How this calculator works
The nucleon modes give the classroom estimate m approximately A u, with A equal to protons plus neutrons. Nuclear binding energy and electron masses mean a real isotope generally differs from that estimate. The measured-mass modes convert directly using the CODATA 2022 atomic mass constant, 1.66053906892e-27 kg per u. They do not round a measured isotope mass into a mass number.
Formula
A = Z + N; estimated m = A u. For a measured value, m(kg) = m(u) x 1.66053906892e-27.
Worked example
Six protons and six neutrons give A = 12 and an estimated mass of 1.992646882704e-26 kg. A measured mass of 3.32107813784e-27 kg converts to 2 u.
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Frequently asked questions
Is the estimate exact for carbon-12?
A neutral carbon-12 atom defines 12 u. The kilogram conversion still has the CODATA measurement uncertainty.
Why are other isotope masses not integers?
Binding energy changes the total mass relative to separated particles.
Does this average isotopes?
No. Use the average atomic mass calculator with isotope masses and abundances.