A Minecraft circle generator answers a placement question: if the centre block is here, which other blocks form a ring of this radius. The outline on this page is the midpoint circle. Radius 8 with thickness 1 is the example. You count from the centre to the edge along a straight row of blocks, and that edge block is included. The table under the grid lists how many blocks each row contains, which is the number you tick off while placing a ring of stone or a ring of glass.
Thick mode unions several outlines. Thickness 2 is the radius you asked for plus the next smaller ring, so the wall is two blocks deep at the widest point and still has a hole. Keep raising thickness and the hole closes. That is the difference between a circular corridor and a filled plaza. Numbers are optional and only label the blocks in angular order so a shared screenshot has something to point at.
Players usually want a circle for a tower floor, a crop ring, a fountain, or a map-art guide. The PNG is the guide. Download it before you change the radius, because the file is whatever is on screen. Larger than about radius 30 is a lot of placing; the tool allows it, and the row count becomes more useful than the picture, which you will have to scroll.
This page starts in circle mode. Ovals are on the ellipse page, and round roofs are on the sphere and dome page. The same generator sits behind all three. A circle is the flat case, one layer, Y ignored.
Do not expect the ring to match a chart that uses a filled disc and then traces the outside by hand. A filled disc of radius 8 contains every block whose distance is inside the circle, which is more blocks than the outline. If you wanted the disc, set the thickness equal to the radius. If you wanted the wall, leave thickness at 1 or 2. Mixing those up is the usual reason two "radius 8" pictures disagree.
Count the radius the way the grid does. The centre cell is a block. Radius 8 means eight blocks out from that centre along a straight row, and the eighth block is part of the outline. The whole width of that row, including the centre and both sides, is seventeen blocks. People who measure a diameter of sixteen and then type 16 as the radius build a circle twice as wide as the room they marked. If you already counted the full width in blocks, subtract one and halve the rest: a seventeen-block width is radius 8. A wall you want outside that ring is thickness, not a larger radius. Thickness 2 on radius 8 keeps the same outer edge and adds one inner ring.
Build order on the ground is dull and reliable. Mark the centre with a different block, place the four axis ends, then fill one row using the count in the table and mirror it to the opposite side. The midpoint outline is symmetric, so the north row and the south row match. If they do not, you have drifted a block off the centre line. The PNG is worth saving at that moment, before you change radius and lose the picture. Circles above radius 20 are easier to place from the row counts than from a zoomed-out screenshot, because the screenshot stops being readable and the count does not.