What this calculator does
The mole ratio is the proportion in which two substances react or form in a chemical equation, read straight off the coefficients once the equation is balanced. In 2H2 + O2 to 2H2O, the mole ratio formula for hydrogen to oxygen is 2:1, meaning two moles of hydrogen react with every one mole of oxygen, regardless of how much of either substance you actually have.
The same ratio also applies to measured amounts once a reaction has been run: if you know how many moles of two substances reacted or formed, dividing one by the other recovers the same ratio the balanced equation predicts. This calculator handles both directions, either from the coefficients you already have or from moles you have measured.
The formula
Divide the value for substance A by the value for substance B, then reduce the fraction to the smallest whole-number ratio using their greatest common factor. The same arithmetic applies whether the two values are equation coefficients or measured mole amounts.
| Term | Meaning |
|---|---|
| Mole ratio | The proportion, in moles, of one substance to another in a reaction: value of A ÷ value of B. |
| Coefficient | The whole number placed in front of a substance in a balanced chemical equation. |
| Stoichiometry | The study of the quantitative relationships between reactants and products in a chemical reaction, of which the mole ratio is the basic building block. |
The inputs explained
| Field | What to enter |
|---|---|
| What you know | Choose whether you are working from a balanced equation's coefficients or from moles you have actually measured. |
| Substance A name | A short label for the first substance, used only to make the result easier to read. |
| Substance A: coefficient or moles | The coefficient of substance A from the balanced equation, or its measured amount in moles. |
| Substance B name | A short label for the second substance. |
| Substance B: coefficient or moles | The coefficient of substance B from the balanced equation, or its measured amount in moles. |
When to use it
Reading a mole ratio straight from a balanced equation
Once an equation is balanced, the mole ratio between any two substances in it is just their coefficients compared, without needing to know how much of either is actually present.
Working out how much of a second reactant is needed
Given the mole ratio and the amount of one reactant in hand, the ratio scales up directly to the amount of a second reactant needed to react completely with it.
Checking a lab result against theory
Comparing the mole ratio calculated from measured amounts of reactants used or products formed against the ratio the balanced equation predicts is a standard check for whether a reaction went as expected.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does the mole ratio scale with the amount of substance A?
A fixed value of 1 for substance B, against a range of values for substance A.
| Substance A value | Mole ratio, A : B | Ratio as a single multiplier (A per B) |
|---|---|---|
| 1.00 | 1.000 : 1.000 | 1.000 |
| 2.00 | 2.000 : 1.000 | 2.000 |
| 3.00 | 3.000 : 1.000 | 3.000 |
| 4.00 | 4.000 : 1.000 | 4.000 |
| 5.00 | 5.000 : 1.000 | 5.000 |
| 6.00 | 6.000 : 1.000 | 6.000 |
Questions
Do I need to balance the equation first?
Yes. Coefficients only give a valid mole ratio once the equation is balanced, since balancing is what makes the coefficients reflect the actual proportions in which atoms combine.
How is a mole ratio different from a mass ratio?
A mole ratio compares the number of particles (in moles) of each substance; a mass ratio compares their weights. Because different substances have different molar masses, the two ratios are usually not the same number, and converting between them needs the molar mass of each substance.
Can a mole ratio be a fraction rather than a whole number?
The simplified ratio between two substances in a properly balanced equation is always a ratio of whole numbers, but the single multiplier (one value divided by the other) can certainly come out as a decimal, such as 1.5 or 0.667.
What is the mole ratio used for in practice?
It is the step that converts a known amount of one substance into the matching amount of another in the same reaction, which is the basis of stoichiometry calculations such as finding a limiting reactant or a theoretical yield.
To convert a mass of a single substance into moles first, use the mole calculator. To identify what type of reaction is involved before working out its ratios, see the types of chemical reactions classifier.