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Fire Pit Calculator Guide
Blocks for a round fire pit — counted on the centreline circumference, which is the only measurement that gives the right answer.
Blocks sit between the inside and outside faces, so count on the middle:
centreline diameter = inside diameter + block depth
A 36in pit with 4in blocks has a 40in centreline → circumference 125.7in → at 7in per block, 18 blocks per course.
Use the inside circumference and you get 17. Use the outside and you get 20. Both wrong.
Centreline, Not Inside or Outside
This is the whole trick, and it's why people come home with the wrong number of blocks.
A block spans from the inner face to the outer face. Its average position — where its middle sits — is on a circle whose diameter is the inside diameter plus one block depth. Count on the inside face and every block is squeezed; count on the outside and they're all gapped. The middle is where the block actually lives.
Trapezoidal Blocks Exist for This
A rectangular block can't form a tight circle — the inner faces collide before the outer faces touch, leaving wedge gaps.
Retaining-wall blocks are made trapezoidal, narrower at the back than the front, so they close up on a curve. Use the average of the two faces as your face length. If you're using straight rectangular blocks, expect visible gaps at the back of each joint, which is fine for a rustic look and not fine if you wanted crisp.
Never Seal It Airtight
This is the safety one and it gets people. A fire pit built as a solid, mortared, sealed ring will trap moisture in the blocks, and heated water turns to steam with nowhere to go.
Concrete and stone can spall — flake off explosively — when that happens. Leave the joints dry-stacked or leave deliberate air gaps at the base, and never build a sealed fire pit over wet ground or a solid concrete pad without drainage.
Use a Steel Insert Ring
Direct flame against concrete block will crack it eventually — thermal cycling opens hairline cracks that grow every burn.
A steel liner takes the heat and protects the masonry, and it's cheap relative to rebuilding. It's also what lets you use ordinary wall blocks rather than firebrick for the structure.
Sizing and Siting
36–44in inside diameter is the sweet spot: big enough for a real fire, small enough that people can sit close and talk. Wall height of 12–18in keeps the fire visible while containing it — taller and you're looking into a chimney.
Check your local rules before building. Many places set minimum distances from structures, fences and overhanging branches, and some ban open burning outright or during dry seasons.
Related: paver calculator, retaining wall calculator, gravel calculator.
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