This limiting reagent calculator tells you which of two reactants runs out first in a chemical reaction and how much of the other reactant is left over. In stoichiometry the limiting reagent is the substance that is fully consumed and therefore caps how much product can form, while the excess reagent is whatever remains once the reaction stops. The method is straightforward. For each reactant you divide its number of moles by its balanced equation coefficient to get a mole ratio, and the reactant with the smaller ratio is the limiting reagent. You enter the moles and the balanced coefficient for reactant A and reactant B, and the tool reports which one is limiting and how many moles of the other reactant remain unreacted. Chemistry students, lab analysts and teachers use this every time they plan a synthesis, predict a yield or work out how much of a costly reagent to buy. Two habits keep your answers correct. First, always start from a balanced chemical equation, because the coefficients are what make the comparison meaningful, and an unbalanced equation will mislead you. Second, make sure every quantity is in moles before you compare, converting from grams using molar mass if needed, since comparing grams directly gives the wrong answer. The excess is found by using the limiting reagent's mole ratio to work out how much of the other reactant is actually used, then subtracting that from the amount you started with. Knowing the limiting reagent lets you calculate the theoretical yield and avoids wasting expensive chemicals on reactants that will simply sit unreacted at the end.
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Reactant B
Limiting reagent
Excess remaining0.667 mol of A
Compare moles / coefficient. Smaller ratio is limiting. Start from a balanced equation.
How it works
Each reactant's moles are divided by its coefficient to give a ratio. Reactant A gives 4 divided by 2, which is 2.000, and reactant B gives 5 divided by 3, which is 1.667. The smaller ratio belongs to reactant B, so B is limiting. The moles of A used equal B's ratio times A's coefficient, and the rest is excess.
Worked example
With 4 moles of A at coefficient 2 and 5 moles of B at coefficient 3, the ratios are 2.000 and 1.667, so B is the limiting reagent. The reaction uses 1.667 times 2, which is 3.333 moles of A, leaving 4 minus 3.333, which is 0.667 moles of A in excess.
