Count the ring the blocks are on
A fire pit is a circle of blocks, and the only hard part is deciding
which circle. It is not the hole in the middle and it is not the outside
of the wall. It is the ring through the blocks' own centres — the inside
diameter plus one block thickness — because that is the line each block
is sitting on.
mean diameter = inside diameter + block thickness
per course = round(π × mean diameter ÷ block length)
courses = ⌈wall height ÷ block height⌉
3 ft inside, 8 in thick, 12 in long, 4 in high
inside face 9.42 ft around
mean ring 11.52 ft around ← this one
outside face 13.61 ft around
12 a course × 5 courses = 60 blocks
Rounded, not rounded up
Every other quantity on this site rounds up, because you cannot buy four
fifths of a bag. A course of a fire pit is the exception: it is a closed
ring, so the count that fits is the nearest whole number and the
joints open or close a fraction of an inch to absorb the difference.
That is exactly what a tapered fire pit block is shaped to do. Rounding
up here would add one block per course that has nowhere to go.
Blocks for an 18 in high pit, 4 in courses
From this calculator, on 8 in thick block. Note how the count follows the circumference: a pit one foot wider takes about three more blocks per course, not one.
Line it, or the blocks flake
Concrete wall block is not a firebrick. Heated and cooled repeatedly, the
moisture in it drives off and the face spalls — pieces pop off, and there
is no fixing it afterwards. Both manufacturers cited below say to line
the inside, either with fire brick or with a steel ring insert. The liner
does a second job as well: on a dry-stacked ring it holds the shape you
built.
Where it can go is a local question
This calculator prints no clearance distance and that is deliberate.
Belgard put a wood-burning feature at fifteen or twenty-five feet from a
structure and say in the same breath that jurisdictions differ; some
municipalities prohibit open wood fires entirely and gas is the only
permitted option. A number here would be wrong somewhere it mattered.
Check your municipality, check your HOA, and look up — eaves and
overhanging branches are what actually catch.
What lands where
- Courses are whole. The tool reports the height whole courses actually reach, which is rarely the height you asked for.
- The bottom course is bedded. Leave it dry so it can be levelled; glue upwards from the second.
- Base depth is yours. No manufacturer publishes one for a fire pit, so this asks instead of inventing.
- Stone inside keeps the fire off the base and lets rain drain. Enter a depth and the tool volumes it.
The rest of the patio
A fire pit is rarely the only thing going in. The
paver calculator handles the
area around it and the base under that,
gravel covers the
compacted stone under the pit itself, and if the pit is part of a low
wall or a seating ring, the
retaining wall calculator
and the
concrete block calculator
count the courses. For what you are going to burn in it, the
firewood calculator
works out what a cord actually is and which species is worth paying for.
Common questions
How many blocks do I need for a fire pit?
Measure the ring the blocks sit on, not the hole in the middle. That ring is the inside diameter plus one block thickness, and its circumference divided by the block's face length is the count per course. A 3 foot pit in 8 inch blocks runs on a 3.67 foot circle, so 12 blocks close a course, and 5 courses of 4 inch block makes 60 in total.
Why not just measure the inside of the pit?
Because the blocks are not on the inside. On the 3 foot example, the inside face is 9.42 feet around, the outside face is 13.61 feet, and the blocks themselves travel the 11.52 foot ring through their middles. Using the inside face leaves you short on every course; using the outside face buys blocks you cannot fit. The mean circle is the only one that counts them correctly, and it is why this tool asks for the block thickness.
What size should a fire pit be?
Three to four feet across the inside is the usual range for a garden pit, and the constraint is how close people can sit to something hot rather than anything structural. Smaller than about three feet and the fire is cramped and hard to feed; much larger and you need a bigger fire to feel it from the edge, which brings its own clearance problem. This tool does not default to a size, because a fire pit is furniture and its size is a decision rather than a calculation.
How far does a fire pit have to be from the house?
Further than most people put it, and the exact figure is local. Belgard put a wood-burning feature at fifteen or twenty-five feet from any structure and note in the same sentence that jurisdictions differ, so this calculator prints no distance. Check your municipality and, if you have one, your HOA — some places prohibit wood-burning outright and gas is the only option. Also look up: overhanging branches and eaves are what actually catch.
Do I need fire brick inside it?
Either fire brick or a steel ring insert. Concrete wall block is not made for direct flame contact — repeated heating and cooling drives moisture out of it, and it spalls, which means the face pops off in flakes. Both manufacturers whose instructions this page cites say to line the interior. The liner also holds the blocks in the shape you built them in, which matters on a dry-stacked ring.
Does the wall height have to land on a whole course?
Yes, and the tool tells you where it lands. Courses come in whole block heights, so an 18 inch wall in 4 inch block is not 18 inches — it is 20, because 5 courses is the nearest you can build without cutting every block in the top ring. Decide which side of your target to land on before you buy, not after.
What goes underneath it?
A compacted granular base with a setting bed on top. Techo-Bloc's published section calls for a one inch setting bed over compacted granular, and this calculator gives you the setting bed volume from the footprint. It does not print a base depth, because neither manufacturer publishes one for a fire pit specifically — Belgard say a standard paver base does the job. Enter the depth you are building to and the tool works out the stone.
Should the blocks be glued?
Above the first course or two, yes. Every manufacturer instruction here specifies a concrete or masonry adhesive between courses. A dry-stacked ring stays put under its own weight until somebody sits on the edge of it or a log rolls against the inside, and the top course is the one that moves. Leave the bottom course dry on its bed so it can be levelled, and glue upwards from there.
