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Limiting Reactant Calculator

Type a chemical equation and how much of each reactant you have. The calculator balances the equation if needed, finds the limiting reactant, the theoretical yield of every product and how much of each excess reactant is left over.

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Use -> or = between sides. Unbalanced equations are balanced automatically.

One line per reactant: formula: amount unit (g, mg, kg, mol, mmol). Or just the numbers in equation order.

Used when a line has no unit

g

Gives the percent yield

Enter your values to see the result.
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How to use the Limiting Reactant Calculator

  1. Type the equation, e.g. N2 + 3H2 -> 2NH3.
  2. Enter an amount for each reactant, one per line, like N2: 28 g.
  3. Optionally add the actual yield of the first product.
  4. Read the limiting reactant, product yields and leftovers.
  5. Check the stoichiometry table or download it as CSV.

How to find the limiting reactant

  1. Balance the equation.
  2. Convert every reactant amount to moles: n = m ÷ M.
  3. Divide each by its coefficient: ξ = n ÷ coefficient.
  4. The smallest ξ is the limiting reactant; that ξ is the reaction extent.
  5. Product formed = ξ × product coefficient × product molar mass.
  6. Excess left = (n − ξ × coefficient) × molar mass.

Worked example: 2H₂ + O₂ → 2H₂O

SubstanceStartMoles÷ coefficientReacts / formsEnd
H₂ (M = 2.016)10 g4.9602.4804 mol = 8.064 g1.936 g left
O₂ (M = 31.998)64 g2.0002.000 (smallest: limiting)2 mol = 64 g0 g
H₂O (M = 18.015)0 g——4 mol formed72.06 g

Mass is conserved: 8.064 g H₂ + 64 g O₂ react to form 72.06 g H₂O. Notice that H₂ is limiting by mass intuition only if you ignore molar masses: 10 g is less than 64 g, yet hydrogen is in excess because its molar mass is so small.

Frequently asked questions

What is a limiting reactant?

The limiting reactant (or limiting reagent) is the reactant that is completely used up first. It stops the reaction and therefore decides how much product can form. The other reactants are in excess and some of them is left over.

How do I find the limiting reactant?

Convert each reactant's mass to moles (mass ÷ molar mass), then divide each by its coefficient in the balanced equation. The reactant with the smallest value is limiting. Comparing raw masses or raw moles without the coefficients gives wrong answers.

How is the theoretical yield calculated?

Take the limiting reactant's moles ÷ its coefficient, multiply by the product's coefficient to get moles of product, then multiply by the product's molar mass. For 10 g H₂ and 64 g O₂ in 2H₂ + O₂ → 2H₂O, O₂ is limiting (2 mol) and the yield is 4 mol = 72.06 g of water.

How much excess reactant is left?

Moles of excess reactant used = reaction extent × its coefficient. Subtract that from the starting moles and multiply by the molar mass. In the water example, 4 mol of H₂ react (8.06 g), leaving about 1.94 g of H₂.

Does the equation have to be balanced?

The math needs a balanced equation. If yours is not balanced, the calculator balances it automatically and tells you so. You can also use the Chemical Equation Balancer first.

Why is my actual yield lower than the theoretical yield?

Real reactions lose product to side reactions, incomplete reaction, purification and handling. Percent yield = actual ÷ theoretical × 100% measures how efficient the experiment was.

Are all the tools on FreeToolCabin tested for quality and accuracy?

Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.

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