Atomic Mass Calculator
Look up any element's standard atomic weight, or rebuild it from isotope masses and abundances.
What is Atomic Mass?
An atomic mass is the average mass of an element's atoms, weighted by how much of each isotope occurs in nature. Chlorine is 75.77% chlorine-35 and 24.23% chlorine-37, and the weighted average of those two masses is 35.45 u — the number on the periodic table. No individual chlorine atom weighs that, which is exactly the point: the table prints an average, and this tool shows both the average and the arithmetic that produced it.
How it works
The lookup tab reads the bundled 118-element table and returns the standard atomic weight, the atomic number and the name. The isotope tab multiplies each isotope's mass by its fractional abundance and sums the results, checking that the abundances sum to 100% (or 1) rather than assuming it — a mis-typed 75.7 is caught at the door. It also accepts element names, so 'chlorine' works as well as 'Cl'.
- Enter what you know. Every field recalculates as you type — there is no submit button. Leave the quantity you want blank and the page solves for it.
- Read the answer and the working. The result appears above the fold; underneath it is the formula with your numbers substituted, so the arithmetic can be checked rather than trusted.
- Check the units and the size. Units are shown on every value and selectable per field, so a result in kJ, kPa or km/h is converted rather than assumed. Sanity-check the magnitude against a known landmark such as 1 g/cm³ for water.
Examples
A textbook problem with a known answer
Every tool here is checked against standard example problems — the molar mass of C6H12O6 is 180.16 g/mol, and free fall from 20 m takes about 2.02 s at g = 9.8 m/s².
A result you want to reason backwards from
Leave the quantity you already know blank instead: given the final answer, most tools recover the starting value — the mass needed for a target concentration, or the height a fall must have had.
Common mistakes
Forgetting that a formula is only as good as its subscripts
H2O and H2O2 are different compounds. The formula field shows its parse — element by element — so a mis-typed subscript is visible before it reaches the arithmetic.
Mixing units in the middle of a calculation
The most common source of a wrong answer is a value in grams next to a formula expecting kilograms, or Celsius next to a gas constant expecting kelvin. This category converts on the way in and labels what it used.
Frequently asked questions
Why is chlorine's atomic mass 35.45 when no chlorine atom weighs that?
Because it is an average. Natural chlorine is 75.77% chlorine-35 (mass 34.9689 u) and 24.23% chlorine-37 (mass 36.9659 u), and the abundance-weighted average is 35.45 u. Enter that pair in the isotope tab and the tool rebuilds the number from its parts.
Should I enter mass numbers (35, 37) or the measured isotope masses?
The measured masses if you have them. With 35 and 37 the average comes out at 35.48 — close, but not the 35.45 on the table, because the mass number is a rounded count of nucleons and the real mass is slightly less than the sum of its parts. The textbook-answer difference is the lesson.
What if my abundances are fractions, not percentages?
Either works. The tool looks at the sum: near 100 means percentages, near 1 means fractions, and anything else is reported rather than guessed at.