What is the difference between atomic mass and average atomic mass?

Atomic mass refers to the mass of a single isotope (e.g., carbon-12 is exactly 12 u). Average atomic mass is the weighted mean of all naturally occurring isotopes of an element, taking into account their relative abundances. This is the value shown on the periodic table.

Do I have to enter abundances as decimals or can I use percentages?

The Average Calculator accepts both. If you enter 75.77 it treats it as 75.77 %, converts it to 0.7577 internally, and computes the weighted mean correctly.

How many isotopes can the tool handle?

The tool can handle any number of isotopes—from two up to hundreds. There is no practical limit; it simply processes every value you enter.

Can I save my isotope data for later?

The tool itself does not save data, but you can bookmark the page with your numbers in the URL (after clicking “Share”) or copy the results and paste them into a spreadsheet or document for safekeeping.

Is the Average Calculator free to use?

Yes, the Average Calculator is completely free. There are no ads, no sign-ups, and no limits on how often you can use it.

What if my abundances don’t add up to 100 %?

The tool will still compute a weighted mean, but it will display a warning so you can double-check your data. Abundances should always sum to 100 % (or very close due to rounding).

How to Calculate Average Atomic Mass in Minutes

The average atomic mass of an element is the weighted mean of the masses of its naturally occurring isotopes, where each isotope’s contribution is proportional to its natural abundance. The formula is: average atomic mass = Σ (isotope mass × abundance as a decimal). For example, chlorine has two stable isotopes: chlorine-35 (mass 34.969 u, abundance 75.77 %) and chlorine-37 (mass 36.966 u, abundance 24.23 %). Multiplying each mass by its abundance and adding the results gives the element’s average atomic mass, which appears on the periodic table.

Calculating this manually for elements with three or more isotopes can be tedious and error-prone. You must convert each percentage to a decimal, multiply by the isotope mass, sum the products, and round to the correct significant figures. A single misplaced decimal or arithmetic slip can throw off the final value. That’s why chemists and students turn to the Average Calculator on Lizely. This free online tool handles the entire calculation instantly: you paste or type the isotope masses and abundances, and the tool returns the weighted mean, median, mode, sum, count, minimum, maximum, and range—all in real time. There’s no need to reformat data or remember formulas, and the results can be copied directly into lab reports or homework.

In the following sections you’ll see exactly how to prepare your isotope data, enter it into the tool, and interpret the results. We’ll also compare the tool’s output with a manual calculation for the same element, so you can verify the accuracy and speed for yourself.

What Isotopic Data You Need

To calculate average atomic mass you need two pieces of information for each isotope:

The exact mass of the isotope (in atomic mass units, u).
The natural abundance of the isotope (as a percentage).

These values are published in periodic tables, isotope databases, and lab manuals. For example, magnesium has three stable isotopes:

Isotope	Mass (u)	Abundance (%)
Magnesium-24	23.985	78.99
Magnesium-25	24.986	10.00
Magnesium-26	25.983	11.01

Before you enter the data, check that the abundances add up to 100 % (or very close, allowing for rounding). If they don’t, re-check your source or the values you transcribed.

How to Enter Isotope Data into the Average Calculator

Open the Average Calculator in any browser.
In the input box, type or paste the isotope masses, one per line. For magnesium you would enter:
```
23.985
24.986
25.983
```
In a second box (or on the same line separated by commas), enter the corresponding abundances as decimals. For magnesium:
```
0.7899
0.1000
0.1101
```
The tool instantly displays the weighted mean (average atomic mass) alongside the median, mode, sum, count, minimum, maximum, and range.
Click the “Copy” button to grab the full summary for your report or spreadsheet.

Manual Calculation vs. Tool Results

To show how the tool matches manual work, let’s calculate magnesium’s average atomic mass step by step.

Formula: average atomic mass = (23.985 × 0.7899) + (24.986 × 0.1000) + (25.983 × 0.1101)

Step 1: 23.985 × 0.7899 = 18.946 u

Step 2: 24.986 × 0.1000 = 2.499 u

Step 3: 25.983 × 0.1101 = 2.861 u

Sum: 18.946 + 2.499 + 2.861 = 24.306 u (rounded to four significant figures)

The Average Calculator returns the same 24.306 u instantly, confirming the tool’s accuracy.

When to Use the Tool Instead of Manual Math

Use the Average Calculator whenever:

You have three or more isotopes—manual multiplication and addition become tedious.
You need to check your manual work for errors.
You want to include the calculation in a lab report or homework and need a clean, copyable summary.
You’re converting between different abundance formats (percent vs. decimal).
You’re working with large datasets from mass spectrometry or geological samples.

The tool also helps if you’re learning the concept: you can tweak the abundances and watch the average atomic mass update in real time, which reinforces how each isotope contributes to the final value.

Common Mistakes and How the Tool Prevents Them

Mistake	How the Tool Prevents It
Forgetting to convert percentages to decimals	The tool accepts either format; if you enter 75.77 it treats it as 0.7577.
Mismatched isotope and abundance lists	The tool counts entries and flags if the lists don’t match in length.
Arithmetic errors in multiplication or addition	The tool computes the weighted sum exactly, eliminating manual math slips.
Incorrect rounding or significant figures	The tool displays the full precision and lets you round as needed.
Omitting an isotope from the calculation	The tool shows the count of values, so you can verify all isotopes are included.

Beyond Average Atomic Mass: Other Uses for the Tool

While this article focuses on average atomic mass, the Average Calculator is versatile. You can also use it to:

Compute the mean, median, and mode of any dataset—grades, lab measurements, or survey responses.
Find the average return on an investment portfolio by entering percentage returns and their weights.
Calculate the average density of a mixture from component densities and mass fractions.
Determine the average molecular weight of a polymer sample from its chain-length distribution.

For more on basic statistics, see our guide How to Calculate Average: Mean, Median & Mode.

How to Cite the Tool in Your Work

If you use the Average Calculator in a lab report or homework, cite it as:

Lizely. (2026). Average Calculator: Mean, median, mode & more from any list of numbers, instantly. Retrieved from https://www.lizely.com/calculator/average-calculator/

For academic papers, check your institution’s citation style (APA, MLA, Chicago) and adapt accordingly.