molecalculator

Atoms & Isotopes

Isotope Atomic Mass Calculator: Mass of One Atom From amu

Calculator No. 07-0032 · Updated · Formula and assumptions shown below

Mass of one atom 1.993 × 10−26 kg
Mass of one atom in grams
1.993 × 10−23 g
= kg × 1000
Molar mass of the isotope
12.000000 g/mol
M = m(1 atom) × N_A
Isotope mass
12 u
Mass in unified atomic mass units, u
You enter
Isotope mass (u)
You get
Mass of one atom (kg), Mass of one atom in grams (g), Molar mass of the isotope (g/mol)
Rounding
3 dp · 6 dp

For study and lab planning. The result follows from the numbers you enter and the formula shown. Check it against your method before you weigh, mix or dilute anything, follow the safety data sheet and your supervisor, and do not use it for clinical, dosing or medication work.

This calculator converts the mass of an isotope in unified atomic mass units (u, also written amu or Da) into the mass of one atom in kilograms and in grams, and gives the molar mass of the pure isotope in g/mol. Enter the isotope mass from a nuclide table.

Formula

matom = m(u) × 1.66053906892 × 10−27 kg; M = matom × NA.

  • 1 u = 1.66053906892 × 10−27 kg, the CODATA 2022 recommended value of the atomic mass constant (one twelfth of the mass of a carbon-12 atom)
  • NA = 6.02214076 × 1023 mol−1, exact
  • M: molar mass in g/mol; numerically it equals the mass in u to better than one part in 109

Worked Example

A carbon-12 atom, whose mass is 12 u by definition of the unit.

  1. Mass in kg: 12 × 1.66053906892 × 10−27 ≈ 1.99264688 × 10−26, shown as 1.993 × 10−26 kg
  2. In grams: × 1000 = 1.993 × 10−23 g
  3. Molar mass: 1.99264688 × 10−23 g × 6.02214076 × 1023 mol−1 ≈ 12.000000 g/mol

Assumptions

  • The mass entered is the mass of the neutral atom (including its electrons), as nuclide mass tables list it.
  • The atomic mass constant is the CODATA 2022 value; earlier editions differ only in the last digits.
  • One isotope only: for an element as found in nature, use its average atomic weight instead.

How to read the result

The mass of one atom is tiny, about 2 × 10−26 kg for carbon-12, which is why chemists count atoms in moles. The molar mass in g/mol and the isotope mass in u are the same number for practical purposes, so 12 u per atom corresponds to 12 g per mole. To average several isotopes, use the weighted average atomic mass calculator.

Limits and common mistakes

  • Confusing the mass number (a whole count of protons and neutrons) with the isotope mass; only carbon-12 has a whole-number mass in u.
  • Dividing by the Avogadro constant once too often: g/mol ÷ NA gives grams per atom.
  • Mixing grams and kilograms: 1 u is 1.6605 × 10−24 g.

Frequently Asked Questions

How many kilograms is 1 amu?

1.66053906892 × 10^−27 kg (CODATA 2022).

What is the mass of one carbon-12 atom?

About 1.993 × 10^−26 kg, or 1.993 × 10^−23 g.

Is amu the same as u and Da?

Yes, the unified atomic mass unit (u) and the dalton (Da) are the same unit; amu is the older name.

Why is the molar mass equal to the mass in u?

The mole and the unit u are scaled so that N_A × 1 u is 1 g/mol to within about one part in a billion.

Formula used
in m | Isotope mass | u | 12 | 0
out kg | Mass of one atom | kg | m * 1.66053906892e-27 | 3
out g | Mass of one atom in grams | g | kg * 1000 | 3
out mm | Molar mass of the isotope | g/mol | g * 6.02214076e23 | 6

Each line is one input (in, sel, list) or one result (out: label, unit, formula, decimals). How the calculators work

Method: the standard formula for this calculation with the constants and conversion factors written on the page, rounded as each result states. See method and constants, the disclaimer or report an error.

More in this section

The whole Atoms & Isotopes section →