Definition
A pie chart is a circular diagram in which a whole is divided into wedge-shaped slices, each slice representing a category. The angle of every slice is proportional to the share that category takes of the total: a category accounting for 25% of the data occupies a quarter of the circle, or 90 degrees of arc. The full circle always represents exactly 100% — the slices must sum to the whole.
The strength of the pie chart is its immediate communication of part-to-whole relationships. A glance at a well-constructed pie chart tells the reader that one segment dominates, or that two are roughly equal, without forcing them to read numbers off an axis.
Why it matters
How it works
Building a pie chart is a two-step arithmetic exercise. First, convert each category's count to a percentage of the total. Second, convert each percentage to degrees of arc by multiplying by 3.6 — since 100% corresponds to 360 degrees, 1% corresponds to 3.6 degrees. A category with a 30% share gets a 108-degree slice. Slices are then drawn around the circle, conventionally starting at the twelve-o'clock position and proceeding clockwise from largest to smallest, so the reader's eye lands on the dominant category first.
The visual weakness of pie charts is a perceptual one. Cleveland and McGill's classic ranking of graphical perception placed angle and area judgments well below position and length judgments — meaning readers extract numbers more accurately from a horizontal bar than from a pie slice. The practical implication is twofold: use pie charts when the goal is to communicate proportions to a non-technical audience, not when the audience must do precise comparisons; and never use them to compare more than five or six categories, because thin slices become indistinguishable. For data with many categories, an ordered bar chart almost always outperforms. The newer donut chart — a pie with the centre cut out — sacrifices the only real perceptual advantage of pies (an angle anchored at the centre) and is best avoided.