Atomic Mass Calculator

Calculate atomic mass from protons and neutrons

Get atomic mass in multiple units: atomic mass units (u), kilograms, electron rest masses, proton rest masses, and neutron rest masses

Last updated: December 5, 2025

CreatorFrank Zhao

Number of protons (Z)

Number of neutrons (N)

Atomic mass

mₚ

Atomic mass (SI)

×10⁻²⁵

Mass number

Introduction / Overview

In many problems, “atomic mass” is used in two different ways: the integer mass number $A$ for a specific isotope, and the decimal “average atomic mass” shown on the periodic table.

Rule of thumb: for a specific isotope, start with $A=Z+N$ .

Related tools:

For weighted-average periodic-table style problems, use Average Atomic Mass Calculator. For particle counts (protons/neutrons/electrons), use Atom Calculator.

How to Use (Quick Start)

Enter the number of protons $Z$ (atomic number).
Enter the neutron count $N$ .
Read the mass number $A$ and the approximate mass.
Use the kg conversion if you need SI units.

Quick note

If you only know an isotope name like “Carbon-14”, that gives you $A=14$ . Combine it with carbon’s $Z=6$ to get $N=8$ .

Worked Examples

Example: Carbon-14

$Z=6$ , $A=14$ → $N=14-6=8$ .

Example: Gold-197

$Z=79$ , $A=197$ → $N=118$ .

Example: Convert u to kg

For $A=12$ , a quick conversion is:

m

\approx

12\cdot 1.66054\times 10^{-27}

\ \mathrm{kg}

Common Scenarios / When to Use

Use this calculator when…

You have protons and neutrons and need $A$ fast.
You have an isotope label (like X-40) and want neutrons via $N=A-Z$ .
You need a quick conversion to kilograms.

Common confusion:

The periodic-table value is usually an average across isotopes, not a single isotope’s mass number.

Tips & Best Practices

Confirm the atomic number

The element identity comes from $Z$ .

Keep track of what’s approximate

For high precision, use tabulated isotope masses rather than integer $A$ .

Use average atomic mass for natural mixtures

If the question mentions natural abundance, you’re likely in weighted-average territory.

Write units in your notes

Distinguish $u$ and $\mathrm{kg}$ to avoid mistakes.

Calculation Method / Formula Explanation

Core relationship for a single isotope:

A = Z + N

Approximate kg conversion:

m_{\mathrm{kg}} \approx A\cdot 1.66054\times 10^{-27}

Average atomic mass (weighted)

\bar{m}

=

\sum_i

f_i

m_i

$f_i$ is fractional abundance; $m_i$ is the isotope mass.

Related Concepts / Background

Isotopes

Same $Z$ , different $N$ .

Mass number vs. atomic weight

Mass number is an integer for an isotope; atomic weight is often a weighted average (decimal).

Mass defect (advanced)

Binding energy makes the true mass slightly smaller than a simple sum.

Frequently Asked Questions (FAQs)

Is atomic mass the same as mass number?

Not always. $A$ is an integer; periodic-table atomic weights are often decimals.

Why ignore electrons?

Electrons contribute very little to total mass compared to nucleons.

How do I get neutrons from isotope notation?

Use $N=A-Z$ .

Limitations / Disclaimers & Sources

This guide is educational. For research-grade precision, use exact isotopic masses and authoritative references.

Suggested references

IUPAC/CIAAW tables for atomic weights and isotopic compositions
NIST reference data for physical constants (including atomic mass unit)

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