The 1.5 IQR rule says a data point is an outlier if it is:
1.5 × IQR below the first quartile (Q1); or
1.5 × IQR above the third quartile (Q3).
In other words, if it's greater than Q1 + 1.5 × IQR or lower than Q1 - 1.5 × IQR, it is outside the lower and upper fences and, therefore, is an outlier. Read more