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Molar Ratio Calculator

Chemistry

Calculate molar ratios between reactants and products in a balanced chemical equation, finding moles of product from moles of reactant instantly.

Reviewed by the thecalcu.com team · Last updated July 2, 2026

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Moles of Substance B

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Molar Ratio (A : B)
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Molar Ratio (B : A)
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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).

Why Use a Molar Ratio Calculator?

The molar ratio is straightforward to derive from a balanced equation, but errors in reading or applying it are extremely common, especially when students write the ratio inverted (A:B instead of B:A) or when the equation has coefficients greater than 1 on both sides. This calculator makes both ratios explicit: A:B and B:A, so the correct conversion factor for either direction is immediately visible.

For multi-product reactions, calculating moles of each product requires applying a separate molar ratio for each product. Applying each ratio in turn, with this calculator for clarity, avoids the compounding errors that come from doing multi-ratio calculations in one step.

In competitive exam practice (JEE, NEET), molar ratio questions often embed a limiting reagent identification step followed by a molar-ratio conversion. Using this calculator for the ratio step lets you focus on the conceptual steps rather than arithmetic.

Who Should Use This Calculator?

Class 11–12 chemistry students covering the stoichiometry chapter need molar ratios for limiting reagent identification and theoretical yield calculations. JEE Main and NEET stoichiometry problems regularly require these calculations under time pressure.

Undergraduate chemistry and chemical engineering students applying stoichiometry to industrial process calculations, combustion reactions, and synthesis planning need rapid, verified molar ratio conversions.

Process chemists and chemical engineers scaling reactions from lab to pilot to manufacturing use molar ratios to calculate raw material requirements for each reactant based on the target product quantity.

Lab technicians and analysts preparing reagent mixtures to defined stoichiometric ratios, such as mixing reagent A and B in the proportion needed for a complete reaction with minimal excess, use molar ratios to convert mass-based weighing quantities.

Teachers and tutors setting stoichiometry problems can use this calculator to verify correct molar ratio steps before presenting examples to students.

What Insights Does the Molar Ratio Calculator Give You?

Moles of Substance B is the primary output, the quantity of substance B that corresponds to the entered moles of A under the balanced equation's stoichiometry. This is the value you carry forward: into the Theoretical Yield Calculator to compute a product mass, or into a subsequent molar ratio step for a different substance pair.

Molar Ratio (A : B) shows how many moles of A correspond to one mole of B, the coefficient of A divided by the coefficient of B. This is the A-to-B conversion factor. If you enter moles of B instead of A, multiply by this ratio to recover moles of A.

Molar Ratio (B : A) shows how many moles of B correspond to one mole of A, the inverse. This is the B-to-A conversion factor and is the ratio you multiply moles of A by to get moles of B directly. Having both ratios side by side removes the need to decide which direction to apply the ratio.

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.
Show formula & methodology ↓Show less ↑

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

What is a molar ratio?
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.
How do I calculate the moles of product from the moles of a reactant?
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.
What is the difference between molar ratio and mole fraction?
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).
How do I use molar ratios to identify the limiting reagent?
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.
What does the molar ratio (A:B) output mean?
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.
Can I use molar ratios for non-integer coefficients?
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.
How do I use the Molar Ratio Calculator?
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.
How does the Molar Ratio Calculator connect to theoretical yield?
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.
Is the molar ratio relevant in Indian chemistry education?
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.
Can I use molar ratios for reactions with multiple reactants and products?
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.