Four calculators, one panel.
Pick a mode below — everything recalculates live as you type, and every result explains itself.
Leave at 0% if this element only has two natural isotopes.
The element's atomic mass from the periodic table.
Protons + neutrons for this specific isotope.
0 for a neutral atom, positive for a cation, negative for an anion.
You can type scientific notation, e.g. 6.022e23.
Must be greater than zero — use the isotope's own mass for a pure isotope sample.
A few inputs to an answer.
No manual required — but here's the entire flow in four moves.
Pick a mode
Atomic mass for isotope masses + abundances → weighted average. Solve abundance for the reverse case. Atomic composition for protons/neutrons/electrons. Mass ↔ moles ↔ atoms for dimensional analysis.
Enter what you know
Type each value into its field — switch between 2 or 3 isotopes, or between mass, moles and atom count, with the toggle at the top. The example chips fill everything in with one tap.
Read the results
The main answer sits up top in large type. Related values sit below, and the exact formula with your numbers substituted sits at the bottom.
Trust the checks
Impossible inputs — negative masses, abundances that can't add up, or a mass number smaller than the atomic number — get flagged with a plain-English reason instead of a wrong or broken answer. Copy any result set with one click.
Four ideas, one periodic table.
Every mode above comes from one of these core isotope relationships.
Average atomic mass
Multiply each isotope's mass by its fractional abundance, then sum across all isotopes.
Two-isotope abundance
Solve the weighted-average equation for the fractional abundance of the lighter isotope.
Atomic composition
Neutrons come from mass number minus atomic number; electrons come from atomic number minus charge.
Questions people actually ask.
The short answers — with the long ones one tap away.
How do you calculate average atomic mass from isotopes?+
Multiply each isotope's mass by its fractional abundance (percent abundance ÷ 100), then add the results together. Chlorine is 75.76% Cl-35 (34.969 amu) and 24.24% Cl-37 (36.966 amu): (34.969 × 0.7576) + (36.966 × 0.2424) ≈ 35.45 amu, matching the periodic table value.
How do I find the percent abundance of two isotopes?+
For a two-isotope element, let x be the fractional abundance of the lighter isotope. Since the weighted average must equal the known atomic mass: M = m1·x + m2·(1 − x). Solving gives x = (M − m2) ÷ (m1 − m2). The Solve abundance tab does this and returns both percentages.
How do I find protons, neutrons and electrons from mass number and atomic number?+
The atomic number (Z) equals the number of protons, and in a neutral atom it also equals the number of electrons. Neutrons = mass number (A) minus atomic number: n = A − Z. For an ion, subtract the charge from Z to get electrons. Carbon-14 (Z=6, A=14) has 6 protons, 8 neutrons, and 6 electrons if neutral.
What's the difference between mass number and atomic mass?+
Mass number is a whole number — the total count of protons plus neutrons in one specific isotope, like 14 for carbon-14. Atomic mass is a decimal value, usually the abundance-weighted average of all naturally occurring isotopes of an element, like 12.011 for carbon on the periodic table.
How do I convert between mass, moles and number of atoms?+
moles = mass ÷ molar mass, and atoms = moles × Avogadro's number (6.02214076 × 10²³). Reverse the steps to go from atoms or moles back to mass. The Mass ↔ moles ↔ atoms tab handles the conversion either way.
Is this tool free, and where does my data go?+
It's completely free and runs entirely in your browser — nothing you type is uploaded, logged, or sent to any server. No account, no sign-up, no limits.