A pie chart calculator does the arithmetic behind every pie chart: it turns your data into a percentage and an angle in degrees for each slice. Enter one category per line with its count, and the table under the chart shows the full working. The finished chart is there to check your hand drawing against.
The example is a classic school exercise: 30 students named their favorite fruit. Here is how to get from the counts to a pie chart you can draw with a compass and a protractor.
What is the formula for a pie chart?
Two formulas cover everything:
- Percent = value ÷ total × 100
- Degrees = value ÷ total × 360 (or percent × 3.6)
Work through the example step by step:
- Add up the total: 9 + 6 + 8 + 4 + 3 = 30 students.
- Work out the fraction for each fruit: apples 9/30, bananas 6/30, and so on.
- Multiply by 360 for the angle: 9/30 × 360 = 108°.
- Check the sum: 108 + 72 + 96 + 48 + 36 = 360°.
| Fruit | Students | Fraction | Percent | Degrees |
|---|---|---|---|---|
| Apples | 9 | 9/30 | 30% | 108° |
| Bananas | 6 | 6/30 | 20% | 72° |
| Grapes | 8 | 8/30 | 26.7% | 96° |
| Oranges | 4 | 4/30 | 13.3% | 48° |
| Strawberries | 3 | 3/30 | 10% | 36° |
| Total | 30 | 30/30 | 100% | 360° |
A shortcut for a total that divides 360 evenly: each student is worth 360 ÷ 30 = 12 degrees, so apples get 9 × 12 = 108°. Totals like 20, 24, 30, 36, 40, 60 and 90 make this mental math easy, which is why textbook exercises use them.
How do you draw a pie chart by hand?
- Draw a circle with a compass and mark the center.
- Draw a radius from the center straight up to 12 o’clock.
- Place the protractor on the center, line it up with the radius, and mark the first angle (108° for apples). Draw the second radius through the mark.
- Measure the next angle from the line you just drew (72° for bananas) and repeat until the circle is full.
- Label each slice with the category and the percent, or add a legend next to the circle.
A tip from math teachers: the last slice is never measured. Whatever is left must equal the last angle (36° here), and if it doesn’t, one of the earlier angles is off.
What if the angles have decimals?
Real data rarely divides into whole degrees. With 7 out of 23 answers, the angle is 7 ÷ 23 × 360 = 109.57°. Round each angle to the nearest whole degree, add them up, and if the total is 359 or 361, fix the largest slice by one degree. A protractor can’t show half a degree anyway, so this changes nothing a reader can see.
The table under the chart shows angles with one decimal, so you can see where the rounding happens.
How do you check a finished pie chart?
- The angles add up to 360° and the percentages to about 100%.
- The biggest number has the biggest slice. If grapes (8) look smaller than bananas (6), an angle was measured from the wrong line.
- A slice of 90° is a perfect quarter, and 180° is a straight line through the center. Use these as visual checkpoints.
Where can you practice more?
Press Random numbers in the tool for a new data set with the answers already worked out, or use the randomizer to make random charts for a whole worksheet. For a single share such as 35%, the percentage pie chart lists the degrees of common fractions. For survey data, the pie chart maker with percentages explains how to make labels add up to exactly 100%.
The calculator is part of the free pie chart tools from Fe/male Switch, a startup game that teaches women to build a company. The same arithmetic shows up in every startup budget and cap table, long after the last math test.
Published 26 October 2026 · by the Fe/male Switch team