Limiting Reagent Calculator
When two reactants combine, one usually runs out before the other. That reactant is the limiting reagent, and it sets the ceiling on how much product can form. To find it you compare the available moles of each reactant against the ratio demanded by the balanced chemical equation. This calculator takes the moles and stoichiometric coefficient of two reactants, divides moles by coefficient for each, and reports which one limits the reaction along with how much of the excess reactant is left over once the reaction completes. Enter your balanced-equation coefficients and the moles you have.
Limiting reagent formula
ratio_A = moles_A / coefficient_A
ratio_B = moles_B / coefficient_B
Limiting reagent = the reactant with the smaller ratio
Excess used = limiting ratio * excess coefficient
Excess left = excess moles - excess used
All coefficients and moles must be greater than zero. Equal ratios mean both reactants are consumed exactly, with no excess left over.
Limiting reagent context
- The limiting reagent caps the theoretical yield of every product in the reaction.
- Comparing moles directly is wrong unless coefficients are equal; always divide by the coefficient.
- The excess reactant remains after the reaction and may need separation or recovery.
- Industrial processes often run a cheap reactant in excess to drive a costly one to completion.
- Use a balanced equation; an unbalanced equation gives the wrong coefficient ratio.
Limiting reagent: frequently asked questions
What is a limiting reagent?
The limiting reagent is the reactant that is fully consumed first in a chemical reaction, stopping the reaction and capping the amount of product. The other reactant is left in excess. Identifying it tells you the maximum yield possible from the amounts you have.
How do I find the limiting reagent?
Divide the moles of each reactant by its stoichiometric coefficient from the balanced equation. The reactant with the smallest ratio is the limiting reagent, because it runs out first relative to what the equation requires.
What is the difference between limiting and excess reagent?
The limiting reagent is consumed completely and determines the maximum product. The excess reagent has more than enough to react with all the limiting reagent, so some of it remains unreacted after the reaction finishes.
Why do I need the balanced equation coefficients?
Reactants combine in fixed mole ratios set by the balanced equation, not one to one. Dividing moles by the coefficient normalizes each reactant to the same basis so they can be compared fairly to find which one limits the reaction.
How much excess reactant is left over?
This calculator computes the moles of the excess reactant consumed using the limiting reagent and the coefficient ratio, then subtracts that from the amount you started with to report the leftover excess.
Official sources
- NIST Chemistry WebBook: reaction and thermochemical reference data.
- IUPAC Gold Book: stoichiometry and amount of substance terminology.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.