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Limiting Reactant Calculator | Find limiting reactant by amounts

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Identify which reactant runs out first in any chemical reaction, how much excess remains, and the theoretical yield it produces. Enter the masses and balanced-equation coefficients and the tool walks through the mole comparisons — a must-have for stoichiometry.

Last updated: June 2026 · Free · No sign-up required

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Quick reference

ReactantMoles ÷ coefficient
Amoles of A ÷ coefficient of A
Bmoles of B ÷ coefficient of B
Limitingthe reactant with the smaller ratio
Excessthe reactant with the larger ratio

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Interpretation guide

OutputWhat it tells you
Limiting reactantRuns out first and caps the product yield.
Excess reactantSome is left over when the reaction finishes.
RatiosThe mole-to-coefficient ratio for each reactant.

lightbulb Worked example — water from 10.0 g of hydrogen and 96.0 g of oxygen

The reaction is 2H₂ + O₂ → 2H₂O. A lab bench has 10.0 g of hydrogen gas (molar mass 2.016 g/mol) and 96.0 g of oxygen gas (molar mass 31.998 g/mol).

Result: Moles of H₂ = 10.0 ÷ 2.016 = 4.96 mol; moles of O₂ = 96.0 ÷ 31.998 = 3.00 mol. Mole-to-coefficient ratios: H₂ 4.96 ÷ 2 = 2.48, O₂ 3.00 ÷ 1 = 3.00. Hydrogen is limiting. Theoretical yield of water = 4.96 mol × 18.015 g/mol = 89.4 g. Oxygen consumed = 2.48 mol × 31.998 g/mol = 79.4 g, leaving 96.0 − 79.4 = 16.6 g in excess.

What this means: Even though oxygen’s mass is nearly ten times larger, hydrogen runs out first because the equation demands two hydrogens per oxygen. The 16.6 g of leftover oxygen is the excess reactant — it simply remains unreacted after the hydrogen is gone.

Formula & methodology

Formula: Compare moles ÷ coefficient for each reactant; the smaller ratio is limiting

How the limiting reactant is found

The method compares each reactant’s mole-to-coefficient ratio: divide the moles you have by the coefficient in the balanced equation, and the smallest ratio identifies the reactant that runs out first.

  • Balance the equation and note each reactant’s coefficient.
  • Convert every given mass to moles (moles = mass ÷ molar mass).
  • Divide each mole value by its coefficient.
  • The smallest ratio is the limiting reactant; the rest are in excess.

Theoretical yield: convert the limiting reactant’s moles to product moles through the equation’s mole ratio, then to grams. Excess remaining: subtract the moles consumed alongside the limiting reactant from what was initially available.

Authoritative source: Wolfram MathWorld

Frequently asked questions

How do I find the limiting reactant?
Enter the required values into the input fields and press the calculate button. The tool applies the formula Compare moles ÷ coefficient for each reactant; the smaller ratio is limiting and returns the result with a short explanation of each step, so you can check the working yourself.
Why do I need the coefficients?
The Limiting Reactant Calculator is free, private, and accurate: it runs entirely in your browser (no uploads, no accounts), applies the standard calculation, and explains each step so you can verify the result. Limiting Reactant Calculator is a free online calculator that helps you work out limiting reactant quickly and accurately, right in your browser. There is no limit on usage, and it works on any device.
What is the excess reactant?
The excess reactant is the reagent left over after the reaction completes, because more of it was supplied than the balanced equation requires. Its remaining amount is the initial quantity minus what the limiting reactant consumed. The calculator shows which reactant is in excess and how much remains, which matters for cost control and for isolating products without unreacted material.
Does the limiting reactant set the yield?
Yes — the limiting reactant determines the theoretical yield, because the reaction stops when it is fully consumed, regardless of how much excess remains. Doubling the limiting reactant (up to the excess available) roughly doubles the product; adding more excess reactant changes nothing. This is why identifying the limiter is the first step in every yield calculation.
Is it free and private to use?
Yes — the Limiting Reactant Calculator. There is no sign-up, no paywall, no trial, and no limit on how many calculations you can run. CalcaTools covers its costs with non-intrusive display advertising, so the calculator itself never asks for payment or restricts any feature.
Can the reactant with the smallest mass still be in excess?
Yes — mass alone never decides it. A 10 g sample of a heavy molecule is far fewer moles than 10 g of a light one, and the balanced-equation coefficients shift the comparison further. That is why the calculation converts masses to moles and divides by the coefficients before comparing.
Why does the formula divide moles by the coefficient instead of comparing moles directly?
Because the coefficients say how much of each reactant the reaction consumes together. In 2H₂ + O₂ → 2H₂O, every oxygen consumed costs two hydrogens, so comparing raw moles would ignore that two-for-one demand. Dividing by the coefficient converts each amount into “how many reaction’s worth” you have, which is the fair comparison.
How do I get the theoretical yield from the limiting reactant?
Convert the limiting reactant’s moles to product moles using the balanced equation’s mole ratio, then multiply by the product’s molar mass. For 4.96 mol of limiting H₂ in 2H₂ + O₂ → 2H₂O, that is 4.96 mol of H₂O × 18.015 g/mol = 89.4 g — the most water that reaction could possibly produce.
What does a percent yield over 100% tell me?
It flags a measurement or weighing error, because you cannot make more product than the limiting reactant allows. The usual culprits are an impure reactant weighed as if it were pure, product contaminated with water or unreacted material, or an arithmetic slip in the mole conversions.

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