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- Enter any one of the radius, diameter, circumference or area.
- Get all the other measurements at once.
- See exact answers in terms of π, such as 25π.
- Shows each formula used.
How to use the circle calculator
Choose what you know
Radius, diameter, circumference or area.
Enter the value and unit
For example a radius of 5 cm.
Read all the measurements
Radius, diameter, circumference and area, with the steps.
Circle formulas
Every measurement of a circle comes from its radius, the distance from the centre to the edge. Once you know the radius, you can find everything else.
The four circle formulas
Diameter: d = 2r
Circumference: C = 2πr = πd
Area: A = πr²
Radius from area: r = √(A ÷ π), from circumference: r = C ÷ 2π
Example 1: radius 5 cm
Find everything else.
- Diameter 10 cm
- Circumference 10π ≈ 31.415927 cm
- Area 25π ≈ 78.539816 cm²
Example 2: area 100 m²
Work backwards to the radius.
- Radius √(100 ÷ π) ≈ 5.641896 m
- Diameter 11.283792 m
- Circumference 35.449077 m
Where the circle calculator helps
Homework
Check circle answers in decimals and in terms of π.
Gardens and patios
Find the area of a round bed or patio.
Crafts and sewing
Work out the edge length of a round tablecloth.
Pipes and wheels
Find the circumference from the diameter.
For other shapes, use the area calculator. Get more digits of π with the pi calculator, or find spheres and cylinders with the volume calculator.
Circle calculator FAQ
How do you find the area of a circle?
Square the radius and multiply by π. A circle with radius 5 has an area of 25π ≈ 78.54.
How do you find the circumference?
Multiply the diameter by π, or the radius by 2π. A radius of 5 gives about 31.42.
How do you find the radius from the area?
Divide the area by π and take the square root.
What is the difference between radius and diameter?
The diameter goes all the way across the circle through the centre, so it is twice the radius.
What does in terms of π mean?
The answer is left with π as a symbol, such as 25π, instead of a rounded decimal.
What units is the area in?
Square units. If the radius is in cm, the area is in cm².