Atomic Mass Calculator

Enter the number of protons and neutrons in a single isotope to get its mass number and an approximate atomic mass in atomic mass units.

🔬 Atomic Mass Calculator
Approximate Atomic Mass
—
Mass Number (A)
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Estimated Mass Defect
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🔬 What is Atomic Mass?

Atomic mass, in the sense this calculator computes it, is the mass of one specific isotope, estimated by adding up the individual masses of its protons and neutrons. Given only a proton count and a neutron count, this calculator returns the mass number (a simple whole-number sum) and an approximate atomic mass in atomic mass units (u), along with the estimated nuclear mass defect, the gap between that raw sum and the isotope's true mass.

This calculation is a core building block in nuclear chemistry and physics: converting between mass number notation (like carbon-14) and an estimated physical mass, introducing the concept of mass defect and nuclear binding energy before deeper study of fission and fusion, and building the arithmetic foundation students need before moving on to isotope abundance and weighted averages.

A common point of confusion is thinking this calculator produces the same number as a periodic table entry. It does not, on purpose. A periodic table's atomic weight is a precisely measured, abundance-weighted average across all naturally occurring isotopes of an element. This calculator instead estimates a single isotope's mass from raw nucleon counts, deliberately skipping the mass defect correction, so its result is always a little higher than the true measured mass.

This distinction matters: this calculator answers "roughly how much does this one isotope's nucleus weigh, from particle counts alone?" while the related Average Atomic Mass Calculator answers a completely different question, "what is the overall average mass of this element as it actually occurs in nature, across multiple isotopes?"

📐 Formula

A  =  Z + N      mass  ≈  Z(1.007825) + N(1.008665)
A = mass number (whole number, protons plus neutrons)
Z = number of protons
N = number of neutrons
1.007825 u = mass of one proton (as a hydrogen-1 atom)
1.008665 u = mass of one neutron
mass defect = (Z(1.007825) + N(1.008665)) − A, the mass "missing" as nuclear binding energy

📖 How to Use This Calculator

Steps

1
Enter the proton count - enter the number of protons (the atomic number Z) in the isotope.
2
Enter the neutron count - enter the number of neutrons in that specific isotope's nucleus.
3
Click Calculate to see the mass number, the approximate atomic mass, and the estimated mass defect.

💡 Example Calculations

Example 1 — Carbon-12

Protons = 6, Neutrons = 6.

1
A = 6 + 6 = 12
2
mass ≈ 6(1.007825) + 6(1.008665) = 12.0989 u
The raw nucleon sum overshoots carbon-12's exact defined mass of 12.0000 u by about 0.099 u, the estimated mass defect.
Try this example →

Example 2 — Oxygen-16

Protons = 8, Neutrons = 8.

1
A = 8 + 8 = 16
2
mass ≈ 8(1.007825) + 8(1.008665) = 16.1319 u
Oxygen-16's true measured mass is about 15.9949 u, again lower than this estimate because of mass defect.
Try this example →

Example 3 — Uranium-238

Protons = 92, Neutrons = 146.

1
A = 92 + 146 = 238
2
mass ≈ 92(1.007825) + 146(1.008665) = 239.9850 u
A large, heavy nucleus like uranium-238 has a much bigger absolute mass defect than light nuclei, consistent with its large total binding energy.
Try this example →

❓ Frequently Asked Questions

What is the difference between mass number and atomic mass?+
Mass number (A) is always a whole number, the simple count of protons plus neutrons in one atom's nucleus. Atomic mass is a decimal number measured in atomic mass units (u), reflecting the actual physical mass of the atom, which is close to but not exactly equal to the mass number, because of the nuclear mass defect.
How do you calculate mass number from protons and neutrons?+
Mass number is simply protons plus neutrons, A = Z + N. For example, an isotope with 6 protons and 8 neutrons has mass number A = 14, this describes carbon-14.
What is nuclear mass defect?+
Mass defect is the difference between the sum of the individual masses of an atom's separate protons and neutrons, and the actual measured mass of the assembled nucleus. That missing mass was converted into nuclear binding energy (via E = mc squared) when the nucleus formed, which is why every real nucleus weighs slightly less than its separate parts would.
Why does this calculator give a slightly different answer than a periodic table?+
A periodic table shows the standard atomic weight, the abundance-weighted average across all of an element's naturally occurring isotopes, each with its own precisely measured mass that already accounts for mass defect. This calculator instead estimates one specific isotope's mass by simply summing individual proton and neutron masses, deliberately ignoring the mass defect, so it always comes out a bit high.
What is the mass of a single proton and neutron used here?+
This calculator uses 1.007825 u for a proton (technically the mass of a hydrogen-1 atom, including its one electron) and 1.008665 u for a neutron, standard reference values. Neutrons are very slightly heavier than protons, a fact that is itself important in nuclear physics and radioactive beta decay.
Does this calculator work for any isotope?+
Yes, this calculator only needs a proton count and a neutron count, it works identically for any element or isotope, common or exotic, real or hypothetical, since it applies the same two nucleon masses regardless of which element the proton count corresponds to.
How is this different from the Average Atomic Mass Calculator?+
This calculator computes the mass of one single, specific isotope from its raw proton and neutron counts, a straightforward arithmetic sum. The Average Atomic Mass Calculator instead combines the precisely measured masses of two or more different isotopes of the same element, weighted by how abundant each one is in nature, to reproduce the decimal atomic weight seen on a periodic table.
Why is neutron mass slightly greater than proton mass?+
A free neutron is about 0.14% heavier than a proton, roughly 1.29 MeV more in rest energy. This small mass difference is why a free neutron can spontaneously decay into a proton, an electron, and an antineutrino (beta decay), while a free proton is stable.
Can mass number ever be negative or non-integer?+
No. Mass number is a count of physical particles (protons plus neutrons), so it is always a positive whole number for any real or hypothetical nucleus. If a calculation gives a non-integer or negative mass number, the proton or neutron count entered was invalid.
Is atomic mass the same thing as molar mass?+
They are numerically almost identical but conceptually distinct. Atomic mass is the mass of one atom, in atomic mass units (u). Molar mass is the mass of one mole (6.022 times 10 to the 23) of atoms, in grams per mole (g/mol), the numeric value carries over because 1 u was defined to make 1 mole of u equal 1 gram.
Tags: Atomic Mass Calculator Mass Number Calculator Protons Neutrons Mass Mass Defect Calculator Nuclear Binding Energy Chemistry Calculator Free Online

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What is the difference between mass number and atomic mass?

Mass number (A) is always a whole number, the simple count of protons plus neutrons in one atom's nucleus. Atomic mass is a decimal number measured in atomic mass units (u), reflecting the actual physical mass of the atom, which is close to but not exactly equal to the mass number, because of the nuclear mass defect.

How do you calculate mass number from protons and neutrons?

Mass number is simply protons plus neutrons, A = Z + N. For example, an isotope with 6 protons and 8 neutrons has mass number A = 14, this describes carbon-14.

What is nuclear mass defect?

Mass defect is the difference between the sum of the individual masses of an atom's separate protons and neutrons, and the actual measured mass of the assembled nucleus. That missing mass was converted into nuclear binding energy (via E = mc squared) when the nucleus formed, which is why every real nucleus weighs slightly less than its separate parts would.

Why does this calculator give a slightly different answer than a periodic table?

A periodic table shows the standard atomic weight, the abundance-weighted average across all of an element's naturally occurring isotopes, each with its own precisely measured mass that already accounts for mass defect. This calculator instead estimates one specific isotope's mass by simply summing individual proton and neutron masses, deliberately ignoring the mass defect, so it always comes out a bit high.

What is the mass of a single proton and neutron used here?

This calculator uses 1.007825 u for a proton (technically the mass of a hydrogen-1 atom, including its one electron) and 1.008665 u for a neutron, standard reference values. Neutrons are very slightly heavier than protons, a fact that is itself important in nuclear physics and radioactive beta decay.

Does this calculator work for any isotope?

Yes, this calculator only needs a proton count and a neutron count, it works identically for any element or isotope, common or exotic, real or hypothetical, since it applies the same two nucleon masses regardless of which element the proton count corresponds to.

How is this different from the Average Atomic Mass Calculator?

This calculator computes the mass of one single, specific isotope from its raw proton and neutron counts, a straightforward arithmetic sum. The Average Atomic Mass Calculator instead combines the precisely measured masses of two or more different isotopes of the same element, weighted by how abundant each one is in nature, to reproduce the decimal atomic weight seen on a periodic table.

Why is neutron mass slightly greater than proton mass?

A free neutron is about 0.14% heavier than a proton, roughly 1.29 MeV more in rest energy. This small mass difference is why a free neutron can spontaneously decay into a proton, an electron, and an antineutrino (beta decay), while a free proton is stable.

Can mass number ever be negative or non-integer?

No. Mass number is a count of physical particles (protons plus neutrons), so it is always a positive whole number for any real or hypothetical nucleus. If a calculation gives a non-integer or negative mass number, the proton or neutron count entered was invalid.

Is atomic mass the same thing as molar mass?

They are numerically almost identical but conceptually distinct. Atomic mass is the mass of one atom, in atomic mass units (u). Molar mass is the mass of one mole (6.022 times 10 to the 23) of atoms, in grams per mole (g/mol), the numeric value carries over because 1 u was defined to make 1 mole of u equal 1 gram.

📌 Quick Tips

💡This calculator finds the mass of a single isotope from raw particle counts, it is not the same as the periodic table's atomic weight, which averages across every naturally occurring isotope, see the Average Atomic Mass Calculator for that.
💡Mass number A is always a whole number, protons plus neutrons, but the approximate atomic mass in amu is never a whole number because proton and neutron masses are each slightly above 1 u.
💡The result here is always a little higher than an isotope's true measured mass, since it ignores the mass defect, the mass converted into nuclear binding energy that holds the nucleus together.
💡Heavier, more tightly bound nuclei (roughly iron and nearby elements) have the largest mass defect per nucleon, this is why nuclear fission and fusion release energy.
💡Electron mass is small enough (about 0.00055 u) that it is usually left out of a quick mass number to atomic mass estimate like this one.