Interquartile Range Calculator

How it works

1. 1Enter your numbers separated by commas or spaces — the calculator sorts them and uses linear interpolation to find Q1 (25th) and Q3 (75th).
2. 2IQR = Q3 − Q1 measures the spread of the central half of your data, making it robust to extreme values unlike the total range.
3. 3Outlier fences follow Tukey’s rule: values below Q1 − 1.5·IQR or above Q3 + 1.5·IQR are typically flagged as outliers.

Use cases

• Detecting outliers in survey responses, test scores, or sensor readings before further analysis.
• Summarising the variability of a distribution in a way unaffected by extreme highs or lows.
• Box-plot construction — IQR defines the box length and the whisker endpoints.

Frequently asked questions

What does the IQR tell me?

IQR = Q3 − Q1 is the width of the interval containing the middle 50% of observations. A larger IQR means more spread; a smaller IQR means the central values are tightly clustered.

How is it different from standard deviation?

Standard deviation uses every value and is sensitive to outliers. IQR only uses the 25th and 75th percentiles, so a few extreme values barely change it — more robust for skewed data.

How do I identify outliers with the IQR?

Apply Tukey’s fences: lower = Q1 − 1.5·IQR, upper = Q3 + 1.5·IQR. Points outside these are mild outliers; beyond Q1 − 3·IQR or Q3 + 3·IQR they are extreme outliers.