Circle Calculator
Calculate a circle’s radius, diameter, circumference and area from any one known value. Shows the formulas with your numbers, a labelled circle diagram, adjustable decimal places and a custom unit label, all in your browser.
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About the Circle Calculator
Circle Calculator works out all four basic measurements of a circle from whichever one you know. Choose radius, diameter, circumference or area, enter the value, and the tool finds the radius first and then calculates the other three using d = 2r, C = 2πr and A = πr².
Each result shows its formula with your numbers substituted, so you can follow or check the working. A labelled diagram marks the radius and diameter, and a text description of the diagram is provided for screen readers. You can set a unit label such as cm, m, in or ft — lengths use that unit and the area uses the squared unit — and choose how many decimal places to display. π is used at full double precision, not 3.14.
How to use
- Choose which measurement you know: radius, diameter, circumference or area.
- Enter the value (area is entered in squared units).
- Type a unit label and choose the decimal places to display.
- Press Calculate to see all four measurements, the working and the diagram.
- Copy or download the summary, or press Reset to start again.
Key features
- Works from radius, diameter, circumference or area.
- Formula breakdown with your values substituted.
- Labelled circle diagram with a text description.
- Full-precision π and a 2–8 decimal place setting.
- Custom unit label with squared units for area.
- Copy Summary and Download Summary as plain text.
- Validation for empty, negative, zero and oversized values.
Best practices
- Measure the diameter across the widest point if the centre is hard to find.
- Keep all measurements in one unit; the label does not convert.
- Choose more decimal places for small parts, fewer for rough estimates.
- Remember area grows with the square of the radius — doubling r quadruples A.
- Round only the final answer, not the intermediate values.
Benefits
- Answers any of the common circle questions from one screen.
- Shows the working, which helps with homework checks and learning.
- Quick for everyday jobs such as round tables, garden beds or lids.
- Private: runs entirely in your browser.
Frequently asked questions
What formulas does the calculator use?
Diameter d = 2r, circumference C = 2πr (or πd) and area A = πr². When you start from circumference it uses r = C ÷ (2π), and from area r = √(A ÷ π).
Which value of π is used?
JavaScript’s Math.PI, 3.141592653589793, which is accurate to about 16 significant digits. The displayed results are rounded to the decimal places you choose.
Why is the area in squared units?
Area measures a surface, so a radius in centimetres gives an area in square centimetres (cm²). The tool adds ² to your unit label automatically.
Does the unit label convert anything?
No. The unit label is text only: it is attached to lengths, with ² for areas and ³ for volumes, so the results read correctly. Enter every measurement in the same unit. To change units, convert the inputs first, for example with the Unit Converter.
What values are accepted?
A positive number greater than zero, written plainly such as 5, 12.75 or 1,250. Lengths can be up to 1,000,000,000 and an area up to 10¹⁸ in the chosen unit. Negative, zero, empty and malformed values are rejected with a message.
Is the diagram to scale?
No. The circle is always drawn the same size so the labels are readable; the labels show your actual values.
How accurate are the results?
The tool uses standard double-precision arithmetic (about 15–16 significant digits) and the full-precision value of π. Results are then rounded to the decimal places you choose, so the last shown digit is rounded and a value computed from rounded figures by hand can differ slightly. Very large or very small values are shown in exponent form.
Is anything I enter saved or sent anywhere?
No. Every calculation runs in your browser on your device. Inputs, settings and results are not uploaded and are not saved in browser storage. Reset clears them from the page.