Circle Calculator

This calculator works out the four key measurements of a circle - radius, diameter, circumference and area - from just one figure you already know. Enter the radius in the input box, in your preferred unit such as millimetres, centimetres, metres or inches, and the calculator instantly returns the diameter, the circumference and the area beneath it. It uses the standard formulas: diameter equals twice the radius, circumference equals two times pi times the radius, and area equals pi times the radius squared, using the true value of pi for accuracy. Figures update live as you type and round to a sensible number of decimal places. It suits geometry homework and practical jobs built around a circular shape: sizing a round garden bed or edging, working out pipe or tank dimensions, or estimating paint or turf for a circular area. Keep an eye on your units: the area comes out in square units, for example square metres if you entered the radius in metres, while the circumference and diameter share the same unit as the radius. If you only know the diameter, circumference or area and need the radius, divide the diameter by two, divide the circumference by two pi, or divide the area by pi and take the square root, then enter that figure to see everything else worked out for you.

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