How to use the circle calculator
Fit the on-screen circle to a small round object, or enter a known diameter to calculate circumference and area. The constant π is the ratio of a circle’s circumference to its diameter. This tool uses JavaScript Math.PI for its calculations.
Calculate circumference and area
For diameter d and radius r, r = d ÷ 2, circumference C = πd, and area A = πr². A circle with a diameter of 4 cm has a circumference of approximately 12.57 cm and an area of approximately 12.57 cm². These numbers happen to match, but represent different quantities and units.
Measure a diameter on your screen
Calibrate your display, then align the left and right handles with the opposite edges of your object. The horizontal line through the center represents its diameter. Drag inside the circle to change its position. Place physical objects gently to avoid damaging the display.
Start with a known radius
Double the radius and enter the result in the diameter field. For a radius of 2.5 cm, enter a diameter of 5 cm. Results are displayed to two decimal places while the underlying calculation retains additional precision.
Match an arc from a larger circle
Hold the slider to the right to keep increasing the diameter; release to stop. Circles larger than the screen appear as a bowl-shaped arc. Calibrate the screen, then manually match a circular object to the curve. This estimates diameter and circumference under a circular-shape assumption. Nearly straight arcs are sensitive to small alignment errors. Images are not detected automatically.
A few common questions
Does pi change for different circles?
No. In the Euclidean plane, π is a constant. A circle’s diameter, circumference and area change with its size.
Can this determine my ring size?
The tool calculates geometric diameter and circumference. It does not replace a regional ring-size chart or a proper measurement of fit.