About the Stoichiometry Calculator
This stoichiometry calculator converts an amount of one substance in a chemical reaction into the amount of any other. Type the equation (it is balanced automatically), choose the substance you know and how much you have in grams, kilograms, milligrams, moles or millimoles, and pick the substance you want. It returns the target mass and moles, the mole ratio used and a table with the stoichiometric amount of every reactant and product.
It follows the grams → moles → mole ratio → moles → grams route taught in every general chemistry course, so it is ideal for checking homework, scaling a lab procedure or estimating how much product a reaction can give. Molar masses use IUPAC 2024 abridged atomic weights.
The result assumes the known substance reacts completely and every other reactant is present in at least the required amount. If you have measured amounts of two or more reactants, use the limiting reactant calculator first; to compare with what you actually isolated, use the percent yield calculator.
With the default inputs, the mass of target is 29.9401 g. Change any value above to recalculate instantly.
How to use the stoichiometry calculator
- 1Type the chemical equation using -> or = (it need not be balanced).
- 2Enter the formula of the substance you know, exactly as written in the equation.
- 3Enter its amount and choose grams, kilograms, milligrams, moles or millimoles.
- 4Enter the formula of the substance you want to find.
- 5Read the mass and moles of the target, and check the table for every species.
Formula and method
Balanced equations relate amounts in moles, not grams. The known mass is converted to moles with its molar mass, multiplied by the ratio of stoichiometric coefficients (target ÷ known) from the balanced equation, and converted back to grams with the target’s molar mass. If the known amount is already in moles the first step is skipped.
The equation is balanced with the smallest whole-number coefficients by solving the element-conservation equations, so you can type it unbalanced. The table applies the same extent of reaction to every species, and the total mass of reactants always equals the total mass of products, which is a useful check on your setup.
- m
- Mass (g)
- n
- Amount (mol)
- M
- Molar mass (g/mol)
- ν
- Stoichiometric coefficient in the balanced equation
Worked examples
Burning 10 g of propane: how much CO₂?
The balanced equation is C3H8 + 5 O2 → 3 CO2 + 4 H2O. 10 g ÷ 44.097 g/mol = 0.2268 mol propane, which gives 3 × 0.2268 = 0.6803 mol CO2, or 0.6803 × 44.009 = 29.94 g.
Haber process: NH₃ from 100 g of H₂
N2 + 3 H2 → 2 NH3. 100 g ÷ 2.016 g/mol = 49.60 mol H2, × 2/3 = 33.07 mol NH3, × 17.031 g/mol = 563.2 g of ammonia.
Iron from 2 mol of iron(III) oxide
Fe2O3 + 3 CO → 2 Fe + 3 CO2, so each mole of oxide gives 2 mol of iron: 4 mol × 55.845 g/mol = 223.38 g.
Oxygen from 12.25 g of potassium chlorate
2 KClO3 → 2 KCl + 3 O2. 12.25 g ÷ 122.545 g/mol = 0.09996 mol, × 3/2 = 0.1499 mol O2 × 31.998 g/mol = 4.80 g of oxygen.
Frequently asked questions
What are the steps of a stoichiometry problem?+
Balance the equation, convert the given mass to moles with its molar mass, multiply by the mole ratio from the coefficients (wanted ÷ given), then convert moles of the wanted substance back to grams with its molar mass.
What is a mole ratio?+
It is the ratio of the coefficients of two substances in a balanced equation. In 2 H2 + O2 → 2 H2O the ratio of H2O to O2 is 2 : 1, so every mole of oxygen consumed makes two moles of water.
Why can I not use grams directly in the ratio?+
Coefficients count particles (moles), and different substances have different molar masses. 1 g of H2 and 1 g of O2 contain very different numbers of molecules, so masses must be converted to moles first.
Do I need to balance the equation first?+
No — this calculator balances it for you using the smallest whole-number coefficients and shows the balanced form. If you balance by hand, check that every element has the same number of atoms on both sides.
What if I know the amounts of two reactants?+
Then one of them may run out first. Work out how much product each reactant could make; the one giving less product is the limiting reactant and determines the theoretical yield. The limiting reactant calculator does this automatically.