A circle is laid out from the inside
The blocks in a ring touch at their inner faces and the gaps open outward. So the count comes from the inner circumference divided by the inner face length: a 36 inch pit has an inner circumference of pi times 36, which is 113.1 inches, and at an 8 inch inner face that is 14.14 blocks. You cannot lay a seventh of a block, so it is 14, and the ring that closes perfectly with 14 blocks is 35.65 inches across rather than 36.
That third of an inch of diameter is 1.1 inches of circumference, and spread over fourteen joints it is 0.078 of an inch each, which nobody will ever see. The lesson is that the diameter is the adjustable number. Pick the diameter that suits the block rather than the one that sounded round, and the ring closes on its own.
| Inside diameter | Inner circumference | Blocks at an 8 in inner face | Exact-fit diameter |
|---|---|---|---|
| 24 in | 75.4 in | 9 | 22.9 in |
| 30 in | 94.2 in | 12 | 30.6 in |
| 36 in | 113.1 in | 14 | 35.7 in |
| 42 in | 131.9 in | 16 | 40.7 in |
| 48 in | 150.8 in | 19 | 48.4 in |
The taper is doing all the work
Here is the part that makes wedge-shaped wall blocks worth their price. The outer circumference of the ring is longer than the inner one by pi times twice the block depth. With an 8 inch deep block that is pi times 16, or 50.3 inches of extra distance to cover, regardless of how big the pit is. Spread across 14 blocks it is 3.6 inches per joint.
A tapered block absorbs that. If the outer face is 11-1/2 and the inner is 8, each block is 3-1/2 inches wider at the back, and fourteen of them cover 49 of the 50.3 inches needed. The 1.3 inches the taper does not cover, plus the 1.1 inches the inner face was already short, is 2.363 inches of outer joint spread over fourteen joints, or 0.169 of an inch each. The ring looks solid. Use a straight-sided block on the same pit and the taper contributes nothing, so the same 2.363 becomes 51.363 and every outer joint opens 3.669 inches, which is the width of a hand.
This is also why a bigger ring is more forgiving of a straight block: the extra circumference at the outside is a fixed 50 inches for that block depth, but there are more joints to spread it over. It never disappears, though, which is why straight retaining wall blocks make bad fire pits and tapered ones make good ones.
Courses, caps and what goes underneath
Courses are simple: the height you want divided by the block height, rounded up, because you cannot lay part of a course. Twelve inches of wall in 4 inch blocks is three courses. Where a ring differs from a straight wall is that the joints in the second course want to sit over the middle of the blocks below, which for a ring of an odd number of blocks does not work cleanly — an even count staggers more neatly.
Caps are counted at the middle of the wall rather than at either face, because that is where a cap block bears. The mid circumference is pi times the inside diameter plus the block depth. On a 36 inch pit with 8 inch blocks that is pi times 44, which is 138 inches, and at 12 inch caps that is 12 of them.
Underneath, the pad wants to be wider than the ring so the outer course is not sitting on the edge of the compacted material. The base gravel and the gravel or sand in the pit floor are both small numbers — under half a cubic yard between them on a normal pit — which is a case for bagged material rather than a bulk delivery. The gravel calculator covers bulk quantities if you are doing a patio at the same time.
The parts that are not arithmetic
Where a fire feature is allowed to go, how far it has to be from a house, a deck, a fence, an overhanging tree or the property line, and whether open burning is permitted at all are local rules. They vary between neighbouring towns and they change with the season and with fire conditions. The fire authority or building department is the place to settle that, before the first pallet is delivered.
Materials matter too, and this page cannot judge them. Whether a specific block is made to take direct flame contact is something the manufacturer states or does not, and there are plenty of concrete and stone products that are perfectly good in a wall and unsuited to the inside of a fire. Damp or porous stone in particular can fracture when it is heated hard. A steel insert is one common answer to that question, which is why the gap between the insert and the block is worth checking before you order either.
If the fire pit is part of a bigger hardscape, the paver patio calculator handles the surface it sits in, and the retaining wall block calculator covers straight runs of the same blocks for a seat wall alongside it.
Questions people ask
How many blocks do I need for a fire pit?
Divide the inside circumference by the inner face length of the block and round to the nearest whole number. A 36 inch pit is 113 inches around the inside, so at an 8 inch inner face it is 14 blocks per ring. Three courses of 4 inch blocks makes a 12 inch wall, which is 42 blocks, or 45 with a bit of waste, plus about 12 caps. Bigger pits scale directly: every extra inch of diameter adds pi inches of circumference.
Can I use straight retaining wall blocks for a round fire pit?
You can lay them, but the outer joints will be wide open. The outside of the ring is longer than the inside by pi times twice the block depth, which for an 8 inch deep block is 50 inches no matter what the diameter is. Spread over the 14 blocks of a 36 inch ring, and adding the small gap already present at the inner face, that is 3.669 inches per outer joint. Tapered blocks exist to absorb exactly that. If you only have straight blocks, a larger ring spreads the gap over more joints but never removes it.
What diameter should a fire pit be?
Practically, the diameter that closes neatly with the block you have bought, which the calculator reports. Beyond that it is a question of how many people sit around it and how much space the patio has, not something with a right answer. What is worth checking before committing is the clearance the local fire rules require to the house, the deck, fences and anything overhanging, because that can constrain the position more than the size.
Do I need a steel ring insert?
That depends on the block, and it is the manufacturer who answers it rather than a calculator. Some concrete products are made for direct fire contact and some are not, and stone that holds moisture can fracture when heated. An insert is a common way to keep flame off the masonry and it also gives the ring a defined inside diameter. If you are fitting one, check the gap between its outside diameter and the inside of the block before ordering, which is what the insert field here is for.
How deep should the base under a fire pit be?
The base gravel depth is a field here rather than a number because it depends on the soil and on what the surrounding patio is built to. What matters more than the depth is that the pad extends past the outside of the block on every side, so the outer course is not bearing on the crumbling edge of the compacted material. On a 36 inch pit with 8 inch blocks and 6 inches of overhang, the pad is 64 inches across, which is about 22 square feet.