Enter the average atomic mass (numerically equal to the element’s molar mass, typically expressed in g/mol) and a chosen reference molar mass (g/mol) into the Atomic Ratio Calculator. The calculator will evaluate and display the (dimensionless) mass ratio AAM/RS.

Atomic Ratio Calculator

Enter any 2 values to calculate the missing variable

Note: This calculator treats “Atomic Ratio” as a unitless ratio of two molar masses (AAM ÷ RS). Both masses must be on the same basis (e.g., both for elements) and in consistent units.

Atomic Ratio Formula

The following formula is used to calculate the Atomic Ratio (a unitless ratio of two molar masses): 

AR = AAM / RS
  • Where AR is the Atomic Ratio (unitless)
  • AAM is the average atomic mass of an element (numerically equal to its molar mass; g/mol) 
  • RS is the reference (standard) molar mass you are comparing to (g/mol) 

To calculate the atomic ratio, divide the average atomic mass by the reference standard (reference molar mass). Because g/mol ÷ g/mol cancels, the result is dimensionless.

How to Calculate Atomic Ratio?

The following example problems outline how to calculate Atomic Ratio.

Example Problem #1:

  1. First, determine the average atomic mass (g/mol).
    • The average atomic mass (g/mol) is given as: 35.45.
  2. Next, determine the reference standard (g/mol).
    • The reference standard (g/mol) is provided as: 12.00.
  3. Finally, calculate the Atomic Ratio using the equation above: 

AR = AAM / RS

The values given above are inserted into the equation below and the solution is calculated:

AR = 35.45 / 12.00 = 2.9542

Example Problem #2: 

For this problem, the variables required are provided below:

average atomic mass (g/mol) = 15.999

reference standard (g/mol) = 12.00

Test your knowledge using the equation and check your answer with the calculator above.

AR = AAM / RS = ?