Molecular Weight Calculator

Common Organic Compounds

Elements of the periodic table

Chemical formula:

# Atom Molar Mass (MM) Subtotal Mass Subtotal Mass
(g/mol) (%) (g/mol)

Total Molecular Weight:

An image illustrating the step-by-step process of using a Molecular Weight Calculator. The image shows a user entering the chemical formula or compound name into the calculator's input field. The next step displays the calculator automatically calculating the molecular weight of the entered compound based on atomic weights of individual elements and the formula. This image visually guides the user on how to use the Molecular Weight Calculator, a helpful tool for determining the molecular weight of a compound or molecule.

Quantifying the molecular weight of chemical compounds facilitates research and industry applications. This guide explains utilizing the molecular weight calculator for summing elemental masses.

Key Takeaways:

  • Use MW to interconvert mass, moles, volume, and concentration
  • Apply MW in stoichiometric calculations and chemical reactions
  • MW allows determining gram equivalent of a compound

Introduction to Molecular Weight

Molecular weight, also termed formula weight or molecular mass, quantifies the total mass of a molecule by summing its composite elemental parts.

Specifically, it represents the:

  • Sum of atomic weights of the atoms within a molecule
  • Grams per mole of the compound

Knowing the weight assists chemical experiments and conversions. The calculator simplifies determining this essential metric.

Using the Molecular Weight Calculator

  1. Select a compound from the database
  2. Choose element(s) constituting it
  3. Click "Calculate"
  4. View formula and molecular weight

For example, calculating water (H20):

  • Hydrogen Atomic Weight: 1.0079 g/mol
  • Oxygen Atomic Weight: 15.999 g/mol
  • H20 Molecular Weight = 18.015 g/mol

This displays the discrete atoms and total weight. Apply the value for downstream usages explained below.

data table showing molecular weights and molar masses of some common substances:


Chemical Formula

Molecular Weight (g/mol)

Molar Mass (g/mol)









Sodium Chloride



Na - 23.00, Cl - 35.45








C - 12.01, H - 1.008

DNA Nucleotide




Key Applications of Molecular Weight

Among numerous uses, molecular weight facilitates:

  • Stoichiometry problems
  • Determining percent composition
  • Calculating grams to moles
  • Drug development and dosing
  • Setting up lab reactions
  • Deriving molar concentration

It quantifies the fundamental "amount of substance" measure enabling diverse chemical calculations as explored below.

Using Molecular Weight for Molar Conversions

Technicians and scientists most often utilize molecular weight for molarity conversions between:

  • Grams ⟷ moles
  • Moles ⟷ liters
  • Mass ⟷ volume

This allows preparing solutions like:

  • 500 grams glucose to 2 liters water
  • 0.5 moles sodium chloride into 125 mL

The calculator makes these translations simple by establishing the basis weight reference point.

Molecular and Molar Mass Differences

Although the terms molecular weight and molar mass both quantify mass per mole, a subtle difference exists:

  • Molecular weight represents the mass of a molecule
  • Molar mass refers to mass per mole of atoms/particles

So water's molecular weight = 18.015 g/mol, while hydrogen's molar mass = 1.0079 g/mol.

IUPAC nomenclature dictates specifying one or the other when reporting compound mass.

Tracking Molecular Weight Changes in Reactions

For chemical reactions involving multiple starting materials and products, molecular weights illuminate the reaction mechanism by revealing the mass changes.

This helps derive reaction stoichiometry for industrial applications. Combining reactants with known molecular weights determines expected products.

For example:

2H2 + O2 → 2H20

Reactant MW = 2(2) + 32 = 36 g/mol

Product MW = 36 g/mol

Balanced as expected!

Limitations of Molecular Weight Calculator

While molecular weight accurately measures mass, some limitations exist:

  • Requires understanding of chemical composition
  • Values depend greatly on isotopic abundance
  • Assumes a set structure which may change
  • Does not consider complex protein configurations

Users should acknowledge these constraints when applying measurements, especially when utilizing the calculator through manual input.


The molecular weight calculator simplifies quantifying the summed atomic mass within chemical substances. This helps answer multifaceted chemistry questions relying on discrete molecular mass foundations. Use the online calculator to enhance research and ensure accurate commercial or industrial usage specifications