Mole Ratio Calculator
Enter the stoichiometric coefficients of two species from a balanced equation and the moles of one — get the moles of the other from the mole ratio, with a live 3D view and charts.
Reviewed by the ToolNestr Editorial Team — July 2026
Enter the balanced-equation coefficients of species A and B, then the moles of one of them.
Two ideas that trip students up
1. The ratio fixes relative pile size
For the Haber process (N₂ + 3H₂ → 2NH₃), a fixed 1:3 mole-ratio pile of N₂ (indigo) vs H₂ (green) — three times as many H₂ spheres, always, no matter the actual amount.
2. Bigger amounts, same ratio
Three fixed points along the moles-B-vs-moles-A line, all sharing one slope (the ratio) — doubling moles of A always doubles moles of B, no matter which point you start from.
Mole ratio graphs
How it works
The core idea in one line: the coefficients of a balanced equation are a fixed exchange rate between moles of any two species — multiply the moles you know by that ratio to get the moles you don't.
molB = molA × (cB / cA)
mole ratio = ratio of balanced coefficients
cAA + cBB + … → products
coefficients c come from the balanced equation
Because coefficients are whole numbers fixed by conservation of mass, the mole ratio cB/cA is exact and constant — it does not depend on how much of A you actually have. Doubling moles of A doubles moles of B; the ratio itself never changes.
Worked example 1 — the Haber process
Given: N₂ + 3H₂ → 2NH₃. If 4.0 mol of H₂ reacts completely, how many moles of NH₃ form?
Worked example 2 — combustion of propane
Given: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. How many moles of O₂ are needed to react with 0.80 mol of C₃H₈?
Mole ratios in common reactions
The ratio is read straight off the balanced coefficients — no other data needed.
| Reaction | Species pair | Mole ratio |
|---|---|---|
| N₂ + 3H₂ → 2NH₃ | H₂ : NH₃ | 3 : 2 |
| 2H₂ + O₂ → 2H₂O | H₂ : H₂O | 2 : 2 (1:1) |
| CH₄ + 2O₂ → CO₂ + 2H₂O | O₂ : CO₂ | 2 : 1 |
| C₃H₈ + 5O₂ → 3CO₂ + 4H₂O | O₂ : C₃H₈ | 5 : 1 |
| 4Fe + 3O₂ → 2Fe₂O₃ | Fe : Fe₂O₃ | 4 : 2 (2:1) |
Mole ratios reduce like any fraction — 2:2 is the same ratio as 1:1.
Where mole ratios actually matter
🏭 Industrial reactor feed ratios
The Haber process feeds N₂ and H₂ in a 1:3 mole ratio because that is what the balanced equation requires — deviating wastes feedstock or leaves reactant unconverted.
⚗️ Lab reagent planning
Before weighing out reagents, chemists convert the target moles of product back through the mole ratio to know exactly how many moles (and then grams) of each reactant to use.
🚀 Propellant mixing
Combustion mole ratios (fuel to oxidizer) determine how much of each propellant component to load so the reaction goes to completion without excess unburned fuel or leftover oxidizer.
Common misconceptions
"Mole ratio and mass ratio are the same thing."
They are not. A mole ratio compares particle counts; converting to a mass ratio requires multiplying each side by its own molar mass. 3:2 moles of H₂:NH₃ is not 3:2 by mass, because H₂ (2 g/mol) and NH₃ (17 g/mol) have very different molar masses.
"You can read mole ratios off an unbalanced equation."
The coefficients must come from a correctly balanced equation. An unbalanced equation's numbers do not reflect a real conservation-of-mass relationship, so any ratio taken from it is wrong.
"The mole ratio tells you which reactant is limiting."
The mole ratio alone does not — you also need the actual moles of each reactant on hand. Comparing the available mole ratio to the required (stoichiometric) mole ratio is what identifies the limiting reagent.
"A 1:1 mole ratio means equal masses react."
Equal moles, not equal masses, unless the two substances happen to share the same molar mass. 1 mol of C (12 g) reacting 1:1 with 1 mol of O₂ (32 g) does not use equal masses.
Formula sources & further reading
The formulas here are standard, traceable to:
- • OpenStax, Chemistry 2e — Chapter 4.2, Reaction Stoichiometry and mole ratios (free, peer-reviewed). openstax.org
- • Brown, LeMay & Bursten, Chemistry: The Central Science — Chapter 3, Stoichiometry: Calculations with Chemical Formulas and Equations.
- • Zumdahl & Zumdahl, Chemistry — Chapter 3, section on stoichiometric calculations and mole ratios.
Mole ratio = ratio of balanced-equation coefficients. This tool solves one species pair (A, B) at a time; results are rounded for display.
How to use this calculator
Enter both coefficients
Type the balanced coefficient of species A and of species B.
Enter one mole value
Fill in moles of A or moles of B — the other solves automatically.
See the ratio visually
The 3D diagrams show a fixed mole-ratio pile comparison and the moles-B-vs-moles-A line.
Related tools
Frequently asked questions
What is a mole ratio?
A mole ratio is the ratio of the coefficients of two species in a balanced chemical equation. It tells you how many moles of one substance react with or produce a given number of moles of another — for N₂ + 3H₂ → 2NH₃, the mole ratio of H₂ to NH₃ is 3:2.
How do I calculate moles of B from moles of A?
Use moles_B = moles_A × (coefficient_B / coefficient_A). The coefficients come from the balanced equation; the ratio of coefficients is the conversion factor between the two species.
Why do I need a balanced equation first?
Mole ratios come directly from the coefficients of a balanced equation, and those coefficients are only meaningful once the equation obeys conservation of mass. An unbalanced equation gives the wrong ratio and therefore the wrong answer.
Is a mole ratio the same as a mass ratio?
No. A mole ratio compares particle counts (moles); a mass ratio also depends on each substance's molar mass. Two moles of H₂ and one mole of O₂ have a mole ratio of 2:1, but because O₂ is about 16× heavier per mole, the mass ratio is very different from 2:1.
Can I use this for more than two species?
This tool solves one pair at a time (A and B). For an equation with more species, apply the same coefficient-ratio method pairwise — moles of A to B, then B to C, chaining the ratios as needed.