Molar Ratio Calculator
The molar ratio between two reactants, which one runs out first, and how much product the reaction can make.
A balanced equation fixes the ratio in which substances react.
How the molar ratio calculator works
A balanced equation fixes the ratio in which substances react. Divide each reactant's moles by its coefficient: the smallest result is the limiting reagent, and it sets the yield.
Everything else is in excess, and the leftover is worth knowing — unreacted excess reagent has to be separated, and in process chemistry that is often the expensive part.
Formula: limiting = min(moles / coefficient)
Worked examples
| Inputs | Limiting reagent | Note |
|---|---|---|
| 2 mol A and 5 mol B, ratio 1:3 | B | B is limiting |
| Exactly stoichiometric | neither — the reagents are exactly stoichiometric | neither is limiting |
| A limiting instead | A | A runs out first |
FAQFrequently asked questions
What is the limiting reagent?
The reactant that runs out first and therefore caps the amount of product.
How do I find it?
Divide each reactant's moles by its coefficient in the balanced equation. The smallest quotient is the limiting one.
Why not just compare moles?
Because the coefficients matter. Five moles of a reagent used three at a time goes less far than two moles used one at a time.
What happens to the excess?
It stays unreacted and has to be separated from the product. In industry, choosing which reagent to put in excess is usually about which is cheaper to recover.
Does this account for yield?
No — this is the theoretical maximum. Actual yield is always lower, and the percent yield calculator handles that.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- Royal Society of Chemistry — the UK chemistry professional body
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.