Enter a radius or diameter to find the area of a circle. Both measurements stay connected, even with different units. View the circumference and follow the calculation steps.
Start with one measurement, choose your units and follow the working to understand your answer.
Fill either field. The other updates automatically: diameter is twice the radius. Editing either measurement changes the circle.
Select the input unit before entering your number. Radius and diameter can use different units. Changing a unit afterward converts the measurement while keeping the same circle size.
Choose your output unit and two, four or six decimal places. Area uses squared units, such as cm²; circumference uses length units, such as cm.
Review the steps for A = πr², then select Copy Result. Use Try an Example to explore connected measurements or Clear to enter another circle.
Already have an area to convert? Use the square feet to square meters converter for ft² and m² values. In this calculator, enter a radius or diameter as a length.
Follow these two examples to see when to use the radius directly and when to divide the diameter first.
A circle has a radius of 5 cm. What is its area?
r = 5 cm
A = πr² = π × 5²
A = π × 25 = 25π cm²
Decimal answer rounded to four places.
A circular surface has a diameter of 12 ft. Find its area.
r = d ÷ 2 = 12 ÷ 2 = 6 ft
A = πr² = π × 6²
A = π × 36 = 36π ft²
Decimal answer rounded to four places.
Use the radius in A = πr², square it before multiplying, and write squared units in your answer. Using the diameter as the radius makes the calculated area four times too large. Try your own values in the calculator.
Choose the formula that matches your known measurement. Use consistent length units before calculating.
| Find | Using radius | Using diameter |
|---|---|---|
| Area (A) | πr² | πd² ÷ 4 |
| Circumference (C) | 2πr | πd |
| Radius (r) | Given radius | d ÷ 2 |
| Diameter (d) | 2r | Given diameter |
Since r = d ÷ 2, substituting into A = πr² gives A = π(d ÷ 2)² = πd² ÷ 4. Squaring the denominator gives 2² = 4. Explore Math Is Fun’s explanation of circle area and circumference for illustrated formula relationships.
An answer such as 25π cm² is exact. Its decimal form, 78.5398 cm², is rounded. If your question specifies 3.14 or 22/7 for π, use that value for your written working.
Ready to apply a formula? Enter your circle measurement to see the calculation steps and results in your chosen units.
Find answers about circle measurements, squared units, rounding and common calculation mistakes.
Divide the diameter by two to find the radius, then use A = πr². You can also use A = πd² ÷ 4 directly. For a diameter of 10 cm, the radius is 5 cm and the area is 25π cm², or approximately 78.5398 cm².
No. Enter either measurement and the calculator updates the other field automatically. Diameter is twice the radius. If you edit the second field, your latest entry becomes the measurement used to calculate the circle.
Yes. Each field has its own unit selector. For example, a radius of 50 cm corresponds to a diameter of 1 m. Changing an input unit converts that measurement while keeping the same circle size.
Area measures a two-dimensional surface. In A = πr², the radius is multiplied by itself, so centimeters become cm² and feet become ft². Circumference measures a distance around the edge, so it uses ordinary length units.
No. Doubling the radius makes the area four times larger, because (2r)² = 4r². The circumference doubles because C = 2πr depends directly on the radius.
Both 3.14 and 22/7 are approximations of π. This calculator uses the browser’s available precision for π, then rounds the displayed result. If a school question asks you to use a particular approximation, use that value in your written working.
Yes. First find the radius using r = C ÷ (2π), then enter that radius into this calculator. Alternatively, calculate the area directly with A = C² ÷ (4π). The current tool accepts radius or diameter as its inputs.
Calculate the full circle’s area and divide it by two: semicircle area = πr² ÷ 2. For a radius of 5 cm, the semicircle area is 12.5π cm², or approximately 39.2699 cm². This calculator displays the full circle area.
Enter a radius or diameter, choose your units and follow the calculation steps. Copy the area and supporting measurements when you’re ready.
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