Circle Calculator
Enter any one of radius, diameter, circumference, or area and get the other three at once.
Known measurement
Use any unit. Length results share that unit, and area is in that unit squared. Everything runs in your browser. Nothing is uploaded.
Decimal places
All four measurements
Calculated with π ≈ 3.14159265.
Everything is calculated in your browser. Nothing is uploaded.
How to calculate a circle
Pick your known measurement
Choose whether you already know the radius, diameter, circumference, or area of the circle.
Enter the value
Type a positive number for that measurement in any unit you like.
Read the other three
The remaining three measurements are computed instantly and shown together in the results.
Copy the results
Click the copy button to put all four values on your clipboard as a plain summary.
Why use this tool
Start from any measurement
Provide the radius, diameter, circumference, or area, and the calculator solves for whichever three you did not enter.
All four values together
Radius, diameter, circumference, and area are shown side by side, with the value you typed clearly marked.
Works backward from area
Enter an area and it recovers the radius by dividing by pi and taking the square root, then fills in the rest.
Adjustable precision
Switch between 2, 4, and 6 decimal places to keep the answers as short or as exact as you need.
Guards bad input
Empty fields, zero, and negative numbers are caught with a clear message instead of a broken or blank result.
Runs entirely in your browser
Every calculation happens on your device. Nothing you type is uploaded.
About this tool
A circle is defined by a single measurement, so knowing any one of its radius, diameter, circumference, or area is enough to work out the rest. This calculator does exactly that. Pick which measurement you already have, type its value, and it derives the other three at once using the standard relationships: the diameter is twice the radius, the circumference is pi times the diameter, and the area is pi times the radius squared.
It works from whichever value you have. Enter a radius and you get diameter, circumference, and area. Enter an area instead and it works backwards to the radius by dividing by pi and taking the square root, then fills in the rest. Values are unit-agnostic, so the results come back in whatever unit you typed, with length measurements sharing that unit and area expressed in its square. You can dial the number of decimal places to keep the answers as tight or as precise as you need.
Everything updates the moment you type, with no button to press, and non-positive or empty input is caught with a clear message instead of a broken result. For general math beyond circles try the scientific calculator, and to move a result between units of length or area use the unit converter.
Frequently asked questions
- How do I find the area of a circle?
- Choose Radius as your known measurement and enter the radius. The area is computed as pi times the radius squared and shown alongside the diameter and circumference. If you know the diameter instead, pick Diameter and the radius is halved first.
- Can I calculate the radius from the area or circumference?
- Yes. Pick Area and enter the value to get the radius from the square root of the area divided by pi. Pick Circumference and the radius is the circumference divided by two pi. Either way the other measurements are filled in for you.
- What units does it use?
- The calculator is unit-agnostic. Enter a value in centimetres, inches, metres, or anything else, and the length results come back in the same unit while the area is expressed in that unit squared.
- What happens with empty, zero, or negative input?
- A circle needs a positive measurement, so empty, zero, and negative values are rejected with a short message rather than a wrong answer. Extremely large numbers that overflow are also flagged instead of showing a broken result.
- Is my data uploaded anywhere?
- No. Everything runs in your browser; nothing is sent to a server.
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