1. Pick an odd diameter
Odd sizes like 15, 21, 31, and 51 have a single center block, which makes the layout easier to mirror.
Minecraft circle building guide
How to Make a Circle in Minecraft
Pick a diameter, mark the center cross, place the numbered row outline, then mirror each quadrant until the circle closes.
Try a circle size21x21
Outline blocks: 56
2. Mark the center cross
Place temporary blocks in the center row and column. This gives you four equal quadrants.
3. Build one quadrant
Follow the chart for one corner first, then mirror the same pattern across the center lines.
4. Remove guide blocks
After the outline closes, remove any temporary center markers and fill floors or walls as needed.