Calculate atomic mass from protons & neutrons or from isotopic abundances
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Understanding atomic mass is essential for chemistry students, researchers, and anyone working with chemical quantities. Our atomic mass calculator provides two powerful calculation modes to meet your needs. In the protons and neutrons mode, you simply enter the number of protons and neutrons to determine the mass number and atomic mass in atomic mass units (amu) and kilograms. In the isotope mode, you can enter multiple isotopes with their masses and abundances to calculate the weighted average atomic mass. This tool is invaluable for students learning about atomic structure, professionals preparing chemical solutions, and educators teaching fundamental chemistry concepts. Toolraxy’s atomic mass calculator runs entirely in your browser, keeping your data secure while delivering accurate results for both simple and complex atomic mass calculations.
Select your preferred calculation mode: Protons & Neutrons or Isotopes.
In Protons & Neutrons mode, enter the number of protons (atomic number Z) and neutrons.
Click Calculate to see the mass number and atomic mass displayed.
In Isotopes mode, add isotopes using the “Add Isotope” button.
Enter each isotope’s label, mass in amu, and natural abundance percentage.
The calculator automatically computes the weighted average atomic mass.
Use quick example buttons to test common elements and isotopes.
Copy results or share them directly from the controls section.
The atomic mass calculator uses two distinct formulas depending on your selected mode.
Protons & Neutrons Formula: Mass Number (A) = Z + N
Atomic Mass: ≈ A atomic mass units (amu)
Isotope Formula: Average Atomic Mass = Σ (Isotope Mass × Fractional Abundance)
For the isotope mode, the calculator normalizes abundances if they don’t sum to exactly 100%. Each isotope’s fractional abundance is calculated as its abundance divided by the total abundance of all isotopes. The calculator then multiplies each isotope’s mass by its fractional abundance and sums the results to give the weighted average atomic mass. In protons and neutrons mode, the atomic mass is approximately equal to the mass number, with conversion to kilograms using the atomic mass unit (1 u = 1.66054 × 10⁻²⁷ kg).
Let’s calculate the atomic mass of carbon-12 using the protons and neutrons mode.
Step 1: Select Protons & Neutrons mode
Step 2: Enter protons (Z) = 6 and neutrons (N) = 6
Step 3: Mass Number (A) = 6 + 6 = 12
Step 4: Atomic Mass = 12.000 atomic mass units (amu)
Step 5: In kilograms: 12 × 1.66054 × 10⁻²⁷ = 1.99265 × 10⁻²⁶ kg
Result: Carbon-12 has a mass number of 12 and an atomic mass of exactly 12 amu. This serves as the standard for atomic mass measurements.
Atomic mass is the actual mass of an atom expressed in atomic mass units, while mass number is the total number of protons and neutrons in the nucleus. For carbon-12, both values are 12, but for most elements, atomic mass differs slightly from the mass number due to isotopic variations.
Multiply the atomic mass in amu by 1.66054 × 10⁻²⁷ kg/amu. For example, carbon-12 (12 amu) equals 1.99265 × 10⁻²⁶ kg. Our calculator performs this conversion automatically.
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They matter because different isotopes have different masses, affecting the average atomic mass and properties like stability and reactivity.
For each isotope, multiply its mass by its fractional abundance (abundance percentage divided by 100), then sum these weighted values. The result is the average atomic mass, as displayed on the periodic table.
This calculator is designed for atomic mass of individual atoms. For molecular mass, you would need to sum the atomic masses of all atoms in the molecule. Our atom and molar mass calculators can help with those calculations.
The atomic mass unit (amu) is defined as exactly one-twelfth the mass of a carbon-12 atom in its ground state. This definition provides a consistent and universal standard for expressing atomic and molecular masses.
Periodic table atomic masses are weighted averages of all naturally occurring isotopes. Since natural isotopic abundances vary slightly, the average mass often has decimal places. For example, chlorine has an average atomic mass of 35.45 amu.
Subtract the atomic number (number of protons) from the mass number (protons + neutrons). For example, carbon-14 has 14 − 6 = 8 neutrons.
Mass defect is the slight difference between the expected mass (sum of protons and neutrons) and the actual nuclear mass. This difference represents nuclear binding energy and is negligible for most practical calculations.
Isotopes have nearly identical chemical properties because chemical behavior depends primarily on electron configuration, not nuclear composition. However, isotopic differences can affect reaction rates in isotope labeling experiments.
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