# Limiting Reagent Calculator

> Find which reactant runs out first in aA + bB → cP, the theoretical yield of product in grams and how much of the excess reactant is left over.

Category: Science & engineering · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/limiting-reagent-calculator

Divide each reactant’s moles by its coefficient in the balanced equation. The smaller result is the limiting reagent, and it sets how much product can form.

Also called: Limiting reactant calculator, Stoichiometry calculator, Theoretical yield calculator.

## Inputs
- Reactant A name (default H2)
- Mass of A (unit g; default 10)
- Molar mass of A (unit g/mol; default 2.016)
- Coefficient of A (a) (default 2)
- Reactant B name (default O2)
- Mass of B (unit g; default 64)
- Molar mass of B (unit g/mol; default 31.998)
- Coefficient of B (b) (default 1)
- Coefficient of the product (c) (default 2)
- Molar mass of the product (unit g/mol; default 18.015)

## Results
- Limiting reagent
- Theoretical yield of product
- Product formed
- Excess reactant left over
- Moles of A
- Moles of B

## Formula

```
n = m ÷ M;  limiting = smaller of n_A ÷ a and n_B ÷ b;  product = c × min(n_A/a, n_B/b);  yield = product × M_P
```

Where:
- a, b, c: coefficients in the balanced equation aA + bB → cP
- n_A, n_B: moles of each reactant
- M_P: molar mass of the product

## Worked example
1. n(H₂) = 10 ÷ 2.016 = 4.9603 mol → ÷ 2 = 2.4802; n(O₂) = 64 ÷ 31.998 = 2.0001 mol → ÷ 1 = 2.0001.
2. O₂ gives the smaller ratio, so it is limiting: water = 2 × 2.000125 = 4.00025 mol × 18.015 = 72.06 g.
3. H₂ used = 4.00025 mol × 2.016 = 8.0645 g, so 10 − 8.0645 = 1.94 g of H₂ is left.

**Result:** In 2H₂ + O₂ → 2H₂O with 10 g H₂ and 64 g O₂, oxygen is limiting; at most 72.06 g of water forms and 1.94 g of hydrogen is left over.

## What the result means

Actual yield is usually lower than theoretical; percentage yield = actual ÷ theoretical × 100.

## Assumptions and limits
- The equation is balanced and the reaction goes to completion.
- Reactants are pure.
- Two reactants and one product of interest; for more reactants, compare every n ÷ coefficient ratio the same way.

## Questions

### Is the reactant with the smaller mass always limiting?

No. Compare moles divided by coefficients, not grams. Here 10 g of hydrogen is in excess over 64 g of oxygen.

### Can I use fractional coefficients?

Yes. Only the ratios between coefficients matter.

## Related calculators
- [Mole to Mass Calculator](https://calculators.geekmidst.com/calculators/mole-to-mass-calculator)
- [Molecular Weight Calculator](https://calculators.geekmidst.com/calculators/molecular-weight-calculator)
- [Molarity Calculator](https://calculators.geekmidst.com/calculators/molarity-calculator)

Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/limiting-reagent-calculator