Molar Ratio Calculator (mole ratio from coefficients, moles and grams of product)

Understanding the molar ratio calculator — concept and principle

This calculator uses the coefficients of a balanced chemical equation to convert an amount of one substance into the amount of another, in moles and in grams. It also shows the mole ratio in simplest form. It is a core tool for stoichiometry homework and for planning lab quantities.

Convert grams to moles, scale by the coefficients 2 : 1, then convert back to grams.

How it is calculated

For a A → b B: n_B = n_A × b ÷ a

If A is given as a mass: n_A = m_A ÷ M_A

Mass of B = n_B × M_B

Coefficients come from the balanced equation; molar masses in g/mol.

The principle

A balanced equation counts particles. Its coefficients say how many molecules or formula units react with or form each other, and since a mole is a fixed number of particles, the same ratio holds for moles. Masses do not follow the coefficients directly, because each substance has a different molar mass, so you always convert mass to moles first, apply the ratio, and convert back.

Worked example

Default inputs: N₂ + 3H₂ → 2NH₃, A = N₂ (coefficient 1), B = NH₃ (coefficient 2), 14.0 g N₂, M(N₂) = 28.014, M(NH₃) = 17.031.

n(N₂) = 14.0 ÷ 28.014 = 0.4998 mol.
n(NH₃) = 0.4998 × 2 ÷ 1 = 0.9995 mol.
Mass of NH₃ = 0.9995 × 17.031 = 17.022 g.
Mole ratio B ÷ A = 2, written 2 : 1.

Each point is the calculator re-run with only “Coefficient of the substance you know (A)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Coefficient of the substance you know (A)	Moles of B (mol)
0.25	3.998
0.5	1.999
0.75	1.3327
1	0.9995
1.25	0.7996
1.5	0.6663
2	0.4998

How to read the result

The answer is the theoretical amount, assuming the other reactants are in excess and the reaction goes to completion. Real yields are lower. If you have amounts for more than one reactant, find the limiting reagent first, because it decides how much product can form.

Common mistakes

Using an unbalanced equation.
Applying the coefficient ratio to grams instead of moles.
Ignoring the limiting reagent.
Using atomic mass of N (14.007) instead of molar mass of N₂ (28.014).

Keep in mind: The result is a theoretical stoichiometric amount. Actual yield depends on reaction conditions and losses.

Frequently asked questions

Why can't I use grams directly with the coefficients?

Coefficients count particles, and different substances have different masses per particle.

What is a mole ratio?

The ratio of coefficients of two substances in a balanced equation.

What if the reaction does not go to completion?

Multiply the theoretical amount by the percent yield.

How do I find the limiting reagent?

Convert each reactant to moles, divide by its coefficient, and the smallest result is limiting.

Sources: OpenStax, Chemistry 2e (2019), section 4.3 "Reaction Stoichiometry"

Checked 9 Nov 2026. Stoichiometry from a balanced equation (mole ratios from coefficients), as in OpenStax, Chemistry 2e (2019), §4.3 “Reaction Stoichiometry” — standard method.

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