Circle Area from Diameter
The circumference is simply π times the diameter. The area needs one extra step: halve the diameter into a radius first, then square it.
Your numbers
Area
314.16cm²
Circumference
62.83cm
Radius
10cm
Formula
Area = π × (Diameter ÷ 2)², Circumference = π × Diameter
Putting the diameter where the radius belongs gives four times the real area. Pipes and discs are almost always labelled by diameter.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Diameter (cm)The longest line across a circle — twice the radius. | 20 | 0 and up |
Step by step
Quick reference table
Results when only Diameter (cm) changes and everything else stays put.
| Diameter (cm) | Area (cm²) | Circumference (cm) | Radius (cm) |
|---|---|---|---|
| 10 | 78.54 | 31.42 | 5 |
| 15 | 176.71 | 47.12 | 7.5 |
| 20 | 314.16 | 62.83 | 10 |
| 30 | 706.86 | 94.25 | 15 |
| 40 | 1,256.64 | 125.66 | 20 |
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 |
| Radius (cm)Centre to edge — half the diameter. | 10 |
Common mistakes
Putting the diameter where the radius belongs gives four times the real area. Pipes and discs are almost always labelled by diameter.
Glossary
- Diameter
- The longest line across a circle — twice the radius.
- Area
- The space a shape covers — length squared.
- Circumference
- The distance once around a circle.
- Radius
- Centre to edge — half the diameter.
Frequently asked questions
Q. How is Circle Area from Diameter calculated?
Area = π × (Diameter ÷ 2)², Circumference = π × Diameter — The circumference is simply π times the diameter. The area needs one extra step: halve the diameter into a radius first, then square it.
Q. Can you walk through an example?
With Diameter 20cm, the answer is Area 314.16cm².
Q. What do I need to enter?
Enter Diameter. 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 Diameter (cm) moves the answer to Diameter (cm) 10 → Area (cm²) 78.54 and Diameter (cm) 40 → 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?
Putting the diameter where the radius belongs gives four times the real area. Pipes and discs are almost always labelled by diameter.
Related calculators
Keep your length units consistent across inputs — mixing centimetres and metres throws results far off.