Atomic Mass Calculator
Look up atomic masses and calculate molecular mass from formulas
Calculator
Composition Breakdown
Molecular Mass
- Molecular mass = sum of atomic masses
- Use element symbols (case-sensitive: Na, not NA)
- Subscripts as numbers: H2O, not H₂O
- Parentheses supported: Ca(OH)2
- 1 mole = 6.022 × 10²³ particles
How to Use
Look up atomic masses or calculate molecular mass
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Select element lookup or molecular mass calculator
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Search for element or type molecular formula (e.g., H2O)
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See atomic/molecular mass with composition breakdown
Molecular Mass
Molecular mass = Sum of (atomic mass x count) for all elements
Sum the atomic masses of all atoms in the molecular formula.
Frequently Asked Questions
Atomic mass (atomic weight) is the average mass of an element's atoms in atomic mass units (amu or u). It accounts for all naturally occurring isotopes weighted by abundance. Carbon's atomic mass is 12.011 u because it is mostly C-12 with some C-13.
Add up the atomic masses of all atoms in the molecule. For H2O: 2(1.008) + 15.999 = 18.015 g/mol. For C6H12O6 (glucose): 6(12.011) + 12(1.008) + 6(15.999) = 180.156 g/mol. Our calculator parses formulas automatically.
Mass number is the total protons + neutrons in a specific isotope (always a whole number). Atomic mass is the weighted average across all isotopes (usually not whole). Carbon-12 has mass number 12, but carbon's atomic mass is 12.011.
A mole is 6.022 x 10^23 particles (Avogadro's number). The atomic/molecular mass in grams equals one mole. So 12.011 g of carbon contains 1 mole (6.022 x 10^23) of carbon atoms. This connects atomic scale to measurable quantities.