Find the limiting reactant, limiting reagent, and excess reactant from a balanced chemical equation with step-by-step stoichiometry calculations. Enter an amount for every reactant, choose the product, and the calculator balances the equation before comparing mole ratios.
Use this calculator when a problem gives more than one reactant amount. For a single known substance and one target substance, use a standard stoichiometry conversion instead.
Step 1
Type an unbalanced or balanced equation such as N2 + H2 -> NH3. The calculator balances it automatically and uses the final coefficients.
Step 2
Enter an amount for every reactant in the equation. Use g, mg, kg, or mol; each mass is converted with that reactant's molar mass.
Step 3
Pick the product whose maximum amount you want to see. The limiting reactant still controls the result.
Step 4
The table shows moles, coefficients, moles ÷ coefficient, limiting status, excess remaining, and step-by-step work.
A limiting reactant, also called a limiting reagent, is the reactant that is consumed first and therefore limits the amount of product that can form.
A balanced equation is a recipe written in moles. If the recipe for ammonia is N₂ + 3H₂ → 2NH₃, then every complete reaction extent needs 1 mole of nitrogen and 3 moles of hydrogen. If hydrogen runs out before nitrogen, the reaction must stop even if some nitrogen remains in the flask.
That is why limiting-reactant stoichiometry compares availability, not just raw mass. A heavier sample is not automatically in excess, and a smaller mole count is not automatically limiting until coefficients are included. The limiting reactant sets the maximum product amount; excess reactants are left over after the limiting reactant is consumed.
To find a limiting reactant or how to find a limiting reagent by hand, reduce each reactant to the same comparison: available moles per balanced-equation coefficient.
Coefficients must conserve atoms before any ratio is meaningful. For N2 + H2 -> NH3, the balanced equation is N₂ + 3H₂ → 2NH₃.
If the amount is in grams, divide by that substance's molar mass. Do this separately for every reactant because N₂, H₂, and NH₃ have different molar masses.
This converts each starting amount into possible reaction extent: n ÷ coefficient.
The smallest ratio runs out first. If two ratios are equal, the reactants are present in exact stoichiometric proportion.
Product moles equal the smallest ratio times product coefficient. Excess consumed equals the same ratio times each excess-reactant coefficient.
The key is to compare possible reaction extent, not grams and not moles alone. In N₂ + 3H₂ → 2NH₃, one mole of N₂ can support one full reaction extent, but one mole of H₂ supports only one-third of a reaction extent because the coefficient on H₂ is 3. That is why the comparison must be moles ÷ coefficient for every reactant.
Once the smallest ratio is found, the rest of the limiting reactant calculation follows from the same number. That smallest ratio is the reaction extent available before something runs out. Multiply it by a product coefficient to get maximum product moles, or multiply it by an excess-reactant coefficient to see how much of that reactant is consumed. The leftover excess reactant is the starting amount minus the consumed amount.
Stoichiometry shortcut
You do not need to guess which reactant is limiting. Convert all supplied reactants to moles, divide by coefficients, and let the smallest number decide.
The limiting reactant formula is the same comparison for every reactant in the balanced equation.
Formula
Rᵢ = nᵢ / νᵢ
nᵢ = available moles of reactant i. νᵢ = coefficient of reactant i in the balanced equation. The smallest value of Rᵢ identifies the limiting reactant.
| Symbol | Meaning |
|---|---|
| Rᵢ | available reaction extent for reactant i |
| nᵢ | available moles of reactant i |
| νᵢ | stoichiometric coefficient from the balanced equation |
Most wrong answers come from skipping one of the comparison steps. Check these before trusting a limiting reagent answer.
A 20 g sample is not automatically more available than a 4 g sample. Different substances have different molar masses, so mass must be converted to moles first.
The reaction may require 3 mol of one reactant for every 1 mol of another. Divide moles by the balanced coefficient before choosing the limiting reactant.
Coefficients from an unbalanced skeleton equation do not conserve atoms. Balance first, then read mole ratios.
Find the limiting reactant first. Theoretical yield comes after the limiting reaction extent is known.
Limiting reactant and limiting reagent are usually synonyms in introductory chemistry.
The word reactant points to the species on the left side of the chemical equation. The word reagent is common in laboratory language, where chemicals are handled as reagents in bottles or solutions. A limiting reagent calculator and a limiting reactant calculator therefore solve the same classroom problem: which starting substance is used up first after the balanced-equation coefficients are considered?
Limiting and excess reactants describe opposite roles after the reaction has gone as far as the available amounts allow.
| Question | Limiting Reactant | Excess Reactant |
|---|---|---|
| Consumed completely? | Yes | No |
| Determines maximum product? | Yes | No |
| Remains after reaction? | No | Usually yes |
The excess-reactant result above uses the same limiting reaction extent. First it calculates how much of each non-limiting reactant must be consumed, then it subtracts that consumed amount from the starting amount.
Here is the full path for the ammonia example, including limiting reactant, excess reactant remaining, and theoretical yield.
N₂ + H₂ → NH₃
N₂ + 3H₂ → 2NH₃
Suppose the problem gives 28.0 g N₂ and 3.00 g H₂. First convert both reactants to moles: 28.0 g N₂ ÷ 28.014 g/mol ≈ 0.9995 mol N₂, and 3.00 g H₂ ÷ 2.016 g/mol ≈ 1.488 mol H₂.
Now divide by the coefficients in the balanced equation. N₂ has coefficient 1, so 0.9995 ÷ 1 = 0.9995. H₂ has coefficient 3, so 1.488 ÷ 3 = 0.496. The smaller value is 0.496, so H₂ is the limiting reactant and N₂ is the excess reactant.
The available reaction extent is 0.496. Because the coefficient of NH₃ is 2, product moles are 0.496 × 2 = 0.992 mol NH₃. Multiplying by the molar mass of NH₃ gives about 16.9 g NH₃ as the theoretical yield. N₂ consumed is 0.496 × 1 = 0.496 mol, leaving about 0.503 mol N₂, or about 14.1 g N₂.
These two limiting reagent examples match common homework setups and can be loaded into the calculator.
Example 1 — N₂ + H₂ → NH₃
N₂ + 3H₂ → 2NH₃
Example 2 — H₂ + O₂ → H₂O
2H₂ + O₂ → 2H₂O
Limiting-reactant work sits between basic equation balancing and yield calculations.
Use the chemical equation balancer when all you need is the smallest whole-number coefficients. Use the stoichiometry calculator when one known substance is given and the other reactants are assumed to be available. Use this limiting reactant calculator when two or more reactant amounts must be compared.
After the limiting reactant is known, the maximum product amount is the theoretical yield. That is shown in the result above; open the theoretical yield calculator when your main question is “how much product can form?” rather than “which reactant runs out first?”
Stoichiometry Calculator
Use one known substance to calculate another substance in the same balanced equation.
Chemical Equation Balancer
Balance equations before using coefficients and mole ratios.
Grams to Moles Calculator
Convert a single mass amount to moles before a stoichiometry problem.
Molar Mass Calculator
Find the molar masses used to convert grams into moles and leftovers back into grams.
Theoretical Yield Calculator
Focus on the maximum amount of product that can form.
Quick answers about limiting reactants, limiting reagents, excess reactants, and theoretical yield.
A limiting reactant, also called a limiting reagent, is the reactant that is consumed first in a chemical reaction. Because it runs out first, it limits how much product can form. Any other reactant with material left after the limiting reactant is used up is an excess reactant.
Balance the equation, convert each given reactant amount to moles, divide each mole amount by its balanced coefficient, and compare the results. The smallest moles ÷ coefficient value identifies the limiting reactant.
Use Ri = ni / νi for each reactant, where ni is the available moles of reactant i and νi is its coefficient in the balanced equation. The smallest Ri is the limiting reactant.
Yes. In introductory chemistry, limiting reactant and limiting reagent usually mean the same thing. Reagent is common in lab language; reactant is common when discussing the balanced chemical equation.
After finding the limiting reactant, use the limiting reaction extent to calculate how much of each other reactant is consumed: consumed moles = reaction extent × coefficient. Subtract consumed moles from starting moles, then convert leftover moles back to grams if needed.
No. You must account for the stoichiometric coefficients in the balanced equation. A reactant with more moles can still be limiting if the reaction requires many moles of it per reaction cycle.
The limiting reactant determines the maximum reaction extent. Multiply that extent by the product coefficient to get product moles, then multiply by product molar mass if you need theoretical yield in grams.