Estimate atomic mass from nucleon count
This atomic mass calculator starts with separate proton and neutron counts, adds them to obtain mass number, and converts that count from unified atomic mass units to kilograms. It is designed for the classroom approximation that one proton or neutron contributes about one u. Keeping the two particle inputs visible helps distinguish a nucleus with six protons and six neutrons from another isotope of the same element with a different neutron count.
The unified atomic mass unit is a convenient scale for atoms. One u is defined from carbon-12 and is approximately 1.66053906892 × 10⁻²⁷ kilograms using the 2022 CODATA atomic mass constant. Kilograms make the physical size of an atomic mass explicit, while u keeps nuclear particle counts readable. The page reports both forms instead of forcing a long string of leading zeros.
Mass number, atomic weight, and isotope mass differ
Mass number is the integer sum of protons and neutrons. Standard atomic weight is usually a weighted average for naturally occurring isotopes and can be a decimal or interval. Exact relative isotope mass is measured and differs slightly from its mass number because the bound nucleus does not have exactly the sum of isolated proton and neutron masses. These three quantities answer different questions and should not be substituted without stating the approximation.
The calculator deliberately labels its output as an estimate. If a problem asks for the mass of a carbon-12 atom using one u per nucleon, entering six protons and six neutrons is appropriate. If it supplies a precise isotope mass such as 34.96885268 u, use that supplied mass in a direct conversion instead. For molar-mass or percent-composition work, use standard atomic weights rather than this nucleon-count shortcut.
Work through a simple conversion
For a nucleus with eight protons and eight neutrons, the mass number is 16. Multiplying 16 by the atomic mass constant gives about 2.6568625 × 10⁻²⁶ kg. The result panel shows the addition first and the kilogram multiplication second. This order mirrors a manual solution and makes it easy to notice if a neutron count was entered in the proton field.
Inputs must be non-negative whole numbers, and at least one nucleon is required. The broad limits allow classroom examples involving known and hypothetical nuclei without accepting infinite, fractional, or unreasonably large values. The calculator does not use electron mass because the selected approximation is specifically proton-plus-neutron mass; electron mass is much smaller but matters in higher-precision atomic-mass work.
Verify the scale and scientific notation
A quick order-of-magnitude check is useful. One nucleon is roughly 1.66 × 10⁻²⁷ kg, so a nucleus near mass number 10 should be near 10⁻²⁶ kg and one near mass number 100 should be near 10⁻²⁵ kg. If the exponent moves in the opposite direction when nucleon count increases, recheck the scientific notation. The coefficient should grow linearly with mass number.
You can also reverse the calculation by dividing the displayed kilogram estimate by the shown atomic mass constant. The quotient should return the mass number within display rounding. This is a stronger check than comparing only the final digits because it confirms both the multiplication and the exponent. Retain several significant digits during the calculation and round only for the requested answer.
Limits, constants, and private use
The CODATA constant is versioned in the calculation, but the one-u-per-nucleon premise remains an approximation. Nuclear binding energy, isotope-specific masses, electron mass, and ionization are outside the model. Do not use the output as precision nuclear data, a radiation calculation, or evidence about isotope stability. Consult evaluated isotope tables when the exact nuclide mass or uncertainty matters.
The values are calculated in the current browser tab. NexaCurrent does not upload the quantities, formulas, chemical names, element choices, or other information entered here, and no account is needed. Copying is a deliberate action after the answer and working are visible. Refreshing or leaving the page clears the current calculation, so keep a copied result if it is needed for later study.