Area of Circle Calculator

Calculator

Input type

Choose what value you have.

Value

Must be greater than zero.

Length unit

Used for radius/diameter/circumference.

π setting

Custom can match your syllabus.

Custom π (optional)

Used only when π setting is Custom.

Decimal places

0–12 decimals supported.

Area unit label

Label only; no unit conversion performed.

Show steps

Tip: For circumference, radius is r = C / (2π).

Formula used

Area: A = πr²
Diameter: d = 2r
Circumference: C = 2πr
Radius from circumference: r = C / (2π)

How to use this calculator

Select what you know: radius, diameter, or circumference.
Enter the value and your preferred unit label.
Choose a π option and set decimal places.
Press Calculate Area to show results above.
Download CSV or PDF from the result box.

Example data

Input type	Value	Unit	π	Computed area	Area unit
Radius	5	cm	π	78.5398	cm²
Diameter	10	m	3.14	78.5000	m²
Circumference	31.416	in	3.1416	78.5400	in²
Radius	2.75	ft	22/7	23.7857	ft²
Diameter	0.60	km	π	0.2827	km²

Example values are rounded for display.

FAQs

1) What is the area of a circle formula?

Area equals π times the radius squared: A = πr². Use radius in length units, and the area will be in squared units.

2) Can I calculate area using diameter?

Yes. The calculator converts diameter to radius using r = d ÷ 2, then applies A = πr² for the final area.

3) Can I calculate area from circumference?

Yes. It derives the radius using r = C ÷ (2π) and then computes A = πr². Your chosen π value affects both steps.

4) Does the tool convert between units?

No. Units are labels only. Enter values in a consistent unit system, and set the area unit label to match your squared unit.

5) Which π option should I choose?

Use built‑in π for best accuracy. Choose 22/7 or 3.14 for classroom work. Use Custom π when you need a specific value.

6) Why are my results slightly different from a textbook?

Differences usually come from rounding and the π approximation used. Increase decimal places or use built‑in π to match more precise calculations.

7) What are the extra outputs besides area?

The calculator also shows radius, diameter, and circumference. These help you cross-check steps and reuse values in other problems.

8) How do I export my result?

After calculating, use the Download CSV or Download PDF buttons shown in the result box. They include your inputs and computed values.