Interquartile Range Calculator

About this calculator

The interquartile range (IQR = Q3 − Q1) measures the spread of the middle 50% of a dataset, the difference between the third quartile (the value below which 75% of observations fall) and the first quartile (below which 25% fall). Because it ignores the extreme top and bottom quarters, IQR is far less sensitive to outliers than the full range or standard deviation, making it a robust measure of variability for skewed or messy data.

Statisticians use the IQR to draw box-and-whisker plots and to flag outliers with Tukey's fences: any value below Q1 − 1.5×IQR or above Q3 + 1.5×IQR is treated as a potential outlier, with 3×IQR marking "extreme" outliers. Data analysts, quality-control engineers screening for defective measurements, and researchers cleaning survey or experimental data all rely on this rule before running further analysis.