Minecraft is built from cubes, so a true circle is impossible. What you can build is a pixel circle: a ring of blocks that approximates the curve closely enough that it reads as round from a few blocks away.
The method that works
Do not eyeball it. Even experienced builders end up with lumpy, asymmetric rings that way, because the eye is bad at judging where a curve should step inward. Use a blueprint instead: our Minecraft circle generator takes a diameter and gives you the exact grid, the total block count, and the number of blocks in each row.
Then build it row by row. Lay out the widest row through the middle first, since that is the easiest to centre, and work outward symmetrically from there. Each quarter of the circle is a mirror of the others, so once one quarter is right the rest follows.
Size changes everything
Under 9 blocks across, a pixel circle looks like a diamond with the corners rounded. It is not a mistake in the blueprint, it is what a circle of that size has to look like on a grid. From about 13 the curve smooths out. At 25 and above it reads as genuinely round, even up close.
Odd diameters have a single centre block, which makes them easier to build outward from and easier to place a pillar or a beacon in. Even diameters centre on a 2×2. For towers and silos, odd numbers are usually the better choice.
Keeping your place
The most common failure on a large circle is losing count halfway round and stepping in one block too early. The generator lets you click each block as you place it, and it remembers your progress in the browser — close the tab, come back tomorrow, and the ring is still marked up where you left off.
Ovals, spheres and domes
Stretch it in one direction with the oval generator for arenas and race tracks. For three-dimensional work, the sphere and dome generators produce the same blueprints layer by layer, with the previous layer ghosted underneath so each ring lines up.