What are quartiles?
Quartiles divide a sorted data set into four equal parts. Q1 (the first quartile, 25th percentile) marks the point below which 25 % of the data falls. Q2 is the median (50th percentile). Q3 (the third quartile, 75th percentile) marks the point below which 75 % of the data falls. Together with the minimum and maximum they form the five-number summary, a compact description of any distribution.
Interquartile range and outlier detection
The interquartile range (IQR) equals Q3 minus Q1 and captures the spread of the middle 50 % of the data. It is the basis of the standard outlier rule: any value below Q1 − 1.5 × IQR or above Q3 + 1.5 × IQR is flagged as an outlier. This calculator computes both fences automatically and lists every outlier so you can decide whether to investigate or exclude those points.
How to use this calculator
Enter at least four numbers separated by commas or spaces—decimals and negatives are accepted. Click Calculate quartiles to see Q1, Q2, Q3, IQR, mean, min, max, range, fences, and outliers. The sorted list is used internally; the result card gives you every statistic at a glance. The layout is responsive: on wide screens the input and result sit side by side; on phones they stack vertically for comfortable reading.
Practical applications
Researchers use quartiles to summarise experimental data without assuming a normal distribution. Financial analysts report quartile returns to compare fund performance. Teachers examine Q1 and Q3 of exam scores to understand how consistently students performed. Quality-control teams track the IQR of measurements to spot production drift. Box-and-whisker plots—one of the most common statistical charts—are built entirely from the five-number summary this calculator provides.
Tips
If the IQR is very small relative to the range, your data likely contains outliers—check the fences section. For small data sets (fewer than about 10 values) quartile estimates can be unstable, so interpret them with caution. This free tool runs entirely in your browser with no sign-up or download. Bookmark it for quick access whenever you need quartile statistics.