Area of a Circle Calculator
Find the area of a circle. Start from whatever you know (the radius, the diameter, the circumference, or the area) and see all the other measurements.
Area
78.5398 cm²
Check the highlighted fields to see your result.
- Radius
- 5 cm
- Diameter
- 10 cm
- Circumference
- 31.4159 cm
Results are estimates for information only and do not constitute financial, medical, or legal advice.
How it works
Everything about a circle depends on its radius. The calculator first works out the radius from the value you enter, then uses it to find the area, the diameter, and the circumference.
The formulas
Area = π × r²
Circumference = 2 × π × r · Diameter = 2 × r
Worked example
A round coaster has a radius of 5 cm.
- Area: π × 5² = π × 25 ≈ 78.54 cm².
- Diameter: 2 × 5 = 10 cm.
- Circumference: 2 × π × 5 ≈ 31.42 cm.
Frequently asked questions
What's the difference between the radius and the diameter?
The diameter is the distance straight across the circle through its center. The radius is half of that: the distance from the center to the edge. Measuring the diameter is usually easier, so the calculator lets you start from either one.
How do I find the area from the circumference?
Divide the circumference by 2π to get the radius, then use π × r². For example, a tree trunk with a 1.5 m circumference has a radius of about 0.239 m and a cross-section of about 0.179 m².
What value of π does the calculator use?
The full precision your browser supports, about 3.141592653589793. Rounding π to 3.14 gives slightly smaller results, which is fine for rough estimates.