Circle Calculator
Get radius, diameter, area, and circumference from any one known value.
Written by Golam Rabbani, Founder & Lead Engineer
How to use this circle calculator
- Select which measurement you already know: Radius, Diameter, Circumference, or Area.
- Enter the known value as a positive number in the input field.
- Press Calculate to see all four circle properties at once.
- Use Copy all values to put the results on your clipboard, or Reset to start over.
About this circle calculator
The circle calculator takes any one known measurement of a circle and instantly computes the remaining three — radius, diameter, circumference, and area — so you never have to juggle multiple formulas at once.
All four properties are derived from the radius. If you enter the diameter, the tool divides it by 2 to get the radius (r = d ÷ 2). If you enter the circumference, it solves r = C ÷ (2π). If you enter the area, it solves r = √(A ÷ π). From that radius it then computes diameter = 2r, circumference = 2πr, and area = πr², rounding every output to six decimal places.
For example, entering a radius of 5 gives a diameter of 10, a circumference of 31.415927, and an area of 78.539816. Alternatively, entering that same area of 78.539816 as the known value recovers the original radius of 5 exactly. This makes the circle calculator useful for engineering layouts, geometry homework, DIY projects, and any task where you have one circle measurement and need the others.
FAQ
- What does the circle calculator compute?
- It computes all four fundamental circle measurements — radius, diameter, circumference, and area — from whichever single value you provide. Enter any one of the four and the tool fills in the rest.
- What formulas does the circle calculator use?
- All results are derived from the radius using the standard formulas: diameter = 2r, circumference = 2πr, and area = πr². When you enter a non-radius value the tool first converts it to a radius, then applies these three formulas.
- How accurate are the results?
- Each output is rounded to six decimal places using JavaScript's floating-point arithmetic and the built-in Math.PI constant (approximately 3.141592653589793). For most practical purposes the precision is more than sufficient.
- What happens if I enter zero or a negative number?
- The calculator requires a positive number. Entering zero, a negative value, or non-numeric text triggers an error message asking you to enter a valid positive number — no result is shown.
- Does this tool store my input data?
- No. The circle calculator runs entirely in your browser. Nothing you enter is sent to a server or saved between sessions — closing the page clears all values.
- Is the circle calculator free to use?
- Yes. It is completely free with no signup, no account, and no usage limit.