Circle Calculator

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

By Konstantin Iakovlev · Updated April 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 and does not constitute tax, financial, or legal advice. Results are estimates based on the information you provide and current rates. Always consult a qualified tax professional or financial advisor for advice specific to your situation.

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. As smart city infrastructure and advanced manufacturing scale up through 2026, accurate circular component design carries more weight than ever for professionals and students alike.

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: Designing a new circular drone landing pad for the 2026 Paris Olympics

  1. 1 A new drone delivery service for the 2026 Paris Olympics requires circular landing pads with a total area of 200 square meters for their autonomous drone fleet.
  2. 2 Input the area (A) as 200 m². The calculator will use the formula r = √(A/π) to find the radius. Then, d = 2r, C = 2πr.
  3. 3 The calculated radius is approximately 7.98 meters, the diameter is 15.96 meters, and the circumference is 50.15 meters.
  4. 4 These measurements provide the exact specifications needed for constructing the landing pads, ensuring they meet the required area for safe drone operations during the Olympic Games, optimizing space and material usage for an estimated construction cost of $15,000 per pad.

Source: Khan Academy · Last updated: April 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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