Probability and Statistics > Descriptive Statistics > How to Choose Bin Sizes in Statistics

Watch the video or read the steps below:

## Choose Bin size: Overview

There isn’t a formula to choose bin sizes in statistics (what is a bin?). However, there are a few general rules:

- Bins should be all the same size. For example, groups of ten or a hundred.
- Bins should include
*all*of the data, even outliers. If your outliers fall way outside of your other data, consider lumping them in with your first or last bin. This creates a rough histogram — make sure you note where outliers are being included. - Boundaries for bins should land at whole numbers whenever possible (this makes the chart easier to read).
- Choose between 5 and 20 bins. The larger the data set, the more likely you’ll want a large number of bins. For example, a set of 12 data pieces might warrant 5 bins but a set of 1000 numbers will probably be more useful with 20 bins. The exact number of bins is usually a judgment call.
- If at all possible, try to make your data set evenly divisible by the number of bins. For example, if you have 10 pieces of data, work with 5 bins instead of 6 or 7.

## Choose Bin size: Steps

Step 1: **Find the smallest and largest data point.** If your smallest and/or largest numbers are not whole numbers, go to Step 2. If they are whole numbers, go to Step 3.

Step 2: **Lower the minimum a little and raise the maximum a little.** For example, 1.2 as a minimum becomes 1, and 99.9 as a maximum becomes 100.

Step 3: **Decide how many bins you need** using your best guess and using the guidelines listed in the intro paragraph above.

Step 4: **Divide your range (the numbers in your data set) by the bin size **you chose in Step 3. For example, if you have numbers that range from 0 to 50, and you chose 5 bins, your bin size is 50/5=10.

Step 5: **Create the bin boundaries** by starting with your smallest number (from Steps 1 and 2) and adding the bin size from Step 4. For example, if your smallest number is 0 and your bin size is 10 you would have bin boundaries of 0, 10, 20…

**Tip:** If you have a large data set, you may want to use Excel to find the smallest and largest point. Type your data into a single column and then use the “Sort” function or type =MIN(A:A) in a blank cell in a different column (i.e. column B) and then type =MAX(A:A) to get the biggest number.