Limiting Reactant Calculator
Identifying the limiting reactant is essential for calculating the theoretical yield of a reaction. The method is to divide the available moles of each reactant by its stoichiometric coefficient in the balanced equation. The reactant giving the smallest quotient is the limiting reactant. Enter the moles of reactant A and B, their stoichiometric coefficients, the product's coefficient, and the product's molar mass to find the limiting reactant and theoretical yield.
Limiting reactant method
ratio A = moles A / coeff A ratio B = moles B / coeff B Limiting = reactant with smaller ratio moles product = ratio(limiting) * coeff(product) theoretical yield = moles product * M(product)
The reactant with the smaller moles/coefficient ratio is the limiting reactant. Theoretical yield is calculated from the limiting reactant's moles and the product's mole ratio.
Worked example: water synthesis
- Reaction: 2 H(2) + O(2) produces 2 H(2)O. Coefficients: A(H2)=2, B(O2)=1, product(H2O)=2.
- Available: 3 mol H(2), 2 mol O(2).
- Ratio A = 3/2 = 1.50, Ratio B = 2/1 = 2.00.
- H(2) has smaller ratio, so H(2) is the limiting reactant.
- Moles H(2)O = 1.50 * 2 = 3.00 mol. Theoretical yield = 3.00 * 18.015 = 54.05 g.
Frequently asked questions
What is a limiting reactant?
The limiting reactant (limiting reagent) is the reactant that is completely consumed first in a chemical reaction, limiting the amount of product that can form. The other reactant is in excess.
How do I find the limiting reactant?
Divide the moles of each reactant by its stoichiometric coefficient from the balanced equation. The reactant with the smaller result is the limiting reactant.
What is the excess reactant?
The excess reactant is the reactant that remains after the limiting reactant is fully consumed. The amount of excess = initial moles - moles consumed by the limiting reactant.
How does the limiting reactant affect theoretical yield?
Theoretical yield is calculated from the limiting reactant only. Multiply the moles of limiting reactant by the mole ratio (product/limiting) and then by the product's molar mass to get theoretical yield in grams.
Can I have more than one limiting reactant?
Technically no. One reactant is consumed first. If both run out simultaneously (the ratio test gives equal values), they are both fully consumed and neither is 'limiting' in the traditional sense, but both determine the yield equally.
Official sources
- IUPAC Gold Book: Limiting reactant definition.
- NIST: NIST Chemistry WebBook.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.