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Molar Ratio Calculator

Chemistry

Calculate molar ratios between reactants and products in a balanced chemical equation. Find moles of product from moles of reactant using stoichiometric coefficients.

2 mol
mol
2
1

Moles of Substance B

1
Molar Ratio (A : B)
2
Molar Ratio (B : A)
0.5

This calculator computes your Moles of Substance B, Molar Ratio (A : B), Molar Ratio (B : A) from the values you enter.

Inputs
Moles of Substance AStoichiometric Coefficient of AStoichiometric Coefficient of B
Outputs
Moles of Substance BMolar Ratio (A : B)Molar Ratio (B : A)

What is a Molar Ratio?

The Molar Ratio Calculator computes how many moles of one substance (B) are produced, consumed, or present in proportion to a known quantity of another substance (A), using the stoichiometric coefficients from a balanced chemical equation. Molar ratios are the fundamental conversion factor in stoichiometry — every calculation involving reactants and products passes through a molar ratio step.

In the balanced equation for the Haber process (N₂ + 3 H₂ → 2 NH₃), the molar ratio of H₂ to N₂ is 3:1, of NH₃ to H₂ is 2:3, and of NH₃ to N₂ is 2:1. If 5 mol of N₂ is available, the molar ratio tells you that 10 mol of NH₃ can be formed — and that you need 15 mol of H₂ to fully react it. The molar ratio is the bridge between the moles of any substance in the equation and the moles of any other.

This calculator is designed to isolate the molar-ratio step from the larger stoichiometry calculation, letting you focus on one pair at a time. For the full pipeline from mass to theoretical yield, connect this tool with the Mole Calculator (mass → moles) and then the Theoretical Yield Calculator (moles × ratio × molar mass → grams).

How to use this Molar Ratio calculator

  1. Balance your chemical equation or obtain the coefficients from a balanced equation in your textbook or reaction database.
  2. Identify substance A (the substance whose quantity you know) and substance B (the substance whose quantity you want to find).
  3. Enter the known quantity of substance A in moles in the Moles of Substance A field. If you have grams, convert to moles using the Mole Calculator first.
  4. Enter the stoichiometric coefficient of A from the balanced equation in Stoichiometric Coefficient of A.
  5. Enter the stoichiometric coefficient of B from the balanced equation in Stoichiometric Coefficient of B.
  6. Read the Moles of Substance B — this is the moles of B that react with, or are produced from, the entered moles of A.
  7. To continue to a theoretical yield calculation, take the moles of B to the Theoretical Yield Calculator.

Formula & Methodology

Core formula:

Moles(B) = Moles(A) × (Coefficient of B ÷ Coefficient of A)

Ratio outputs:

Molar Ratio (A:B) = Coefficient(A) / Coefficient(B) Molar Ratio (B:A) = Coefficient(B) / Coefficient(A)

Worked example — combustion of propane (C₃H₈):

Balanced equation: C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O

(a) Moles of CO₂ from 2.4 mol of C₃H₈:

Coefficient of C₃H₈ = 1, Coefficient of CO₂ = 3 Moles(CO₂) = 2.4 × (3 / 1) = 7.2 mol CO₂

(b) Moles of O₂ needed to react with 2.4 mol of C₃H₈:

Coefficient of O₂ = 5 Moles(O₂) = 2.4 × (5 / 1) = 12.0 mol O₂

(c) Limiting reagent check (2.4 mol C₃H₈, 10.0 mol O₂ available):

Quotient for C₃H₈: 2.4 / 1 = 2.4 Quotient for O₂:   10.0 / 5 = 2.0  O₂ has the smaller quotient → O₂ is the limiting reagent. Moles of C₃H₈ that react = 10.0 × (1/5) = 2.0 mol Moles of CO₂ produced = 10.0 × (3/5) = 6.0 mol

The example shows that the molar ratio used depends on which substance is the limiting reagent — applying it in the wrong direction is the most common stoichiometry error.

Frequently Asked Questions

A molar ratio is the ratio of moles of one chemical species to moles of another, derived from the stoichiometric coefficients in a balanced chemical equation. In the reaction 2 H₂ + O₂ → 2 H₂O, the molar ratio of H₂ to O₂ is 2:1, meaning 2 moles of hydrogen react for every 1 mole of oxygen. Molar ratios are the fundamental tool of stoichiometry — they allow chemists to convert between the quantity of any substance in a reaction and the quantity of any other substance.
Use the molar ratio: Moles of Product = Moles of Reactant × (Coefficient of Product ÷ Coefficient of Reactant). Both coefficients are read directly from the balanced chemical equation. For example, in N₂ + 3 H₂ → 2 NH₃, the ratio of NH₃ to H₂ is 2:3. If 1.5 mol of H₂ is available, moles of NH₃ = 1.5 × (2/3) = 1.0 mol.
A molar ratio compares the moles of one substance to the moles of a specific other substance in the balanced equation — it is a ratio of two counts. A mole fraction compares the moles of one substance to the total moles of all substances in a mixture — it is a fraction between 0 and 1 and always sums to 1 across all components. Molar ratios come from stoichiometry (balanced equations); mole fractions describe composition (mixtures and solutions).
Convert all reactant quantities to moles, then divide each by its stoichiometric coefficient. The reactant with the smallest quotient is the limiting reagent. For example, in 2A + 3B → products with 4 mol A and 5 mol B: quotient for A = 4/2 = 2; quotient for B = 5/3 = 1.67. B has the smaller quotient, so B is the limiting reagent. You can then use the moles of B and the molar ratio to B in the product to calculate the theoretical yield.
The molar ratio (A:B) output shows how many moles of substance A correspond to one mole of substance B, derived from the stoichiometric coefficients you entered. A ratio of 2.0 means that for every one mole of B produced or reacted, two moles of A are consumed or produced. This ratio is the stoichiometric conversion factor — multiply any moles of A by (1 ÷ ratio A:B) to get the equivalent moles of B.
Balanced chemical equations always have integer (or simple fractional) coefficients, but the Molar Ratio Calculator accepts any positive number as the coefficient. Fractional coefficients appear when equations are written with a coefficient of 1 for one substance (e.g., ½ N₂ + ³⁄₂ H₂ → NH₃). The calculator handles decimal inputs for both coefficients. If you enter fractional coefficients, the ratio remains mathematically correct even though convention typically multiplies through to clear fractions.
Enter the known moles of substance A in the 'Moles of Substance A' field. Enter the stoichiometric coefficient of A from the balanced equation in 'Stoichiometric Coefficient of A'. Enter the stoichiometric coefficient of B in 'Stoichiometric Coefficient of B'. The calculator returns the moles of substance B, the molar ratio A:B (coeffA ÷ coeffB), and the ratio B:A (coeffB ÷ coeffA) for use in either direction.
The molar ratio is one of the three inputs needed to calculate theoretical yield: Theoretical Yield = Moles(limiting reagent) × Molar Ratio (product:reagent) × Molar Mass(product). The Molar Ratio Calculator solves the middle step — converting moles of limiting reagent to moles of product. The [Theoretical Yield Calculator](/theoretical-yield-calculator/) incorporates this ratio internally, but when you need to check the ratio step independently or use it across multiple products, this calculator is the right tool.
Yes — molar ratios and stoichiometry appear in Class 11 and 12 NCERT Chemistry (Chapter: Some Basic Concepts of Chemistry), and stoichiometry numericals based on molar ratios are a regular feature of JEE Main, JEE Advanced, and NEET examinations. Questions typically ask for the moles of one product given a starting mass of a reactant, requiring identification of the limiting reagent, conversion to moles using the [Mole Calculator](/mole-calculator/), application of the molar ratio, and conversion back to grams.
This calculator handles one pair of substances (A and B) at a time. For a reaction with multiple reactants and products, apply the molar ratio independently for each pair: use the limiting reagent and each product coefficient to calculate the moles of each product. Similarly, use the limiting reagent and each excess reagent's coefficient to calculate how much excess remains after reaction. Running the calculator multiple times — once per pair — covers any reaction.