How to read and build a box-and-whisker plot
Quick answer: A box plot (or box-and-whisker plot) is a compact summary of a distribution. The box runs from the first quartile (Q1) to the third quartile (Q3) with a line at the median; the whiskers extend to the most extreme values within 1.5 × IQR of the box, and any points beyond them are drawn as outliers.
A box plot is built directly from the five-number summary — minimum, Q1, median, Q3 and maximum. The box spans Q1 to Q3, so it covers the middle 50% of the data; its length is the interquartile range (IQR). The line inside the box marks the median. The whiskers stretch from the box to the furthest data points that still lie within 1.5 × IQR of the quartiles. Anything past the whiskers is plotted as an individual point — an outlier.
A wide box means the central half of the data is spread out; a narrow box means it is tightly clustered. The position of the median line shows skew: if it sits toward the bottom of the box, the data is right-skewed (a longer tail of high values); toward the top means left-skewed. Long whiskers or isolated outlier points flag extreme values worth investigating.
Find the five-number summary, draw an axis covering the range of the data, then draw the box from Q1 to Q3 with the median line inside. Add whiskers out to the last values within 1.5 × IQR, and mark any remaining points as outliers. The box plot maker does all of this automatically and lets you download the chart.
Box plots shine when you place two or more side by side. Because each one compresses a whole distribution into the same simple shape, differences in center (median lines), spread (box and whisker lengths) and skew jump out immediately — for example, comparing test scores between classes or delivery times before and after a change.
The box spans the first quartile (Q1) to the third quartile (Q3), covering the middle 50% of the data. Its length is the interquartile range.
The whiskers reach to the smallest and largest values that are still within 1.5 × IQR of the quartiles. Points beyond them are shown as individual outliers.
A modified box plot is the common version that separates outliers out as individual points rather than extending the whiskers to the true minimum and maximum.