Enter the number of protons in the isotope and the number of neutrons in the isotope into the Relative Mass Calculator. The calculator will evaluate and display the Relative Mass. 

Relative Mass Formula

The following formula is used to calculate the Relative Mass. 

RM = P + N

  • Where RM is the Relative Mass (amu)
  • P is the number of protons in the isotope 
  • N is the number of neutrons in the isotope 

How to Calculate Relative Mass?

The following example problems outline how to calculate Relative Mass.

Example Problem #1:

  1. First, determine the number of protons in the isotope. 
    • The number of protons in the isotope is given as: 5.
  2. Next, determine the number of neutrons in the isotope. 
    • The number of neutrons in the isotope is provided as: 7.
  3. Finally, calculate the Relative Mass using the equation above: 

RM = P + N

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

RM = 5 + 7 = 12.00 (amu)


Example Problem #2: 

For this problem, the variables needed are provided below:

number of protons in the isotope = 8

number of neutrons in the isotope = 3

This example problem is a test of your knowledge on the subject. Use the calculator above to check your answer. 

RM = P + N = ?