Minecraft circle generator

Reading the grid

Each square is one block seen from above. Outline mode gives you a ring — the wall of a tower, the kerb of a pond — and the block count is what one layer costs. Multiply by height for the whole build.

Fill mode gives a disc, which is what you want for a floor, a roof, or the widest slice of a dome.

Why circles are hard on a grid

There is no such thing as a circle on a square grid, only an approximation, and which approximation you get is a choice made by whoever wrote the algorithm. This one tests the centre of each block against the ellipse equation, which keeps the result symmetrical in both axes for odd and even diameters alike.

The practical consequence is that some diameters simply look better than others. Odd numbers have a true centre block. Certain sizes — 9, 11, 21 — land cleanly on the grid and read as convincingly round; others sit between two approximations and look slightly polygonal whatever you do.

Building from it

Count from the centre outward rather than following the edge around. Place the four axis points first, then fill the quadrants — they are identical, so you only need to read one eighth of the grid and mirror it. Getting one quadrant right and copying it is much faster than walking the circumference, and far less likely to drift by a block.

Above about 50 across, use WorldEdit. This tool will tell you what the shape should be; placing three thousand blocks by hand is a different problem.