegylated drugs) use average MW, not a single formula parse.
Include water of hydration in the formula: CuSO4·5H2O or CuSO4.5H2O adds 5 × 18.015 = 90.075 to the anhydrous MW. Anhydrous vs hydrate forms have different MW and different mass required for the same molar concentration — the COA specifies which form is supplied.
PubChem and ChemCalc provide authoritative MW for known structures — useful for verification. For novel intermediates, salts not in databases, or label reconciliation, parse the formula directly. GMP batch records should cite the COA MW for the specific lot, not a generic database value.
Mass (g) = M × V (L) × MW. Example: 500 mL of 150 mM NaCl — MW 58.44, mass = 0.150 × 0.500 × 58.44 = 4.383 g. Use the Molarity Calculator after obtaining MW here for multi-mode mass/volume/concentration solving.
Different salt forms of the same API have different MW but the same molar amount of active moiety when converted correctly. Bioequivalence and labelling may reference free acid/base equivalents — use MW ratio to translate between salt dose and free-base activity for comparative efficacy studies.
Standard IUPAC average atomic weights (e.g., C 12.011, Cl 35.453, H 1.008). Isotopic purity and monoisotopic mass differ slightly for high-resolution mass spectrometry — this tool reports average MW suitable for stoichiometry and pharmacy calculations, not exact monoisotopic MS matching.
Parentheses group atoms with a subscript multiplier: Ca(OH)2 has 1 Ca, 2 O, 2 H. Nested groups and hydrates (dot notation) are supported. Enter formulas without spaces; subscripts follow element symbols directly (C9H8O4 for aspirin).
Do not parse MW from formula for polydisperse biologics (heparin, pegylated proteins), undefined mixtures, or proprietary polymers. Use manufacturer-reported average MW or SEC-MALS data. Heparin sodium in the reference table is listed as complex (~15,000 Da) for this reason.

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