Atomic Mass - Mass Number, amu, and Kilograms

The atomic mass calculator is a free chemistry and physics tool that turns the proton and neutron counts of any element from hydrogen to oganesson into the mass number, the nucleon mass sum in amu and kilograms, and the resolved isotope notation.

• • Identify a stable isotope: type the proton and neutron counts for carbon-12, oxygen-16, or uranium-238 and read the matching element symbol and isotope notation in one step.
• • Estimate per-atom mass in kilograms: set protons and neutrons for a common atom, then read the nucleon mass sum in kilograms for a kinetic-energy or stoichiometric equation.
• • Compare light and heavy atoms: switch between hydrogen-1, carbon-12, iron-56, and uranium-238 to see how the amu and kg rows scale across orders of magnitude.
• • Check a textbook answer: enter protons and neutrons from a homework question, read the mass number and nucleon mass sum, and compare with the answer key while keeping the binding-energy distinction in mind.

The result panel returns the nucleon mass sum, the CODATA sum of proton and neutron rest masses, not the IUPAC atomic mass: the measured atomic mass is lower by the binding-energy mass defect. For matching protons, neutrons, electrons, and ion-charge breakdown, the Atom Calculator returns the full subatomic-count row next to the mass number.

The atomic mass calculator sums the proton and neutron counts to get the mass number, multiplies each count by its CODATA 2018 rest mass in atomic mass units, and converts the resulting nucleon mass sum into kilograms. The result is the nucleon mass sum, not the IUPAC atomic mass: the measured atomic mass is lower by the binding-energy mass defect (E = mc^2).

• protons: Number of protons Z, the atomic number. Valid 1 to 118.
• neutrons: Number of neutrons N in the nucleus. Valid 0 to 177.
• includeElectrons: Toggle. When yes, the calculator adds Z * 0.000548579909 u for the CODATA electron rest mass; when no, it uses the Z + N nucleon mass sum.
• CODATA rest masses: PROTON 1.00727646688 u, NEUTRON 1.00866491600 u, ELECTRON 0.000548579909 u.
• AMU_TO_KG: CODATA 2018: 1 u = 1.66053906660 x 10^-27 kg.

According to NIST CODATA 2018, one unified atomic mass unit equals 1.66053906660 x 10^-27 kilograms, and the proton and neutron rest masses are 1.00727646688 u and 1.00866491600 u, the values used in the formula box.

When the per-atom mass is needed in grams per mole instead of atomic mass units, the Mole & Molar Mass Calculator turns the same per-atom value into molar mass for stoichiometry.

Four ideas drive every result this atomic mass calculator prints: the proton count, the neutron count, the mass number they form, and the rest-mass unit, with the binding-energy correction that separates the nucleon mass sum from the actual atomic mass.

The nucleon mass sum is the value to use when the problem treats nucleons as free particles, and the Bohr Model Calculator uses the matching nuclear mass to compute orbit radius and electron energy for the same proton count.

Set the proton and neutron counts, pick whether to add the electron mass, then read the mass number, the nucleon mass sum in amu and kilograms, and the resolved isotope notation from the result panel.

1. 1 Enter the number of protons: Type Z in the first field. The calculator accepts integers from 1 to 118 inclusive, the full IUPAC range of known elements.
2. 2 Enter the number of neutrons: Type N in the second field. The calculator accepts integers from 0 to 177 inclusive, the range observed in known isotopes.
3. 3 Decide whether to include electrons: Leave the toggle on No for the standard Z + N nucleon mass sum, or switch to Yes to add Z electrons at the CODATA electron rest mass.
4. 4 Read the mass number first: The mass number A is the integer at the top of the result panel and is the value used in isotope notation.
5. 5 Read the nucleon mass sum rows: The amu row is the CODATA sum of proton plus neutron (and optional electron) rest masses, and the kg row is the same value times 1.66053906660 x 10^-27 kg/u.
6. 6 Note the resolved isotope: The element and isotope rows confirm which atom the inputs represent, the fastest way to catch a Z or N typo before trusting the result.

A student wants the nucleon mass sum for uranium-238. They enter 92 protons, 146 neutrons, and leave the electron toggle on No. The result panel shows mass number 238, nucleon mass sum 239.9345 u, 3.9842 x 10^-25 kg, element U, and isotope notation U-238. The measured atomic mass of U-238 is about 238.0508 u, lower than the nucleon mass sum.

After the per-atom mass has been read in grams per mole, the Grams to Moles Calculator converts a measured sample mass in grams into moles for stoichiometry.

Using the atomic mass calculator for a proton-plus-neutron calculation gives a result panel that covers the mass number, the nucleon mass sum in amu and kilograms, and the resolved isotope, with the binding-energy distinction called out so the value is not mistaken for the measured atomic mass.

• • Mass number A from Z plus N: the result panel shows the integer mass number A immediately, the value used in isotope notation like C-12, O-16, or U-238.
• • Nucleon mass sum in amu and kilograms: the same calculation reports the rest-mass sum of the free constituents in unified atomic mass units and in kilograms, covering the unit conventions used in chemistry and physics classes.
• • CODATA 2018 proton and neutron rest masses: the amu row uses the CODATA 2018 proton rest mass of 1.00727646688 u and the neutron rest mass of 1.00866491600 u, so the result is traceable to the international recommended values.
• • Optional electron mass contribution: the electron toggle adds Z electrons at the CODATA 2018 electron rest mass of 0.000548579909 u each, raising the precision to include the electron rest mass.
• • Element symbol and isotope notation: the resolved element and isotope rows confirm which atom the inputs represent, catching a Z or N typo before the answer is written into a homework or lab report.
• • Validation against the IUPAC range: the calculator warns when Z is outside 1 to 118 or N is outside 0 to 177, so out-of-range inputs do not produce a misleading number.

The same residue-mass addition logic that builds the nucleon mass sum from individual particle rest masses also drives molecular-weight calculations in biochemistry, and the Protein Molecular Weight Calculator applies it to an amino-acid sequence.

Three small choices drive the result panel: the proton count Z, the neutron count N, and whether the electron mass contribution is added. The result is the nucleon mass sum, so the binding-energy mass defect is the main caveat.

• • The atomic mass calculator returns the nucleon mass sum, the CODATA proton plus neutron (and optional electron) rest masses, not the IUPAC atomic mass. The measured atomic mass is lower because nuclear binding energy releases mass via E = mc^2, about 0.1 percent for most atoms.
• • The element and isotope rows resolve Z to a fixed IUPAC element symbol and A to an integer, so the calculator does not replace a standard atomic-weight or isotopic-composition table. Average atomic weights and natural isotopic abundances are a different calculation.
• • The proton count is clamped to the IUPAC known-element range of 1 to 118, and the neutron count is clamped to 0 to 177. Inputs outside those ranges return a validation warning rather than a misleading result.

According to IUPAC Periodic Table, the currently known range of atomic numbers runs from 1 (hydrogen) to 118 (oganesson), which sets the validation range used to keep proton inputs inside the known-element set.

According to IUPAC Gold Book, atomic mass is the rest mass of a neutral atom at rest, equal to the proton, neutron, and electron rest masses minus the binding-energy mass defect, which is why the nucleon mass sum from this calculator is higher than the measured atomic mass of the same isotope.

When per-atom values of every atom on each side of a reaction need to line up for a mass-balance check, the Chemical Equation Balancer Calculator balances the equation and the mass check uses measured atomic weights.