Atomic Mass Calculator
Use this atomic mass calculator to estimate mass number and approximate atomic mass from the numbers of protons, neutrons, and electrons in an atom or ion.
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Run the calculator.
What This Atomic Mass Calculator Helps You Do
This page is useful when you want the bookkeeping side of atomic mass, not just a periodic-table value. It shows how proton, neutron, and electron counts translate into a mass number and an approximate particle-sum mass.
That makes it a good bridge between atomic-structure questions and mass concepts, while also clarifying why measured isotope masses differ slightly from the simple free-particle total.
How to Calculate Atomic Mass Calculator
- Enter particle counts: Use whole numbers for protons, neutrons, and electrons.
- Calculate the mass number: The mass number comes from protons plus neutrons only.
- Estimate the particle-sum mass: The tool multiplies each particle count by its approximate particle mass in atomic mass units.
- Read the kg conversion: The result also converts the estimated atomic mass into kilograms.
Atomic Mass Calculator Formula
| Variable | Meaning | Unit |
|---|---|---|
| p | Number of protons | count |
| n | Number of neutrons | count |
| e | Number of electrons | count |
| u | Atomic mass unit | 1 u = 1.66053906660 x 10^-27 kg |
Use the worked examples below to check how the formula behaves with real values. If the result looks unexpected, verify the unit assumptions and the meaning of each variable before interpreting the answer.
Worked Examples
- Protons: 6
- Neutrons: 6
- Electrons: 6
Result: Mass number = 12 and approximate particle-sum mass = 12.0989 u.
The particle-sum mass is slightly above the tabulated isotope mass because real nuclei lose mass through binding energy.
- Protons: 11
- Neutrons: 12
- Electrons: 11
Result: Mass number = 23 and approximate particle-sum mass = 23.1901 u.
This gives the bookkeeping mass from separate particles rather than an exact isotopic mass table entry.
- Protons: 17
- Neutrons: 18
- Electrons: 18
Result: Mass number = 35 and approximate particle-sum mass = 35.2895 u.
Adding one extra electron changes the mass only slightly because electron mass is tiny compared with nucleon mass.
- Protons: 26
- Neutrons: 30
- Electrons: 24
Result: Mass number = 56 and approximate particle-sum mass = 56.4623 u.
Ionic charge has only a very small effect on the estimated atomic mass compared with the nucleons.
How to Interpret Your Results
| Range | Meaning | Action |
|---|---|---|
| Mass number | Counts nucleons only. | Use it for isotope notation and basic atomic structure problems. |
| Particle-sum mass | Adds separate particle masses in u. | Treat it as an approximation rather than a tabulated exact isotope mass. |
| Binding energy effect | Real atoms weigh slightly less than the simple sum of separate particles. | Use measured isotope masses when high precision matters. |
Frequently Asked Questions
References
Last reviewed: March 2026