Molar Ratio Calculator
Enter the coefficients from a balanced chemical equation to get the molar ratio between any two substances. Switch to moles mode to convert a known quantity of one substance into moles of another, or to mass mode to work in grams using each substance's molar mass. The full calculation is shown step by step so you can follow the stoichiometry at every stage.
What is a molar ratio?
A molar ratio is the proportion between the amounts (in moles) of any two substances involved in a chemical reaction. It is read directly from the coefficients of a balanced chemical equation. For example, in the reaction N2 + 3H2 -> 2NH3, the molar ratio of H2 to NH3 is 3 : 2, meaning three moles of hydrogen are needed to produce two moles of ammonia. Molar ratios are the core tool of stoichiometry: they act as conversion factors that let you calculate how much of any other substance you need or produce when a known amount of one substance reacts.
How to find and use a molar ratio
Finding a molar ratio requires three steps. First, write and balance the chemical equation so that atoms are conserved on both sides. Second, read the stoichiometric coefficients for the two substances you are comparing (these are the numbers in front of each formula). Third, write the ratio as coefficient B divided by coefficient A - this is your conversion factor. To convert moles of A to moles of B, multiply moles of A by (coefficient B / coefficient A). To convert grams of A to grams of B, divide the mass of A by its molar mass to get moles of A, apply the molar ratio to get moles of B, then multiply by the molar mass of B to get grams of B.
Worked example: Haber-Bosch process
The balanced equation for ammonia synthesis is: N2(g) + 3H2(g) -> 2NH3(g). Suppose you start with 5.83 moles of hydrogen and want to know how many moles of ammonia are produced (assuming excess nitrogen). The molar ratio of H2 to NH3 is 3 : 2, giving a conversion factor of 2/3. Multiply: 5.83 mol H2 x (2 mol NH3 / 3 mol H2) = 3.89 mol NH3. If you have 10 g of H2 instead, first find moles: 10 g / 2.016 g/mol = 4.96 mol H2. Then apply the ratio: 4.96 x (2/3) = 3.31 mol NH3, which corresponds to 3.31 x 17.031 = 56.4 g of ammonia.
Molar ratio vs. mass ratio
Molar ratios reflect the number of particles (atoms, molecules) involved, while mass ratios reflect the actual weights of those particles. These are related but different. In 2H2 + O2 -> 2H2O, the molar ratio of H2 to O2 is 2 : 1, but the mass ratio is (2 x 2.016) : (1 x 32.00) = 4.032 : 32.00, roughly 1 : 7.94 by mass. Mass-based conversions require the extra step of multiplying by each substance's molar mass. When working in the laboratory you almost always measure mass, so the mass-to-mass pathway (enter grams of A, get grams of B) is the most practically useful form of the calculation.
Common molar ratios in well-known reactions
| Reaction | Substances | Molar ratio (A : B) | Notes |
|---|---|---|---|
| 2H2 + O2 -> 2H2O | H2 to O2 | 2 : 1 | Hydrogen combustion |
| 2H2 + O2 -> 2H2O | H2 to H2O | 1 : 1 | Equal moles produced |
| N2 + 3H2 -> 2NH3 | H2 to NH3 | 3 : 2 | Haber-Bosch process |
| N2 + 3H2 -> 2NH3 | N2 to NH3 | 1 : 2 | Nitrogen to ammonia |
| CH4 + 2O2 -> CO2 + 2H2O | CH4 to O2 | 1 : 2 | Methane combustion |
| CH4 + 2O2 -> CO2 + 2H2O | CH4 to H2O | 1 : 2 | Methane combustion |
| 2NaCl + H2SO4 -> Na2SO4 + 2HCl | NaCl to HCl | 1 : 1 | Salt acid reaction |
| C3H8 + 5O2 -> 3CO2 + 4H2O | C3H8 to O2 | 1 : 5 | Propane combustion |
Stoichiometric coefficients from standard balanced equations. Ratios depend on the balanced form of the equation.
Frequently asked questions
What is a molar ratio in chemistry?
A molar ratio is the ratio of the stoichiometric coefficients of two substances in a balanced chemical equation. It tells you how many moles of one substance are consumed or produced for every mole of another. For example, in 2H2 + O2 -> 2H2O, the molar ratio of H2 to O2 is 2 : 1 - for every two moles of hydrogen that react, exactly one mole of oxygen is consumed.
How do I calculate the molar ratio between two substances?
Balance the chemical equation first. Then identify the stoichiometric coefficient of each substance you want to compare. Write the ratio as (coefficient of B) / (coefficient of A) to get the conversion factor. Simplify by dividing both numbers by their greatest common divisor if they are whole numbers. This calculator does all of that automatically once you enter the two coefficients.
Can I use molar ratios with grams instead of moles?
Yes, but you need each substance's molar mass as an extra step. First convert grams of A to moles of A by dividing by its molar mass (g/mol). Then multiply by the molar ratio (coeffB / coeffA) to get moles of B. Finally multiply by the molar mass of B to get grams of B. This three-step pathway (grams A -> moles A -> moles B -> grams B) is called the mass-to-mass or gram-to-gram conversion, and it is the most common form of stoichiometric calculation in a laboratory setting.
Does a molar ratio have to be a whole number?
Molar ratios derived from a balanced equation always have integer coefficients, so the ratio itself is always a ratio of whole numbers (like 3 : 2 or 1 : 4). However, the conversion factor (coeffB / coeffA) is often a decimal - for a 3 : 2 ratio the factor is 0.6667. When you multiply a real-world mole quantity such as 5.83 mol by this factor, the result (3.89 mol) is not a whole number. That is normal and correct.
What is the difference between molar ratio and mole fraction?
A molar ratio compares two substances directly (e.g. 3 mol H2 per 1 mol N2) and is derived from stoichiometric coefficients. A mole fraction is the ratio of one substance's moles to the total moles of all substances in a mixture, so it always falls between 0 and 1 and the mole fractions of all components sum to exactly 1. Molar ratios are used in reaction stoichiometry to predict yields; mole fractions are used in thermodynamics and mixture calculations.
How does this calculator help find the limiting reagent?
The limiting reagent is the reactant that runs out first. To find it, convert all reactant quantities to moles and divide each by its stoichiometric coefficient. The reactant with the smallest quotient is the limiting reagent. Once you know the limiting reagent's moles, use the molar ratio conversion in this calculator to find the theoretical yield of any product or the amount of other reactants consumed.