Molar Ratio Calculator

How to use this molar ratio calculator

What this calculator does. It converts the coefficients of a balanced chemical equation into a simplified three-part molar ratio, then scales that ratio from one known mole amount. Use it to plan stoichiometric quantities, check homework, compare reagent requirements, or estimate the theoretical moles of a product. It does not balance an equation for you, identify a limiting reagent from multiple available amounts, or predict real laboratory yield.

When to use it. It is useful after balancing a reaction, when preparing a reagent plan, when converting one known amount into theoretical amounts of the other participants, and when checking whether coefficient ratios have been interpreted correctly. The calculator opens with the balanced water-formation example 2H₂ + O₂ → 2H₂O, so the results and Excel workbook are immediately available.

How to calculate.

1. Replace Substance A, Substance B, and Substance C with formulas or short names. Each is required and may contain up to 40 characters.
2. Enter Coefficient A, Coefficient B, and Coefficient C as positive numbers taken from the balanced equation. Whole numbers are customary, but proportional decimal coefficients are accepted.
3. Choose the Known substance, then enter its Known amount (mol) as a positive decimal using a period as the decimal separator. Scientific notation and comma-decimal input are rejected to prevent ambiguity.
4. Read the Simplified molar ratio, Scale factor, Known basis, and the table. Select Download Excel to export the current typed inputs and calculated values. Reset clears the demonstration data and disables export until a complete valid state is entered again.

Input guide. The three substance-name fields are text labels only; changing a name changes labels, not arithmetic. Each coefficient is required, must be finite and greater than zero, and controls that substance's share of the ratio. For example, coefficients 2, 1, and 2 produce 2:1:2. A missing, zero, negative, comma-formatted, or nonnumeric coefficient is rejected. Known substance determines the coefficient used as the scaling denominator. Known amount (mol) is the actual mole basis, such as 10 mol H₂; increasing it increases every scaled amount proportionally.

Output guide. Simplified molar ratio is an exact proportional identity derived from the coefficients after removing their greatest common factor when they are effectively integers. Scale factor equals known moles divided by the selected substance's coefficient. Known basis repeats the selected label and amount so the interpretation remains explicit. In the table, Coefficient is the balanced-equation value, Ratio to known substance is coefficient divided by the known coefficient, and Scaled amount (mol) is that ratio multiplied by the known amount. A larger coefficient produces a larger required or theoretical amount at the same scale.

Worked example. For 2H₂ + O₂ → 2H₂O, choose H₂ as the known substance and enter 10 mol. The scale factor is 10 ÷ 2 = 5. Multiplying each coefficient by 5 gives 10 mol H₂, 5 mol O₂, and 10 mol H₂O. The displayed ratio is 2:1:2, and the table shows ratios to H₂ of 1.0000, 0.5000, and 1.0000.

Learn more. The Chemistry LibreTexts explanation of mole ratios shows how balanced coefficients become conversion factors, while the NIST overview of the mole and amount of substance explains the SI quantity used here.