Outlier Detector (1.5 IQR)

About this calculator

The 1.5×IQR rule, popularised by statistician John Tukey, flags outliers in a data set using the interquartile range (IQR), the spread between the first quartile (Q1, the 25th percentile) and third quartile (Q3, the 75th percentile). It sets a lower fence at Q1 minus 1.5 times the IQR and an upper fence at Q3 plus 1.5 times the IQR, and any data point falling outside those two fences is treated as an outlier.

This method is a staple of exploratory data analysis and is the same rule that produces the whiskers on a box-and-whisker plot; it's widely used because, unlike rules based on the mean and standard deviation, it isn't itself thrown off much by extreme values, making it a robust first pass for spotting data-entry errors, sensor glitches, or genuinely unusual observations before deeper statistical analysis.

Enter a list of numbers and this calculator computes the quartiles, the lower and upper fences, and lists any values that fall outside them.