Stoichiometry Calculator: Limiting Reagent, Theoretical Yield, Excess and Percent Yield

Find which reactant runs out first, how much product forms and how much you actually got, as a percent.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Balance the equation first. Enter the coefficients of reactants A and B and of the product.

Enter the molar mass and the mass you have of each reactant (the mole calculator gives the molar mass from a formula).

Divide the moles of each by its coefficient: the smaller number is the limiting reagent. The calculator gives the theoretical yield, the excess left over and the percent yield from the mass you collected.

Formulas used

  • n = \frac{m}{M}
  • \xi = \min\left(\frac{n_A}{a},\frac{n_B}{b}\right)
  • n_P = c\,\xi
  • \text{percent yield} = \frac{\text{actual}}{\text{theoretical}}\times100

Worked example

With the default numbers in the calculator above:

  1. Moles of each reactant you have: 1.98 mol of A and 0.50 mol of B
  2. Moles of each reactant divided by its coefficient (the number of times the reaction can run): 0.99 for A and 0.50 for B
  3. Limiting reagent: the one that gives the smaller number: Reactant B is the limiting reagent
  4. Amount of product that can be made: 1.00 mol
  5. Theoretical yield (the mass of product): 18.02 g
  6. Mass of the excess reactant A left over: 1.98 g of A
  7. Percent yield: 83.26 %

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

What is the limiting reagent?

The reactant that is used up first. It limits the amount of product, because the reaction stops when it runs out.

How do I find the limiting reagent?

Convert each mass to moles, divide each by its coefficient in the balanced equation and compare. The smaller result is the limiting reagent.

What is percent yield?

The mass of product you actually collected divided by the theoretical yield, times 100. It is below 100 because of side reactions and losses.

Can the yield be over 100 percent?

Not truly. A result above 100 means the product is wet or impure or a measurement is wrong.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Any first-year chemistry text (stoichiometry and limiting reagents).

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