How to Draw a Circle With HTML5 Canvas

By: O. Wolfson

Draw a circle in a cavas html element using the arc method.

An arc drawn to the HTML5 canvas.

ctx.arc(canvas.width / 2, canvas.height / 2, 100, 0, Math.PI * 2, false);
ctx.strokeStyle = "blue";

The arc canvas API method takes five (or optionally 6) arguments.

arc( x, y, r, sAngle, eAngle )

x The x-coordinate of the center of the circle, y The y-coordinate of the center of the circle, r The radius of the circle, sAngle The starting angle, in radians (0 is at the 3 o’clock position of the arc’s circle), eAngle The ending angle.

Note that the start / end angle of the arc is expressed in radians. That is why the end angle is 2*PI. 2*PI is equivalent to 360 degrees.

Here is a link to the HTML and JavaScript for this app. View page source to see the code.