IQR Calculator

About this calculator

The interquartile range (IQR) measures the spread of the middle 50% of a dataset: it is simply Q3 (the 75th percentile) minus Q1 (the 25th percentile). Unlike the standard deviation, which every extreme value pulls on, the IQR ignores the top and bottom quarters of the data entirely, which makes it a far more robust measure of spread when a dataset contains skew or outliers — a property statisticians call resistance.

The IQR’s best-known application is Tukey’s outlier fences: any value below Q1 − 1.5×IQR or above Q3 + 1.5×IQR is flagged as a potential outlier, the exact rule that draws the whiskers on a box-and-whisker plot. Data analysts use it to screen sensor readings or survey responses for anomalies, teachers use it to summarize test-score distributions without letting one exceptional score distort the picture, and quality-control engineers use it to spot production measurements that fall outside expected tolerances.

This calculator takes your Q1 and Q3 values and returns the IQR along with the lower and upper outlier fences, so you can immediately see which values in your dataset warrant a closer look.