Pharmaceutical Chemistry

Molecular Weight Calculator

Type a chemical formula to get its molecular weight, with the mass and percentage each element contributes.

Molecular weight

18.0150g/mol

H₂O · 2 unique elements

Formula

Case matters: Co is cobalt, CO is carbon plus oxygen.

Chemical formula

Element symbols followed by atom counts. Group repeats go straight after “(”, e.g. (2NH4)SO4.

Try an example

Add a common element

Tap an element to add it to the end of the formula.

Element breakdown

Each element's atomic weight × atom count, and its share of the total mass.

H₂O

Element	Atomic weight	Count	Contribution	% Composition
HHydrogen	1.008	2	2.0160	11.19%
OOxygen	15.999	1	15.9990	88.81%
Total		3	18.0150	100%

Working

MW = (1.008 × 2) + (15.999 × 1) = 18.0150 g/mol

Molar mass18.0150 g/mol

Unique elements2

Total atoms3

Quick calculations

Common conversions using this molecular weight.

Mass of 1 mole18.02 g

Moles in 10 mg5.551e-1 mol

Moles in 1 g0.0555 mol

Mass for 1 L of a 1 mM solution0.0180 g

MW = Σ (atomic weight × atom count)

% composition = (element mass ÷ total mass) × 100%

Mass of 1 mole in grams = MW in g/mol

Each element's contribution is its standard atomic weight multiplied by how many atoms of it the formula contains; the molecular weight is the sum of those contributions. Dividing one contribution by the total gives that element's share of the mass. Elements are listed by atomic number.

mg/mL ↔ Molarity CalculatorUse the molecular weight to turn a mass concentration into molarity.

References

International Union of Pure and Applied Chemistry. Compendium of Chemical Terminology (the 'Gold Book'). Source
Bureau International des Poids et Mesures. The International System of Units (SI Brochure). 9th ed. Sèvres: BIPM; 2019. Source

Common questions

Numerically, yes. Molecular weight is quoted in atomic mass units per molecule and molar mass in grams per mole, but the number is the same — water is 18.015 amu per molecule and 18.015 g/mol.

This tool does not read the hydrate dot. Calculate the anhydrous salt and the water separately (CuSO4, then H2O) and add five times the water's weight to the salt's.

The parser reads a group's repeat count straight after the opening bracket, not after the closing one. Expand the group instead (CaO2H2), or write the count first: (2OH)Ca.

Symbols are case-sensitive. Co is cobalt (58.933); CO is one carbon and one oxygen (28.010). Always start each symbol with a capital letter.

Older textbooks use rounded or older atomic weights (for example Cl = 35.5 or S = 32.07). This calculator uses the IUPAC 2021 standard values, so the last decimal places can differ.

About molecular weight

Molecular weight (MW) is the sum of the atomic weights of every atom in a molecule. It is expressed in atomic mass units (amu) or, per mole, in grams per mole (g/mol) — the same number either way.

Use it when

Doing stoichiometric calculations

Preparing solutions of a given molarity

Working through analytical chemistry problems

Checking the drug content of a pharmaceutical formulation

How to type a formula

Element symbols with the first letter uppercase, the second lowercase: Na, Mg, Fe, Au

A number after a symbol is its atom count: H2O, CO2, C6H12O6

Write a repeated group with its count straight after the opening bracket: (2NH4)SO4

No spaces, dots or charges

Limitations

Isotopic variations are not considered — average atomic weights are used

Hydrates need a separate calculation: add the water mass yourself

Brackets with the count after them, such as Ca(OH)2 or (NH4)2SO4, are not read — expand them (CaO2H2)

For ionic compounds the result is the formula weight

Atomic weights: IUPAC 2021 standard values.

Prepared by the PharmaWallah student team. Verify results with your textbook or instructor. Report an error · Editorial policy