Stoichiometry Calculator
Stoichiometry is the bookkeeping of a chemical reaction: it tells you how much product a given amount of reactant can make. This calculator takes the mass of a reactant, its molar mass, the balanced-equation coefficients of the reactant and product, and the product molar mass, then returns reactant moles, product moles, and the theoretical product mass. The method is the classic mass to moles to moles to mass conversion that conserves atoms. Look up molar masses from the NIST atomic weights tables and take coefficients from your balanced equation.
Stoichiometry formula
Reactant moles = reactant mass / reactant molar mass
Mole ratio = product coefficient / reactant coefficient
Product moles = reactant moles * mole ratio
Product mass = product moles * product molar mass
This is the standard dimensional-analysis route through a balanced equation. It conserves atoms because the mole ratio comes from balanced coefficients. No empirical constant is assumed beyond the molar masses you supply.
Working with reactions
- Always start from a balanced equation so atoms are conserved on both sides.
- Molar masses are atomic-weight sums from the NIST and IUPAC tables.
- The result is the theoretical yield, the maximum if the reaction goes to completion.
- To find the limiting reactant, run the calc for each reactant and take the smallest product amount.
- Percent yield is actual product mass divided by this theoretical mass, times 100.
Stoichiometry: frequently asked questions
How does stoichiometry convert reactant to product?
First convert reactant mass to moles by dividing by its molar mass. Then multiply by the mole ratio, the product coefficient divided by the reactant coefficient from the balanced equation. Finally convert product moles back to mass by multiplying by the product molar mass.
Where do the coefficients come from?
Coefficients come from the balanced chemical equation. For 2 H2 plus O2 yields 2 H2O, the coefficient of H2 is 2 and of H2O is 2. Balancing ensures atoms are conserved. Enter the integer coefficients from your balanced equation; the calculator forms the ratio for you.
What molar masses should I enter?
Use the molar mass of each species in grams per mole, the sum of the atomic weights of its atoms from the NIST or IUPAC tables. For example water (H2O) is about 18.02 g/mol and carbon dioxide (CO2) is about 44.01 g/mol.
Does this handle limiting reactants?
This tool computes the theoretical yield from one specified reactant assuming it is fully consumed. To find a limiting reactant, run the calculation for each reactant separately; the one giving the smallest product amount is limiting and sets the true yield.
What is theoretical yield?
Theoretical yield is the maximum product mass possible if the reaction goes to completion with no losses, which is exactly what this calculator returns. Actual yield is usually lower; percent yield is actual divided by theoretical times 100.
Official sources
- National Institute of Standards and Technology: Atomic Weights and Isotopic Compositions.
- NIST: Avogadro constant and the mole definition.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.