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Circle Calculator

Enter any one measurement โ€” get radius, diameter, circumference, and area.

rr = 5
Radius (r)
5
Diameter (d)
10
Circumference (C)
31.4159
Area (A)
78.5398

Step 1: Radius given: r = 5

Step 2: Diameter d = 2r = 10

Step 3: Circumference C = 2ฯ€r = 31.4159

Step 4: Area A = ฯ€rยฒ = 78.5398

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Circle Formulas You Actually Need

Every circle measurement flows from the radius. Double it for the diameter, multiply by 2ฯ€ for the circumference, square it and multiply by ฯ€ for the area. Know any one of the four and you can unlock the rest โ€” that is exactly what this calculator does.

The Four Formulas

  • Diameter: d = 2r
  • Circumference: C = 2ฯ€r = ฯ€d
  • Area: A = ฯ€rยฒ
  • Going backwards: r = d/2 = C/(2ฯ€) = โˆš(A/ฯ€)

Where Circles Show Up

Wheels, pipes, pizzas, running tracks, and cylinders all reduce to circle math. Engineers size pipes by diameter, bakers scale recipes by area (a 12-inch pizza has 44% more area than a 10-inch one), and runners convert track laps via circumference.

Working with other shapes? Try our area and perimeter calculator.

Frequently Asked Questions

How do you find the area of a circle from the diameter?

Halve the diameter to get the radius, then use A = ฯ€rยฒ. For a diameter of 10, r = 5 and A = ฯ€ ร— 25 โ‰ˆ 78.54.

What is the formula for the circumference of a circle?

C = 2ฯ€r, or equivalently C = ฯ€d. A circle with radius 5 has circumference 2 ร— ฯ€ ร— 5 โ‰ˆ 31.42.

What is the difference between radius and diameter?

The radius runs from the center to the edge; the diameter runs edge to edge through the center and is exactly twice the radius.

Is this circle calculator free?

Yes โ€” every tool on Math Solver Free is free with no sign-up, and all calculations run in your browser.