Calculate average atomic mass from isotope masses and abundances, or solve for a missing isotope mass or abundance, using percent or decimal inputs.

Required: mass and abundance for each isotope.

Isotope 1
Calculated from the target
Calculated from the target
Isotope 2
Calculated from the target
Calculated from the target
Isotope 3
Calculated from the target
Calculated from the target
Isotope 4
Calculated from the target
Calculated from the target
Isotope 5
Calculated from the target
Calculated from the target

Isotope Mass Formula

The following formulas are used to calculate a molecule’s monoisotopic (exact) mass and average mass from its chemical formula.

Mₘₒₙₒ = Σᵢ nᵢ m₍i,mono)\ M₍avg) = Σᵢ nᵢ mᵢ
  • Where Mmono is the monoisotopic (exact) mass (u or Da)
  • Mavg is the average mass based on natural isotopic abundances (u or Da)
  • ni is the number of atoms of element i in the formula
  • mi,mono is the exact mass of the most abundant isotope of element i (e.g., 12C, 1H, 16O)
  • m̄i is the average atomic mass of element i (weighted by natural abundances)

To calculate a monoisotopic mass, multiply each element’s atom count by that element’s monoisotopic isotope mass and sum the results. To calculate an average mass, do the same using the element’s average atomic mass. The isotopic distribution is built by combining (convolving) the isotope distributions of each element in the formula.

How to Calculate Isotope Mass?

The following example problems outline how to calculate monoisotopic (exact) mass from a chemical formula.

Example Problem #1:

  1. First, determine the chemical formula.
    • The formula is given as: C6H12O6.
  2. Next, identify the monoisotopic isotope masses to use (most abundant isotope of each element).
    • 12C = 12.00000000000 u, 1H = 1.00782503223 u, 16O = 15.99491461957 u
  3. Finally, calculate the monoisotopic (exact) mass using the equation above.

Mmono = Σ ni·mi,mono

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

Mmono = 6(12.00000000000) + 12(1.00782503223) + 6(15.99491461957) = 180.063388 (u)

Example Problem #2: 

For this problem, the variables needed are provided below:

chemical formula = H2O

Find the monoisotopic (exact) mass (u) and the abundance (%) of the monoisotopic isotopic composition.

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

Mmono = Σ ni·mi,mono = ?