Circle Calculator

Calculate radius, diameter, circumference, and area from any one circle measurement.

By Konstantin Iakovlev · Updated September 2026 · Source: Khan Academy

Solve From

Radius

5.0000

Diameter

10.0000

Circumference

31.4159

Area

78.5398

Formulas Used

π (pi)3.14159265
C = 2πr31.4159
A = πr²78.5398

Use the Circle Calculator above to calculate your results. Enter your values and see instant results — all calculations run in your browser.

Disclaimer: This calculator is for informational purposes only. Results are estimates based on the information you provide and the assumptions described on this page.

How It Works

Give this tool a single measurement and it returns the rest: radius, diameter, circumference, and area of any circle. That reach makes it useful well beyond the classroom, spanning engineering and architecture down to weekend DIY work where dimensions have to be right.

Everything flows from a handful of geometric relationships. Knowing the radius (r), you get the diameter (d) as 2r, the circumference (C) as 2πr, and the area (A) as πr². Work backward just as easily: a known diameter gives radius d/2, a circumference gives C/(2π), and an area gives √(A/π).

Because π is irrational, the answers are approximations by nature; 3.14159 covers most practical work, though this calculator carries more digits for tighter accuracy. The classic slip is mixing up radius and diameter, an error that throws every downstream figure off, so confirm which measurement type you're entering before you read the results.

Example: Sizing a Circular Landing Pad

  1. 1 A circular landing pad needs an area of 200 square meters. What radius, diameter and circumference does that take?
  2. 2 Choose 'Area' under Solve From and enter 200. The calculator finds the radius as r = √(A/π) = √(200/π) = √63.662, then d = 2r and C = 2πr.
  3. 3 The calculator shows a radius of 7.9788 m, a diameter of 15.9577 m and a circumference of 50.1326 m, with the area back at 200.0000 as a check.
  4. 4 Rounded for a site plan: a radius of 7.98 m, a little under 16 m across, with about 50.13 m of edging around the rim.

Source: Khan Academy · Last updated: September 2026

Frequently Asked Questions

How do you find the area of a circle?
Area equals pi times radius squared (A = πr²). For a circle with radius 5, the area is π x 25 = 78.54 square units.
How do you calculate circumference from diameter?
Circumference equals pi times diameter (C = πd). For a circle with 10-inch diameter, the circumference is π x 10 = 31.42 inches.
How do you find the radius from the area?
Radius equals the square root of (area / pi). For an area of 100 square inches, the radius is sqrt(100/π) = sqrt(31.83) = 5.64 inches.

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