A pie chart turns numbers into slices of a circle. It's pretty, it's intuitive — and statisticians quietly hate it, because your eye is terrible at comparing angles.
A pie chart shows parts of a whole as wedges of a circle. Each slice's angle is proportional to its share of the total.
Budget breakdowns, market share, survey results, time use — anywhere you're showing how 100% divides up.
A category that's 25% of the total gets a 90° wedge.
A budget: Rent 50%, Food 25%, Transport 15%, Other 10%. What angle is the Food slice?
When does a pie chart fail?
With many small slices of similar size — say, 8 categories all near 10-15%. Your eye can't rank them. A bar graph would make the order obvious instantly.
Don't use a pie chart for data that doesn't sum to a meaningful whole. 'Favourite colour' percentages from a multi-select survey can exceed 100% — a pie chart of that is nonsense.
Rule of thumb: 5 slices or fewer, with clearly different sizes. More than that, or close sizes — switch to a bar graph.
- Pie slices show parts of a whole; angle = share × 360°.
- Best with few, clearly different categories.
- When in doubt, a bar graph is usually clearer.