Molar Ratio Calculator
Enter two balanced-equation coefficients and the known moles to find the moles (and optional mass) of the other substance, plus the simplified ratio.
⚗️ What is a Molar Ratio?
A molar ratio (also called a mole ratio) is the proportion between the coefficients of two substances in a balanced chemical equation. It is the conversion factor chemists use to move from an amount of one substance, expressed in moles, to the corresponding amount of any other substance appearing in the same reaction. This calculator takes the coefficients of a known and an unknown substance plus the moles you already have, and returns the moles of the unknown, its simplified coefficient ratio, and, if you supply a molar mass, its mass in grams.
Mole ratios show up constantly in real laboratory and classroom work: figuring out how many moles of oxygen are needed to completely burn a known amount of fuel, predicting how much product a reaction will yield from a measured amount of one reactant, scaling a recipe-style synthesis up or down while keeping the same proportions, and checking whether an experimental yield is consistent with the theoretical stoichiometry of the reaction.
A common point of confusion is treating the mole ratio as if it depends on how much material you happen to be using. It does not. The ratio is fixed entirely by the coefficients in the balanced equation, two moles of hydrogen and one mole of oxygen combine in a 2:1 ratio whether you are working with a few millimoles in a test tube or many kilomoles in an industrial reactor. What changes with the amount of starting material is only the final calculated quantity of product, never the underlying ratio itself.
This calculator is useful precisely because it isolates that one conversion step, moles of A to moles of B, cleanly. Rather than working through an entire multi-reactant stoichiometry problem, you can quickly answer "if I have this many moles of one substance, how many moles of this other substance does the balanced equation imply?" and, optionally, convert that answer straight into grams.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Ammonia Synthesis
N₂ + 3H₂ → 2NH₃. You have 4.0 mol of N₂, coefficient of N₂ = 1, coefficient of NH₃ = 2, molar mass of NH₃ = 17.03 g/mol.
Example 2 — Iron Rusting (Simplified Ratio)
4Fe + 3O₂ → 2Fe₂O₃. You have 10 mol of Fe, coefficient of Fe = 4, coefficient of Fe₂O₃ = 2, molar mass of Fe₂O₃ = 159.69 g/mol.
Example 3 — Propane Combustion (Moles Only)
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. You have 2.5 mol of O₂, coefficient of O₂ = 5, coefficient of CO₂ = 3, no molar mass supplied.
❓ Frequently Asked Questions
🔗 Related Calculators
What is a mole ratio in chemistry?
A mole ratio is the ratio between the coefficients of two substances in a balanced chemical equation. It tells you exactly how many moles of one substance react with, or are produced from, a given number of moles of another substance, regardless of how much of either you actually have.
How do you calculate mole ratio from a balanced equation?
Divide the coefficient of the substance you want (the unknown) by the coefficient of the substance you already know the moles of. For 2H2 + O2, the mole ratio of O2 to H2 is 1/2, meaning every 2 moles of H2 needs 1 mole of O2.
Why do I need to balance the equation before finding the mole ratio?
The coefficients in a balanced equation reflect the fixed whole-number proportions in which atoms actually combine, according to the law of conservation of mass. An unbalanced equation has coefficients that do not reflect real reacting proportions, so any mole ratio taken from it is meaningless.
Can the mole ratio ever simplify to a non-whole number?
The ratio between two coefficients (like 3:2) is often left as a fraction (1.5) when used in a calculation, but the underlying coefficients themselves are always whole numbers in a properly balanced equation. This calculator both gives you the decimal ratio for arithmetic and the simplified whole-number form, like 3:2, for reference.
How do I convert moles of one substance to grams of another?
First use the mole ratio to convert moles of the known substance into moles of the unknown substance, then multiply that answer by the unknown substance's molar mass in g/mol. This calculator does both steps at once when you fill in the molar mass field.
What is the difference between this calculator and a full stoichiometry calculator?
This calculator isolates just the mole ratio conversion step, moles of A to moles of B, using only two coefficients. A full stoichiometry calculator additionally parses the entire balanced equation, converts between grams and moles using molecular weights, and identifies the limiting reagent among multiple reactants.
Does the mole ratio change if I have more or less of the starting substance?
No. The mole ratio is a fixed property of the balanced equation itself, set entirely by the coefficients. Having 2 moles or 200 moles of the known substance does not change the ratio, it only changes the final amount of the unknown substance you calculate.
What if my two coefficients are already in simplest form, like 1 and 3?
The calculator still shows the simplified ratio, it will just be identical to what you entered, for example 1:3 stays 1:3, since dividing both numbers by their greatest common factor of 1 changes nothing.
Can I use this for reactants and products, or only reactants?
Either. The mole ratio works between any two species in the same balanced equation, reactant to reactant, reactant to product, or product to product. Simply enter the two relevant coefficients from the balanced equation in the order known substance, then unknown substance.
Why is my calculated mass different from what I expected?
Double check that the molar mass you entered matches the unknown substance, not the known one, and that the coefficients were taken from a correctly balanced equation. A common mistake is swapping which coefficient belongs to the known substance versus the unknown substance.