Circle Calculator

Circle Calculator works from whichever measurement you have. Radius, diameter, circumference and area each determine the other three, so you enter the one you know and read off the rest. Going backwards from area to radius -- dividing by pi and taking the square root -- is the step people most often need and least often remember.

The sector fields handle the part of a circle rather than the whole. An angle gives the arc length, the sector area and the chord across it, all as simple proportions of the full figures. Sixty degrees is a sixth of a circle, so a sixty-degree sector has a sixth of the area, and the working shows that explicitly.

Results are converted into square metres and square feet alongside your chosen unit, because a circular area is usually being measured in order to buy something: turf, paint, paving or fabric, all of which are sold by the square metre or the square foot depending on where you are.

The page shows its working, not just its answer. Where there is a formula it is printed with your own numbers substituted in, so you can check the result rather than take it on trust -- which is the difference between a calculator you can use for something that matters and one you cannot.

Everything runs inside your browser. What you type is never uploaded, never logged and never stored, and the calculator keeps working if your connection drops after the page has loaded.

• Shows radius, diameter, circumference, area and sector area together, so you are not running it twice
• A results table you can read across
• Prints the formula with your own figures substituted in
• Updates as you type, with no button to press and no page reload
• Copy the result in one click
• Free, no sign-up, and it runs entirely in your browser

The circle calculator is used by writers, developers, students, marketers and anyone else who needs the job done once without installing software. Common cases include:

• Finding the area of a circular patio or pond
• Getting the radius from a known area
• Working out the arc length of a curved section
• Converting a diameter into a circumference for materials