Circle Area Calculator
Area is π times the radius squared; circumference is 2π times the radius. Double the radius and the circumference doubles while the area quadruples.
Your numbers
Area
314.16cm²
Circumference
62.83cm
Diameter
20cm
Formula
Area = π × Radius², Circumference = 2π × Radius
If you have the diameter, halve it first — entering the diameter gives four times the real area.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Radius (cm)Centre to edge — half the diameter. | 10 | 0 and up |
Step by step
Quick reference table
Results when only Radius (cm) changes and everything else stays put.
| Radius (cm) | Area (cm²) | Circumference (cm) | Diameter (cm) |
|---|---|---|---|
| 5 | 78.54 | 31.42 | 10 |
| 7.5 | 176.71 | 47.12 | 15 |
| 10 | 314.16 | 62.83 | 20 |
| 15 | 706.86 | 94.25 | 30 |
| 20 | 1,256.64 | 125.66 | 40 |
What each result means
| Result | At default values |
|---|---|
| Area (cm²)The space a shape covers — length squared. | 314.16 |
| Circumference (cm)The distance once around a circle. | 62.83 |
| Diameter (cm)The longest line across a circle — twice the radius. | 20 |
Common mistakes
If you have the diameter, halve it first — entering the diameter gives four times the real area.
Glossary
- Radius
- Centre to edge — half the diameter.
- Area
- The space a shape covers — length squared.
- Circumference
- The distance once around a circle.
- Diameter
- The longest line across a circle — twice the radius.
Frequently asked questions
Q. How is Circle Area Calculator calculated?
Area = π × Radius², Circumference = 2π × Radius — Area is π times the radius squared; circumference is 2π times the radius. Double the radius and the circumference doubles while the area quadruples.
Q. Can you walk through an example?
With Radius 10cm, the answer is Area 314.16cm².
Q. What do I need to enter?
Enter Radius. The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Radius (cm) moves the answer to Radius (cm) 5 → Area (cm²) 78.54 and Radius (cm) 20 → Area (cm²) 1,256.64. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
If you have the diameter, halve it first — entering the diameter gives four times the real area.
Related calculators
Keep your length units consistent across inputs — mixing centimetres and metres throws results far off.