Stoichiometry Calculator
Type a chemical equation and the amount of any one substance you know, in grams or moles. The calculator balances the equation if needed and works out how many moles and grams of every other reactant and product are involved, showing each mole ratio step.
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Use -> or = between the sides. Unbalanced equations are balanced automatically. Hydrates (CuSO4·5H2O) and brackets work.
Formula exactly as in the equation (reactant or product). Empty = first reactant
Highlighted as the main result. Empty = first product
Scales the products to an expected real yield
How to use the Stoichiometry Calculator
- Type the equation, e.g. C3H8 + 5O2 -> 3CO2 + 4H2O. Coefficients can be left out.
- Enter the known substance and its amount in grams or moles.
- Optionally pick the substance to find and a percent yield.
- Read the main result and the amounts of every other species.
- Follow the steps to see each mole ratio, or download the table.
The stoichiometry road map
Every stoichiometry problem follows the same path through moles, because the balanced equation counts particles, not grams:
grams of A → (÷ molar mass of A) → moles of A → (× coef. B ÷ coef. A) → moles of B → (× molar mass of B) → grams of B
Worked example: how much carbon dioxide forms when 44.0 g of propane burns? C3H8 + 5O2 → 3CO2 + 4H2O.
- Moles of propane: 44.0 g ÷ 44.097 g/mol = 0.9978 mol.
- Mole ratio CO2 : C3H8 = 3 : 1, so CO2 = 0.9978 × 3 = 2.993 mol.
- Mass of CO2: 2.993 mol × 44.009 g/mol = 131.7 g.
The same 0.9978 mol of propane needs 4.989 mol (159.6 g) of O2 and makes 3.991 mol (71.9 g) of water. The reactants total 203.6 g and the products total 203.6 g, as conservation of mass requires.
Common mole ratios
| Reaction | Balanced equation | Useful ratio | Grams per gram |
|---|---|---|---|
| Water formation | 2H2 + O2 → 2H2O | H2O : O2 = 2 : 1 | 1 g O2 → 1.126 g H2O |
| Haber process | N2 + 3H2 → 2NH3 | NH3 : N2 = 2 : 1 | 1 g N2 → 1.216 g NH3 |
| Methane combustion | CH4 + 2O2 → CO2 + 2H2O | CO2 : CH4 = 1 : 1 | 1 g CH4 → 2.743 g CO2 |
| Limestone decomposition | CaCO3 → CaO + CO2 | CaO : CaCO3 = 1 : 1 | 1 g CaCO3 → 0.560 g CaO |
| Thermite | Fe2O3 + 2Al → Al2O3 + 2Fe | Fe : Fe2O3 = 2 : 1 | 1 g Fe2O3 → 0.699 g Fe |
Tips for getting stoichiometry right
- Never use a mass ratio from the coefficients. The coefficients are mole ratios; 2H2 + O2 does not mean 2 g of hydrogen per 1 g of oxygen.
- Write formulas carefully. Element symbols start with a capital letter: CO is carbon monoxide, Co is cobalt. Use brackets for groups, as in Ca(OH)2.
- Diatomic elements: H2, N2, O2, F2, Cl2, Br2 and I2 are written with a subscript 2 when they appear as elements.
- Keep extra digits until the end and round to the significant figures of your data only in the final answer.
- Gases at STP: to convert moles of a gas to volume, multiply by 22.71 L/mol (0 °C, 1 bar) or 22.41 L/mol (0 °C, 1 atm).
Frequently asked questions
What is stoichiometry?
Stoichiometry is the calculation of the quantities of reactants and products in a chemical reaction. It relies on the balanced equation: the coefficients give the ratio in which substances react and form, measured in moles.
How do you do a grams-to-grams stoichiometry problem?
Use three steps: convert the known mass to moles (grams ÷ molar mass), multiply by the mole ratio from the balanced equation (wanted coefficient ÷ known coefficient), then convert back to grams (moles × molar mass of the wanted substance). For 44 g of propane in C3H8 + 5O2 → 3CO2 + 4H2O: 44 ÷ 44.097 = 0.998 mol, × 3/1 = 2.993 mol CO2, × 44.009 = 131.7 g of CO2.
What is a mole ratio?
A mole ratio is the ratio of the coefficients of two substances in a balanced equation. In N2 + 3H2 → 2NH3 the ratio of H2 to N2 is 3 : 1 and NH3 to N2 is 2 : 1, so 2 mol of N2 needs 6 mol of H2 and makes 4 mol of NH3.
Does the equation need to be balanced?
Yes, the mole ratios only work with a balanced equation. If yours is not balanced, the calculator balances it automatically with the smallest whole-number coefficients and tells you. You can also check it first with the Chemical Equation Balancer.
Can I start from a product?
Yes. Enter a product as the known substance to find how much of each reactant you need. For example, to make 100 g of water by 2H2 + O2 → 2H2O you need 5.55 mol of H2O, so 2.775 mol (88.8 g) of O2 and 5.55 mol (11.2 g) of H2.
What if I know the amounts of two reactants?
Then one of them may run out first. This calculator assumes the other reactants are present in exactly the needed amounts. To find which reactant limits the reaction and how much excess is left, use the Limiting Reactant Calculator.
Which atomic masses are used?
Standard atomic weights from IUPAC (for example H 1.008, C 12.011, N 14.007, O 15.999), with mass numbers of the most stable isotope for radioactive elements. Results can differ slightly in the last decimal from a textbook that uses rounded values.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.