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Atomic Mass Calculator

Look up the atomic mass of any element by its atomic number. Get the standard atomic weight in atomic mass units (amu) along with element details.

Enter an atomic number between 1 (H) and 118 (Og)

What is a Atomic Mass Calculator?

The Atomic Mass Calculator allows you to instantly look up the standard atomic mass of any element in the periodic table. Atomic mass (also known as atomic weight) is the weighted average mass of all naturally occurring isotopes of an element, expressed in atomic mass units (amu) where 1 amu is defined as exactly 1/12 the mass of a carbon-12 atom. The atomic mass listed on the periodic table reflects the natural abundance of each isotope. For example, chlorine has an atomic mass of approximately 35.45 amu because it exists as a mixture of Cl-35 (75.77%) and Cl-37 (24.23%). Understanding atomic mass is fundamental to chemistry calculations including molar mass determination, stoichiometry, and understanding nuclear properties. This calculator provides the standard atomic weight, element name and symbol, number of protons, and approximate number of neutrons for any of the 118 known elements.

Formula

A≈Z+NA \approx Z + N

The mass number A is approximately equal to the number of protons (Z) plus the number of neutrons (N). The standard atomic mass is a weighted average across all natural isotopes and may not be a whole number.

How to Calculate

  1. 1

    Enter the atomic number (Z) of the element you want to look up, between 1 and 118.

  2. 2

    Click Calculate to retrieve the element's standard atomic mass.

  3. 3

    The result shows the element name, symbol, atomic mass in amu, and the approximate number of protons and neutrons.

  4. 4

    The atomic mass displayed is the IUPAC standard atomic weight based on natural isotope abundances.

Worked Examples

Atomic Mass of Hydrogen (Z=1)

Input: Atomic Number: 1

  1. Element: Hydrogen (H)
  2. Atomic Number: 1
  3. Standard Atomic Mass: 1.008 amu
  4. Protons: 1, Neutrons: 0

Result: 1.008 amu (Hydrogen)

Atomic Mass of Iron (Z=26)

Input: Atomic Number: 26

  1. Element: Iron (Fe)
  2. Atomic Number: 26
  3. Standard Atomic Mass: 55.845 amu
  4. Protons: 26, Neutrons: ~30

Result: 55.845 amu (Iron)

Atomic Mass of Gold (Z=79)

Input: Atomic Number: 79

  1. Element: Gold (Au)
  2. Atomic Number: 79
  3. Standard Atomic Mass: 196.97 amu
  4. Protons: 79, Neutrons: ~118

Result: 196.97 amu (Gold)

Frequently Asked Questions

Atomic mass (or atomic weight) is the weighted average mass of all naturally occurring isotopes of an element, measured in atomic mass units (amu). One amu is defined as 1/12 the mass of a carbon-12 atom.
Mass number is the total number of protons and neutrons in a specific isotope (always a whole number). Atomic mass is the weighted average across all natural isotopes (usually not a whole number).
Atomic mass is a weighted average of all naturally occurring isotopes based on their relative abundances. Since elements typically have multiple isotopes in different proportions, the average is almost never a whole number.
An atomic mass unit (amu), also called a dalton (Da), is defined as exactly 1/12 the mass of a carbon-12 atom. It equals approximately 1.66054 × 10⁻²⁷ kg.
For a specific isotope, subtract the atomic number (protons) from the mass number (nearest whole number of the atomic mass) to get the number of neutrons: N = A - Z.

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