Know one measurement of a circle? Enter it to get the radius, diameter, circumference and area, with the formula for each.
Circle formulas
| To find | From the radius r | From the diameter d |
|---|
| Diameter | d = 2r | – |
| Circumference | C = 2πr | C = πd |
| Area | A = πr² | A = π(d ÷ 2)² |
Circumference from the diameter
Multiply the diameter by π. A circle 10 inches across has a circumference of 10π ≈ 31.4159 inches. Its radius is 5 inches and its area is 25π ≈ 78.5398 square inches.
Working backwards
From the circumference: d = C ÷ π. A tree trunk 94.2 inches around is about 94.2 ÷ π ≈ 29.98 inches across. From the area: r = √(A ÷ π). A circle of 50 square feet has a radius of about 3.9894 feet.
How we checked this
Before every update, this calculator is run in a real browser with these known answers, and it must match all of them:
- Diameter 10: circumference 10π = 31.41592654 ✓
- Diameter 10: area 25π = 78.53981634 ✓
- Circumference 94.2: diameter 29.98479128 ✓
Last checked October 26, 2026. Spotted something wrong? The math is done in your browser with the formula shown on this page, so you can check any result by hand.
Questions people ask
How do you find the circumference with the diameter?
Multiply the diameter by π (about 3.14159). C = πd, so a 6 ft diameter gives about 18.85 ft.
What is half the diameter called?
The radius. It runs from the center to the edge, and the diameter is twice as long.
How do you find the diameter from the circumference?
Divide the circumference by π. 31.4 cm around is about 10 cm across.
Why is the area πr²?
Cut a circle into thin slices and lay them head to tail: they make a shape close to a rectangle half the circumference long (πr) and one radius high (r), so the area is πr × r = πr².