Chemistry
Atomic Mass Calculator
Estimate the mass number and the mass of a single atom in unified atomic mass units and kilograms.
Your inputs
Change any value to update the result instantly.
Mass number (A)
12 u (approx.)
Approximate atomic mass
12 u
Mass of one atom
19.9265 ×10⁻²⁷ kg
Nucleons
12
The mass-number approximation neglects electron mass and nuclear binding-energy defect, so it differs slightly from the measured isotopic mass.
What this calculator does
Estimates the mass number of a nuclide and the mass of one atom in unified atomic mass units and kilograms.
What your result means
The mass number counts nucleons; the atomic mass in kilograms uses the unified atomic mass unit u = 1.66053906660 × 10⁻²⁷ kg.
Formula and working
A ≈ Z + N; mass ≈ A × u
- 1
Collect the inputs
Protons (Z) = 6; Neutrons (N) = 6
- 2
Apply the formula
A ≈ Z + N; mass ≈ A × u
- 3
Result
Mass number (A) = 12 u (approx.)
Frequently asked questions
Symbols and assumptions
- Z
- Protons
- N
- Neutrons
- A
- Mass number
- u
- Unified atomic mass unit
- • Electron mass is neglected.
- • Nuclear binding-energy mass defect is neglected.
Sources and review
Reviewed 2026-09-22 · Next review 2027-09-22
- • Measured isotopic masses differ from the whole-number mass approximation by a fraction of a percent.
Important limitation
This is an educational estimate based only on the values and formula shown. Verify current rates, rules, units, and professional standards before relying on the result. It is not personalized financial or investment advice.
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