Stoichiometry Ratio Calculator

Background.

A balanced equation fixes the ratio in which every species is consumed or produced, and this calculator converts between any two of them. Enter both species with their balanced-equation coefficients, tell it which one you know, and it returns the other — in grams and in moles, with the mole ratio and the mass ratio shown so you can check the work.

What this page is FOR, and how it differs from its neighbours. Reactant-to-product with the limiting reactant known is the theoretical yield question, and the theoretical yield calculator is the better page for it — it carries the assumptions, the ceiling language and the hand-off into percent yield. This page exists for the questions that page cannot answer: reagent requirements, where both species are reactants ('how many grams of oxygen do I need to burn 50 g of propane?'), and reverse calculations, where you start from a target amount of product and work back to how much starting material to charge ('how much propane must I burn to produce 100 g of CO₂?'). The two pages share the mass-mole-mass chain, and that overlap is stated openly here rather than hidden — use whichever framing matches your question, and the pages link each other.

The method is three steps and only one of them is chemistry. Convert the known mass to moles by dividing by its molar mass. Multiply by the mole ratio — the coefficient of the species you want divided by the coefficient of the species you have. Convert back to grams by multiplying by the other molar mass. The mole ratio is the whole of the chemistry; the divisions and multiplications on either side are unit conversion. The 'mass ratio' output collapses all three into one number: for propane burning in oxygen it reads 3.6281, so every gram of propane demands 3.6281 g of oxygen, and you never need to touch moles again for that reaction.

UNITS AND CONVENTIONS, stated where the answer is. Masses are in grams, molar masses in g/mol, amounts in moles; both ratio outputs are dimensionless. There is no temperature or pressure basis anywhere on this page — a mole ratio is valid at any T and p, and species are entered as masses precisely so that no STP-versus-SATP reference state has to be chosen. If your quantity is a gas volume, convert it to moles with the ideal gas law calculator (which does need T and p, and declares its own conventions) and then multiply by the molar mass. Molar masses come from the CIAAW Abridged Standard Atomic Weights 2024 table — the IUPAC Atomic Weights 2021 recommendation with the 2024 revisions to gadolinium, lutetium and zirconium — and elements with no IUPAC standard atomic weight are refused with an explicit error rather than given an invented value.

WHAT THE NUMBER IS NOT. It is a stoichiometric requirement at exact proportions, not a bench recipe. Charging exactly the calculated amount of a second reagent is unusual in practice: real preparations use an excess to drive an equilibrium, to compensate for a reagent that decomposes, or to make sure the expensive component is the one fully consumed. Published industrial data show second-reagent charges running from about 120 percent of stoichiometry up to 2650 percent. Treat the figure here as the floor you scale up from, and use the limiting reagent calculator to see what that excess leaves behind.

AND WHAT IT CANNOT CHECK. The tool takes your coefficients entirely on trust; it has no way to know whether the equation is balanced. If the coefficients are wrong, every number below is wrong in a way that looks completely plausible. Balance on paper first. Rounding happens once, at the end, to ten decimal places — molar masses, moles and the ratio are all carried at full precision through the middle of the calculation — and you should report to the significant figures your least precise input justifies.

What is stoichiometry ratio calculator?

The stoichiometric ratio, or mole ratio, between two species in a balanced chemical equation is the ratio of their coefficients. For C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O, the mole ratio of oxygen to propane is 5:1, of carbon dioxide to propane is 3:1, and of water to oxygen is 4:5. It is a ratio of AMOUNTS, in moles, never of masses — 5:1 in moles is 3.628:1 in grams for that pair, because oxygen and propane have different molar masses. The formal basis is the IUPAC extent of reaction ξ. Every species X in a reaction has a stoichiometric number ν(X), negative for reactants and positive for products, and any change in amount obeys Δn(X) = ν(X) × ξ. Divide that relation for two species and ξ cancels, leaving Δn(B)/Δn(A) = ν(B)/ν(A) — the mole ratio. That is why the ratio holds at every point in the reaction and not only at completion, and why it is independent of temperature, pressure, concentration and how far the reaction has gone. Converting a mole ratio into a mass ratio requires both molar masses: mass ratio = mole ratio × M(B)/M(A). A mass ratio is specific to one pair of species and one balanced equation, which is why it is worth computing once and reusing rather than deriving repeatedly.

How to use this calculator.

1. 01Balance your equation on paper first. Everything here depends on the coefficients being right, and the calculator cannot check them.
2. 02Decide which species you know and which you want. Set 'Solve for' accordingly — the mass field of the species you are solving for is ignored.
3. 03Enter both formulas in Hill/IUPAC notation, with the coefficient each one carries in the balanced equation.
4. 04Enter the mass you know, in grams, in that species' mass field.
5. 05Read the answer at the top, and check the mole ratio in the breakdown against the coefficients you meant to type.
6. 06Use the 'mass ratio' output as a shortcut for repeat work — multiply any mass of A by it and you have the mass of B directly.
7. 07If you are computing a reagent requirement, remember this is the exact-stoichiometry floor. Decide your excess separately, then use the limiting reagent calculator to see what is left over.
8. 08For a gas given as a volume rather than a mass, convert with the ideal gas law calculator first — that step, unlike this one, does need a temperature and pressure.

The formula.

For a balanced equation containing species A with coefficient ν_A and species B with coefficient ν_B:

STEP 1 — mass to moles. n(A) = m(A) / M(A). Dimensionally g ÷ (g/mol) = mol.

STEP 2 — the mole ratio. n(B) = n(A) × (ν_B / ν_A). This follows directly from the IUPAC extent of reaction: Δn(X) = ν(X) × ξ for every species, so dividing the relation for B by the relation for A cancels ξ and leaves Δn(B)/Δn(A) = ν_B/ν_A. Because ξ cancels, the ratio holds at every point during the reaction, not just at completion, and it does not depend on temperature, pressure or concentration.

STEP 3 — moles to mass. m(B) = n(B) × M(B), giving mol × g/mol = g.

Running backwards simply inverts the middle step: n(A) = n(B) / (ν_B/ν_A).

Collapsed, m(B) = m(A) × (ν_B/ν_A) × M(B)/M(A). That whole factor is reported as the MASS RATIO. For propane and oxygen it is 5 × 31.998 / 44.097 = 3.6281, so 50 g of propane needs 50 × 3.6281 = 181.41 g of oxygen. Note how different the mass ratio is from the mole ratio of 5 — this is exactly why mole ratios must never be applied to masses directly.

ROUNDING STAGE. No intermediate rounding. Molar masses, moles, the mole ratio and the mass ratio are all carried at arbitrary decimal precision; every returned value is rounded exactly once, at the return boundary, to ten decimal places. Report to the significant figures your least precise input justifies — 50 g entered as two significant figures makes 180 g the honest answer, not 181.4068984285 g.

INVALID DOMAIN. A coefficient of zero is a true singularity, because the mole ratio divides by ν_A, and it is refused along with negative coefficients. The mass of the species you are SOLVING FOR is ignored entirely, so leaving it at zero is correct and does not raise an error; the mass of the species you KNOW must be greater than zero. Unknown element symbols, unbalanced parentheses, stray characters and elements with no IUPAC standard atomic weight raise a field error naming the fault rather than returning a plausible-looking wrong molar mass. Non-integer coefficients are accepted, because half-equations such as C₂H₆ + 3.5 O₂ → 2 CO₂ + 3 H₂O are legitimate.

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. SECOND, INDEPENDENT AUTHORITY consulted for this page: 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 this calculator's independent test case — it must return 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.