Circle radius and diameter
Circle radius and diameter: give any one of radius, diameter, circumference or area and the other three are computed at once; values are rounded to 6 decimals and an unknown quantity or non-positive value is rejected.
Runs in your browserEvery computation happens in your browser — your data never leaves this device.
Result
One quantity per line, ready to copy.
Enter a value to calculate
What this tool does
- Recover the diameter or radius from a measured circumference, e.g. when you wrapped a tape around a pipe.
- Work back from an area to the radius to check a drawing annotation against a real part.
- Convert freely between radius, diameter, circumference and area when making round parts.
Example
Input
2
Output
半径 r：2 直径 d = 2r：4 周长 C = 2πr：12.566371 面积 A = πr²：12.566371 长度单位 u，面积 u²，体积 u³
Given a circumference of 5: radius = 5/(2π) ≈ 0.795775, diameter ≈ 1.591549 and area ≈ 1.989437. Given an area of 12.566371 the radius comes back as 2, the diameter 4 and the circumference 12.566371.
Frequently asked questions
Which quantity should I pick?
Pick whichever one you measured: radius, diameter, circumference or area. After switching, the input is read as that quantity and the other three appear immediately.
Why is the radius 0.795775 for a circumference of 5?
C = 2πr, so r = C/(2π) = 5/6.283185 ≈ 0.795775. Six decimals are kept, so multiplying back may differ in the last digit — normal floating point behaviour.
What if the known quantity is invalid?
If the selector is not one of the four values (say a script corrupted it) the tool throws invalid-known instead of silently assuming a radius.
Keywords：半径直径circle radius直径计算周长反推半径几何计算圆的直径