Quick answer: A circle with radius 5 has diameter 10, circumference 2 × π × 5 = 31.415927 and area π × 5² = 78.539816.
How it's calculated
Everything follows from the radius r: the diameter is d = 2r, the circumference (perimeter) is C = 2πr = πd, and the area is A = πr².
Going the other way, r = d ÷ 2, r = C ÷ 2π, or r = √(A ÷ π). π (pi) is about 3.14159265; the calculator uses its full double-precision value.
Areas come out in square units: a radius in cm gives an area in cm². For other shapes see the area calculator.
Worked examples
- Radius 5: d = 10, C = 31.415927, A = 78.539816
- Circumference 100: r = 100 ÷ 2π = 15.915494, A = 795.774715
- Area 50: r = √(50 ÷ π) = 3.989423
Frequently asked questions
What is the difference between radius and diameter?
The radius goes from the centre to the edge; the diameter goes right across through the centre, so it is twice the radius.
How do I find the area from the diameter?
Halve the diameter to get the radius, then square it and multiply by π: A = π(d ÷ 2)² = πd² ÷ 4.
Can I use 3.14 or 22/7 for π?
They are approximations. The calculator uses π to about 15 digits, so its answers can differ slightly from hand calculations with 3.14.
What is the area of a circle with a 5 cm radius?
About 78.54 cm². Its circumference is about 31.42 cm.
How do I find the radius from the circumference?
Divide by 2π, roughly 6.28. A 100 cm circumference means a radius of about 15.9 cm.
How do I find the area of a semicircle?
Work out the full circle and halve it. A semicircle with a 5 cm radius is about 39.27 cm², and a quarter circle is half of that again.