How many cinder blocks for a fire pit?

Concrete block makes the quickest fire pit there is: no cutting, no mortar, a couple of courses dry-stacked on a level base. The square, pinwheel layout is the easy way to get a closed ring from 16 in blocks.

How the pinwheel works

In a pinwheel, each side is a run of whole blocks that butts against the end of the next run, like the blades of a pinwheel, so no block needs cutting. That only closes if every side has the same number of blocks, so a course is always a multiple of four. With three blocks of 15⅝ in per side and blocks 7⅝ in deep, laid dry, each run is 46⅞ in: the inside is 46⅞ − 7⅝ = 39¼ in and the outside 54½ in. Two blocks per side gives a small 23⅝ in bowl (8 per course); four per side gives about 55 in inside (16 per course). Turn the pinwheel the other way on the next course so the joints don’t line up.

Two courses make a wall about 15 in high — a good height for a pit you sit around. Three courses (about 23 in) shelter the fire better but put it further below your feet.

Fire safety with block

Standard concrete block isn’t made for fire. Exposed directly and repeatedly to flames, it can crack or spall, and blocks with trapped moisture can do so suddenly. A steel fire ring or a lining of firebrick inside the blocks keeps the flames off them. Check local burn rules and setbacks before you build. The block itself is described on 8×8×16 block actual size, and round pits on how many blocks for a 36 inch fire pit and how many blocks for a 48 inch fire pit.