Stoichiometry Ratio Calculator

Background.

How many grams of oxygen does it take to burn 50 g of propane? Not 250 g, even though the balanced equation, C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O, puts a 5 in front of the oxygen. That 5 counts molecules, and an oxygen molecule is lighter than a propane molecule, so the answer is 181.41 g. Getting from one to the other is the whole job: grams of what you have into moles, moles through the coefficients, moles back into grams of what you want.

The chain works between any two species in an equation, in either direction. Two reactants give a reagent requirement. A reactant and a product give a yield or, run backwards, the starting charge needed to reach a target amount of product. Two products give a by-product ratio, such as the water a burner condenses for every gram of carbon dioxide it releases. You type the formulas and coefficients; molar masses are summed from the IUPAC standard atomic weights, and the mole and mass ratios appear beside the answer.

What the calculator cannot check is whether the equation is balanced. Wrong coefficients produce answers that look entirely reasonable and are wrong.

What is stoichiometry ratio calculator?

A stoichiometric ratio, or mole ratio, is the ratio of two species' coefficients in a balanced equation. In propane combustion, oxygen to propane is 5 : 1, carbon dioxide to propane 3 : 1 and water to carbon dioxide 4 : 3. IUPAC grounds it in the extent of reaction ξ: each species changes by Δn = ν ξ, where ν is its stoichiometric number, negative for reactants and positive for products. For any two species ξ cancels, leaving the ratio of their coefficients. It therefore holds at every stage of the reaction and is untouched by temperature, pressure or concentration, which decide how far a reaction goes but not the proportions in which it consumes and produces. The calculator uses the coefficients as written, without signs.

How to use this calculator.

1. 01Balance the equation on paper first and keep it in front of you.
2. 02Under 'Solve for', choose the species you want: B when you know A's mass, A when you know B's. The mass box of the species you are solving for is ignored, so whatever it holds does not matter.
3. 03Type each formula with a capital letter starting every element symbol, since CO is carbon monoxide and Co is cobalt. Brackets work, as in Ca(OH)2 or Al2(SO4)3; leave out spaces, charges, state labels and the dots of hydrates.
4. 04Enter each species' coefficient from the balanced equation. Fractions are accepted, as in C2H6 + 3.5 O2 → 2 CO2 + 3 H2O.
5. 05Enter the known mass in grams, then read the answer and compare the mole ratio shown with the coefficients you meant to enter.
6. 06For a gas measured by volume, convert it to moles with the ideal gas law, multiply by its molar mass and enter the grams.

The formula.

Three steps, and only the middle one is chemistry. First, n_A = m_A ÷ M_A turns the known mass into moles. Second, n_B = n_A × ν_B ÷ ν_A applies the coefficients. Third, m_B = n_B × M_B turns moles back into grams. Solving for A runs the same chain in reverse, dividing by the mole ratio instead of multiplying.

Collapsed into a single factor, m_B = m_A × (ν_B ÷ ν_A) × (M_B ÷ M_A). That factor is the mass ratio output: once you have it for a reaction, any mass of A multiplied by it gives the mass of B with no mole arithmetic at all.

Molar masses are summed from the atom counts in each formula using the conventional values of the CIAAW abridged standard atomic weights, 2024 revision: C 12.011, H 1.008, O 15.999, N 14.007, Si 28.085 and so on. Elements without a standard atomic weight, such as technetium, promethium, polonium, radon, radium, actinium and everything beyond uranium, are refused rather than given an invented mass. Both coefficients and the known mass must be greater than zero.

A worked example.

Frequently asked questions.

References& sources.

1. [1]IUPAC. Compendium of Chemical Terminology (the Gold Book), 'stoichiometric number, ν' (S06025) and 'extent of reaction, ξ' (E02283). PRIMARY SOURCE for this page: stoichiometric numbers are positive for products and negative for reactants, and dξ = dn_B / ν_B. S06025 is sourced there to Pure Appl. Chem. 1996, 68, 149 ('A glossary of terms used in chemical kinetics, including reaction dynamics', IUPAC Recommendations 1996) at p. 187 and to the Green Book 2nd ed. p. 42. This is what makes the mole ratio ν_B/ν_A a consequence of a single extent of reaction, and therefore independent of temperature, pressure and how far the reaction has gone. Retrieved 2026-07-29 via the legacy IUPAC host (the current goldbook.iupac.org returns HTTP 403 to automated retrieval). Open access.
2. [2]Flowers, P., Theopold, K., Langley, R. & Robinson, W. R. (2019). Chemistry 2e, Sections 4.3–4.4. OpenStax, Rice University. Peer-reviewed, CC BY 4.0. It works the identical mass–mole–mass conversions arithmetically, without invoking the extent of reaction, and agrees. Example 4.12 (3 Si + 2 N2 → Si3N4 from 2.00 g Si and 1.50 g N2) is used as an independent check: this calculator returns a nitrogen requirement of 1.3300 g, which is less than the 1.50 g charged, consistent with OpenStax finding silicon limiting. Retrieved 2026-07-29. Open access.
3. [3]Meija, J. et al. (2021). 'Atomic weights of the elements 2021 (IUPAC Technical Report)'. Pure and Applied Chemistry 93(5), 573–600, doi:10.1515/pac-2019-0603, as maintained in the CIAAW table 'Abridged Standard Atomic Weights 2024' (named revision: 2024, incorporating the Gd, Lu and Zr revisions on the Atomic Weights 2021 base). The named, versioned source of every molar mass this calculator computes — C 12.011, H 1.0080, O 15.999, Si 28.085, N 14.007. Retrieved 2026-07-29. Open access.
4. [4]Constable, D. J. C., Curzons, A. D. & Cunningham, V. L. (2002). 'Metrics to "green" chemistry — which are the best?' Green Chemistry 4(6), 521–527, doi:10.1039/B206169B. Royal Society of Chemistry, peer-reviewed. Table 1 column 'Stoichiometry of B mol (%)' records the actual second-reagent charge across twenty-eight industrial reaction classes, ranging from 120% of stoichiometry (N-alkylation) to 2650% (N-dealkylation). This is the evidence for this page's statement that the stoichiometric requirement is a floor rather than a recipe. Retrieved 2026-07-29. Publisher page paywalled; an open copy was consulted.
5. [5]IUPAC (2007). Quantities, Units and Symbols in Physical Chemistry ('the Green Book'), 3rd edition (2nd printing 2008), section 2.12 'Chemical reactions' — the formal definitions of stoichiometric number, extent of reaction and amount of substance from which the mole ratio is derived on this page. International Union of Pure and Applied Chemistry / RSC Publishing. Retrieved 2026-07-29. Bibliographic reference: a print and PDF edition rather than a live web page.