Molar Ratio Calculator — Stoichiometry
Enter the mass and molar mass of two substances to find their mole amounts and the molar ratio nA : nB — the key step in any stoichiometry or limiting-reagent calculation.
g
g/mol
g
g/mol
Moles of A per mole of B
- 1
Moles of A = mA ÷ molar mass A
36 ÷ 18.015 = 1.9983 - 2
Moles of B = mB ÷ molar mass B
28 ÷ 28.014 = 0.9995 - 3
Molar ratio nA ÷ nB
1.9983 ÷ 0.9995 = 1.9993Compare this ratio to stoichiometric coefficients to identify the limiting reagent.
How does this calculator work?
Moles = mass / molar mass. The molar ratio nA : nB = (mA/MA) : (mB/MB) tells you how many moles of A there are per mole of B. Compare this ratio to the balanced equation's stoichiometric coefficients to determine the limiting reagent and theoretical yield.
Formula
How this is calculated
A molar ratio expresses how many moles of one substance are present relative to another. It is the bridge between the balanced equation (which gives ratios of moles) and the actual masses of reactants and products you weigh in the lab.
The conversion is straightforward: divide each mass by its molar mass (found from the periodic table or a formula weight) to get the number of moles — nA = mA / MA and nB = mB / MB. The ratio nA : nB tells you, for every mole of B, how many moles of A are present. Compare this ratio to the stoichiometric coefficients in the balanced equation to identify the limiting reagent: whichever reactant provides the smaller multiple of its stoichiometric coefficient is consumed first.
Common molar masses for reference (g/mol, 2025 IUPAC values): H₂O 18.015, NaCl 58.44, CO₂ 44.009, H₂SO₄ 98.07, NaOH 39.997, HCl 36.461, C₆H₁₂O₆ (glucose) 180.16, N₂ 28.014, O₂ 31.998. The calculator assumes pure substances with exact molar masses; impure samples or hydrated salts require adjustment.
Frequently asked questions
Divide each substance's mole count by its stoichiometric coefficient from the balanced equation. The substance with the smallest result is the limiting reagent — it runs out first and determines how much product can form.
Add the atomic masses (from the periodic table) for every atom in the formula. For example, H₂O: 2×1.008 + 15.999 = 18.015 g/mol. Online molar mass tools and formula-weight calculators can handle complex formulas instantly.
In that case moles = molarity (mol/L) × volume (L). You do not need the mass or molar mass — plug the calculated moles directly into the ratio. The molar-ratio concept is the same; this calculator covers the mass-based route.
Also known as
TG we-Calculate Editorial Team. (2026). Molar Ratio Calculator — Stoichiometry [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/molar-ratio-calculator
TG we-Calculate Editorial Team. "Molar Ratio Calculator — Stoichiometry." TG we-Calculate. 2026. https://we-calculate.com/calculator/molar-ratio-calculator.
TG we-Calculate Editorial Team, "Molar Ratio Calculator — Stoichiometry," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/molar-ratio-calculator
@misc{wecalculate_molar_ratio_calculator, title = {Molar Ratio Calculator — Stoichiometry}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/molar-ratio-calculator}}, year = {2026}, note = {TG we-Calculate} }
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