What it is
A pie chart divides a circle so that each slice’s angle — and so its area and arc — is proportional to that category’s share of the total. The whole circle is 100%.
Pies are good at one thing: showing that a part is about a half, a quarter or a third of the whole. They are poor at comparing slices of similar size, or comparing across several pies.
✓ Use it when
- You have two to six parts that add up to one meaningful whole.
- The message is a simple share: “more than half of our budget is rent”.
- The values are all positive.
✗ Avoid it when
- There are more than six slices — readers can’t tell the small ones apart. Use a bar chart.
- The slices are close in size and the differences matter.
- You want to compare several wholes — use a 100% stacked bar.
- The values don’t add up to a whole (survey questions where people could pick several answers, for example).
Tips for a better chart
- Start the largest slice at twelve o’clock and go clockwise, largest to smallest — unless the categories have an order.
- Label slices directly with name and percentage; skip the legend.
- Separate slices with a thin gap in the background colour rather than outlines.
- Fold tiny slices into “Other” — Chart Buffet does this automatically beyond eight.
The maths
A slice for value vi spans an angle θi = 360° · vi / Σ v. Area and arc length are proportional to the angle, so all three carry the same information.
Experiments disagree on just how bad pies are. Cleveland and McGill ranked angle judgements below position along a common scale (Cleveland & McGill 1984), while Spence and Lewandowsky found pies about as accurate as bars for judging proportions of a whole (Spence & Lewandowsky 1991). Both agree that comparing similar slices is hard.
A little history
William Playfair drew the first known pie charts in The Statistical Breviary (Playfair 1801), showing the territories of the Turkish Empire across three continents. In 1858 Florence Nightingale used a related polar-area diagram, popularly called the “coxcomb”, to show that soldiers in the Crimean War were dying more from disease than from wounds.
The pie’s reputation has swung ever since. Walter Crosby Eells tested circles against bars as early as 1926 (Eells 1926); the argument continues, but the rule of thumb has held: a few slices, one whole, one message.
Questions
How many slices can a pie chart have?
Up to about six. Beyond that, use a bar chart or fold the smallest into “Other”.
Pie chart or doughnut chart?
They show the same thing. A doughnut leaves room in the middle for the total, and judging arcs is about as accurate as judging angles.
References
- Playfair, W. (1801). The Statistical Breviary. London: J. Wallis (printed by T. Bensley).
- Eells, W. C. (1926). The relative merits of circles and bars for representing component parts. Journal of the American Statistical Association 21(154), 119–132.
- Spence, I. & Lewandowsky, S. (1991). Displaying proportions and percentages. Applied Cognitive Psychology 5(1), 61–77.
- Cleveland, W. S. & McGill, R. (1984). Graphical perception: theory, experimentation, and application to the development of graphical methods. Journal of the American Statistical Association 79(387), 531–554.