Atomic Mass Calculator
Calculate the atomic mass of an atom using the number of protons and neutrons. Results in atomic mass units (amu) and kilograms.
Atomic number (number of protons in nucleus)
Number of neutrons in the nucleus
Leave empty for neutral atom (electrons = protons)
What is atomic mass?
Atomic mass is the mass of an atom, typically expressed in atomic mass units (u or amu). It is approximately the sum of protons and neutrons (mass number A = Z + N) because electrons have negligible mass (~1/1836 of proton mass). 1 u = 1.660539 × 10⁻²⁷ kg.
How do you calculate atomic mass from protons and neutrons?
The approximate atomic mass in amu is A = Z + N (protons + neutrons). For more precision, use: mass = Z×m_p + N×m_n + (Z-N)×m_e - BE/c², where BE is binding energy. For most purposes, A = Z + N is sufficient, giving mass in amu.
What is the difference between atomic mass and mass number?
Mass number (A) is the sum of protons and neutrons (always an integer). Atomic mass is the actual mass of the atom in amu, which is slightly less than A due to nuclear binding energy (mass defect). Carbon-12 has exactly 12 amu by definition.
How do you convert atomic mass to kilograms?
Multiply atomic mass in amu by the conversion factor: 1 amu = 1.660539 × 10⁻²⁷ kg. For example, carbon-12 has mass 12 amu = 12 × 1.660539 × 10⁻²⁷ = 1.9926 × 10⁻²⁶ kg.