Limiting Reagent Calculator

Background.

Heat 2.00 g of silicon with 1.50 g of nitrogen and the silicon is used up first. That surprises people, because there is more silicon by mass and more of it in moles — 0.0712 mol against 0.0535 mol. The balanced equation explains it: 3 Si + 2 N2 → Si3N4 consumes three silicon atoms for every two nitrogen molecules, so silicon has to be present at 1.5 times the moles of nitrogen just to keep pace. Here it is present at only 1.33 times. Silicon runs short, the reaction stops when it is gone, and 0.170 g of nitrogen is left over.

That is the whole idea of a limiting reagent: whichever reactant runs out first caps the amount of product, and anything charged beyond it is excess that has to be separated afterwards. The reliable test is not grams and not moles but moles divided by the reactant's coefficient. Chemists call that quotient the extent of reaction, ξ, measured in moles — the IUPAC quantity that tracks how far a reaction has gone. Each reactant on its own would let the reaction run to its own ξ; the smaller of the two is how far it actually runs, and every other amount follows from ξ by a single multiplication.

You supply two reactants and one product, each as a formula and a coefficient from the balanced equation, plus the grams of each reactant you actually weighed out. Back come the verdict, the moles charged, the mole ratio you used beside the ratio the equation needs, ξ, the theoretical yield of the product in grams, and the leftover of the excess reagent in moles and grams. Molar masses are worked out from the formulas using the CIAAW 2024 abridged standard atomic weights, and because everything is a mass balance, gases are entered by mass and no temperature or pressure is needed.

One thing no calculator can do is check that your equation is balanced. Coefficients are taken as typed. Enter 1, 1 and 1 for the silicon nitride reaction and the verdict flips to nitrogen, with a theoretical yield of 7.51 g of product from 3.50 g of starting material — a figure that breaks conservation of mass. Balancing on paper first is the only defence, and comparing the yield with the total mass charged is a quick way to catch a slip.

What is limiting reagent calculator?

The limiting reagent, or limiting reactant, is the reactant that is entirely consumed in a reaction and therefore sets the maximum amount of product that can form. Every other reactant is in excess: part of it is still present when the reaction stops. In IUPAC terms, each species in a balanced equation has a stoichiometric number ν — negative for reactants, positive for products — and every change in amount obeys Δn = ν × ξ, where ξ is the extent of reaction. Since no amount can fall below zero, the reaction can advance only until the first reactant reaches zero, at ξ = min(nA/a, nB/b), and that reactant is the limiting one. The theoretical yield of any product is calculated from this ξ, which is why identifying the limiting reagent is the first step in almost every quantitative stoichiometry problem.

How to use this calculator.

1. 01Balance the equation on paper and keep it in front of you; every coefficient you enter comes from it.
2. 02Type each formula in plain Hill notation with exact capitals — Co is cobalt, CO is carbon monoxide. Parentheses work, as in Al2(SO4)3. Write a hydrate as CuSO4(H2O)5 rather than with a dot, and enter diatomic gases as N2, O2, H2 or Cl2.
3. 03For each reactant, enter its coefficient and the mass in grams actually charged. Fractional coefficients are fine, so C2H6 + 3.5 O2 can be entered as written.
4. 04Enter the product you want a yield for and its own coefficient. If the reaction makes several products, choose the one you plan to isolate.
5. 05Read the verdict, then check it against the two ratios: if the A:B mole ratio you charged is below the ratio the equation needs, A is limiting; if it is above, B is.
6. 06Before trusting the yield, add the theoretical yield to the leftover excess mass. For a single-product reaction the sum should equal the total mass you charged.

The formula.

Write the balanced equation as aA + bB → pP. The first step converts each mass to moles, nA = mA ÷ M(A) and nB = mB ÷ M(B). The molar masses are summed atom by atom from the CIAAW Abridged Standard Atomic Weights 2024, the IUPAC Atomic Weights 2021 values with that year's revisions to gadolinium, lutetium and zirconium: silicon 28.085, nitrogen 14.007, oxygen 15.999 g/mol and so on. Lithium appears as 6.94 rather than the 6.941 printed in older textbooks, which moves lithium results in the fourth significant figure.

The second step divides by the coefficients. Reactant A alone would let the reaction run to ξ = nA/a and reactant B alone to nB/b. The actual extent is the smaller of the two, ξ = min(nA/a, nB/b), and the reactant that sets it is limiting. Comparing nA/a with nB/b is the same as comparing the ratio you charged, nA/nB, with the ratio required, a/b, which is why both ratios are shown.

The third step multiplies out. The product formed is ξ × p moles, so the theoretical yield is m(P) = ξ × p × M(P) grams. The excess reagent X has nX − ξ × νX moles remaining, where νX is its coefficient, and that leftover is converted back to grams with its molar mass. If the two candidate extents are exactly equal, the mixture is stoichiometric, the verdict reads neither, and the leftover is zero.

Scaling a balanced equation does not change the chemistry. Writing ethane combustion as 2 C2H6 + 7 O2 → 4 CO2 + 6 H2O instead of C2H6 + 3.5 O2 → 2 CO2 + 3 H2O halves ξ but leaves the limiting reagent, yield and leftover unchanged. The model assumes the reaction goes to completion by one route and that all the product is recovered, so the theoretical yield is the most you could ever isolate. Elements with no standard atomic weight, such as technetium, promethium, radon, radium and everything beyond uranium, are not accepted, since a natural-abundance molar mass does not exist for them.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Flowers, P., Theopold, K., Langley, R. & Robinson, W. R. (2019). Chemistry 2e, Section 4.4 'Reaction Yields', Example 4.12. OpenStax, Rice University. Peer-reviewed, CC BY 4.0. PRIMARY SOURCE for this page: defines the limiting reactant as the substance 'entirely consumed' and thus 'limiting the amount of product that may be generated', and works the exact problem reproduced here — 3 Si(s) + 2 N2(g) -> Si3N4(s) from 2.00 g Si and 1.50 g N2, with a provided Si:N2 mole ratio of 1.33 against a required 1.5, giving Si as limiting. Retrieved 2026-07-29. Open access.
2. [2]IUPAC. Compendium of Chemical Terminology (the Gold Book), entries 'stoichiometric number, ν' (S06025) and 'extent of reaction, ξ' (E02283). S06025 is sourced 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: stoichiometric numbers are positive for products and negative for reactants, and dξ = dn_B / ν_B. This agrees with the textbook 'divide moles by coefficient' rule and supplies the sign convention and the ξ = min(nA/a, nB/b) formulation used by this calculator. Retrieved 2026-07-29. Open access.
3. [3]IUPAC (2007). Quantities, Units and Symbols in Physical Chemistry ('the Green Book'), 3rd edition, section 2.12 'Chemical reactions' — the formal definitions of stoichiometric number, extent of reaction and amount of substance on which this page's ξ formulation rests. International Union of Pure and Applied Chemistry / RSC Publishing, 2007 (2nd printing 2008). Retrieved 2026-07-29. Bibliographic reference: a print and PDF edition rather than a live web page.
4. [4]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' (revision: 2024, incorporating the Gd, Lu and Zr revisions on the Atomic Weights 2021 base). This is the named, versioned source of every molar mass this calculator computes: Si 28.085, N 14.007, Cu 63.546, S 32.06, O 15.999, Zn 65.38. Retrieved 2026-07-29. Open access.
5. [5]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 tabulates the molar excess of the second reactant actually used across twenty-eight industrial reaction classes (from 120% for an N-alkylation to 2650% for an N-dealkylation), which is the evidence behind this page's statement that deliberate excess is normal industrial practice and that the leftover mass is a real separation cost. Retrieved 2026-07-29. Publisher page paywalled; an open copy was consulted.
6. [6]Sheldon, R. A. (2018). 'Metrics of Green Chemistry and Sustainability: Past, Present, and Future'. ACS Sustainable Chemistry & Engineering 6(1), 32–48, doi:10.1021/acssuschemeng.7b03505. Peer-reviewed. Source for this page's remark that atom economy 'assumes the use of exact stoichiometric quantities of starting materials', which is why an excess reagent degrades real material efficiency below the atom-economy figure. Retrieved 2026-07-29. Publisher version paywalled; the author's TU Delft repository copy is open.