Stoichiometry Calculator

Convert between moles and grams of any two substances in a balanced chemical equation. Enter the stoichiometric coefficients, molar masses, and the known quantity — then solve for the unknown moles or grams.

How to use: Find a balanced equation (e.g., 2H₂ + O₂ → 2H₂O). Pick two substances — the one you know (Given) and the one you want to find (Find). Enter the coefficient of each from the balanced equation, their molar masses, and the amount you have.

A Given Substance

Name / Formula (optional)

Stoichiometric Coefficient

Molar Mass (g/mol)

Use the Molar Mass Calculator if needed.

Known Amount

B Find Substance

Name / Formula (optional)

Stoichiometric Coefficient

Molar Mass (g/mol)

Load an example reaction:

What Is Stoichiometry?

Stoichiometry (from the Greek stoicheion = element, metron = measure) is the branch of chemistry that deals with the quantitative relationships between reactants and products in chemical reactions. In a balanced equation, the coefficients represent the mole ratios of each substance involved.

For example, in the combustion of hydrogen: 2H₂ + O₂ → 2H₂O, the coefficients tell us that 2 moles of hydrogen react with 1 mole of oxygen to produce 2 moles of water. If you start with 4 moles of H₂, you need 2 moles of O₂ and will produce 4 moles of H₂O.

The Mole Road Map

The standard stoichiometry problem uses a three-step "road map":

Convert grams → moles of Given: moles = grams ÷ molar mass
Convert moles of Given → moles of Find using the mole ratio (coefficients from the balanced equation): moles_B = moles_A × (coefficient_B / coefficient_A)
Convert moles → grams of Find: grams = moles × molar mass

This calculator performs all three steps automatically and shows the work in a step-by-step format.

Finding Molar Masses

Each substance's molar mass is the sum of the atomic masses of all atoms in its formula. For water (H₂O): 2 × 1.008 + 15.999 = 18.015 g/mol. You can look up individual atomic masses on the periodic table, or use our Molar Mass Calculator — just type the formula (e.g., H2O, Ca(OH)2) and it computes the molar mass automatically, with a full breakdown by element.

Limiting Reactants

This calculator assumes you have enough of the other reactants — it solves the theoretical yield for one substance-to-substance pair. In real lab problems where you have both reactants and need to find the limiting reactant, you would run this calculation twice (once for each reactant) and take the smaller result as the theoretical yield.

Frequently Asked Questions

What is a mole in chemistry?

A mole is the SI unit of amount of substance. One mole contains exactly 6.02214076 × 10²³ entities (Avogadro's number). For elements and compounds, one mole weighs exactly its molar mass in grams. For instance, one mole of water (18.015 g/mol) weighs 18.015 grams.

What if the coefficient is 1?

A coefficient of 1 is almost always written implicitly — meaning if there's no number in front of a substance in a balanced equation, the coefficient is 1. Leave the coefficient field at 1 in those cases.

Can this handle percent yield?

This calculator gives the theoretical yield (100% efficiency). Actual yield is usually less. Percent yield = (actual yield / theoretical yield) × 100%. To use this calculator for percent yield: compute the theoretical yield, then divide your actual lab yield by that number and multiply by 100.

Why do I need a balanced equation?

The Law of Conservation of Mass states that atoms cannot be created or destroyed in a chemical reaction. A balanced equation ensures the same number of atoms of each element appears on both sides of the arrow. Without a balanced equation, the mole ratios (and therefore your stoichiometry calculation) will be wrong.