Circle Area and Circumference Calculator
A circle is defined by its radius. Area is πr², circumference (the perimeter) is 2πr, and diameter is 2r. Enter a non-negative radius in any unit; the outputs share that unit (and unit² for area). π here is the usual IEEE Math.PI, not 22/7.
Area (πr²)78.5398
Circumference (2πr)31.4159
Diameter (2r)10
How this calculation works
Diameter = 2 × radius. If you were given a diameter, type half of it as the radius.
Circumference = 2 × π × r. That is the distance around, also called the perimeter of the circle.
Area = π × r × r. Doubling the radius quadruples the area because r is squared.
Units: r in meters → circumference in meters, area in square meters. No unit conversion is applied.
Worked example
Radius 1 → diameter 2, circumference 2π ≈ 6.2832, area π ≈ 3.1416. Radius 10 → area ≈ 314.16, circumference ≈ 62.83.
Frequently asked questions
Is π 3.14 or 22/7?
This uses Math.PI (about 3.141592653589793). 22/7 is a fraction approximation (~3.1429) that is fine for rough work, not this page.
What if I only have diameter?
Radius = diameter ÷ 2. Type that radius.
Arc length or sector area?
Not on this page. Sector area is (θ/360)×πr² for degrees.
Sphere surface?
A sphere’s surface is 4πr² — four times this circle’s area. Different object.
Can radius be zero?
Yes: area, circumference, and diameter are all 0.
Are inputs stored?
No. Browser only.
This tool is provided for general information only. Verify important figures independently. · Last reviewed: August 25, 2026