How it works
The calculator counts the blocks around one course from the inside size, multiplies by the number of courses, and adds caps and the gravel pad if you enter them.
Round – Blocks per course = ⌈ π × Inside diameter ÷ Block face length ⌉
Square – Blocks per course = 4 × ⌈ (Inside side + Block depth) ÷ Block length ⌉
Courses = ⌈ Wall height ÷ Block height ⌉
Blocks = ⌈ Blocks per course × Courses × (1 + Spare ÷ 100) ⌉
Outside size = Inside size + 2 × Block depth
Caps = ⌈ Centerline length ÷ Cap length ⌉
Gravel pad = Pad area × Pad depth
⌈ ⌉ means rounding up to whole blocks or courses. The centerline runs through the middle of the wall: π × (inside diameter + block depth) for a round pit, four times (inside side + block depth) for a square one.
Round rings and wedge blocks
Straight blocks set in a circle touch at their inside corners and leave wedge-shaped gaps on the outside. Wedge or trapezoid blocks close those gaps, but only at the ring size their taper was made for: RumbleStone’s trapezoid is 6½ in on the inner face, 10¼ in on the outer face and 7 in deep, which closes up at about 12 blocks to a ring roughly 24 in inside. For larger rings, kits add small spacer blocks between the wedges – RumbleStone’s 45.8 in kit uses 12 trapezoids and 12 minis in each course. The calculator counts one block size along the inside circle; for a kit, use its piece list.
Square pits
A square pit with pinwheel corners puts one block end against the side of the next at each corner, so every side is the inside length plus one block depth long. The granite square fire pit kit in the sources lays 12 blocks per course for its 21 in size and 16 for its 30 in size, with joints staggered course by course.
Pad, adhesive and liner
Kit instructions start from a level, compacted pad – Keystone builds 6 in of crushed stone in two 3 in layers, and a granite kit’s instructions make the pad about 6 in larger than the pit. Each course is glued to the one below with exterior construction adhesive. Keystone recommends lining the inside with a steel shield or high-strength brick or paver units to protect the blocks from direct heat. Where and how you may burn, and how far the pit must be from buildings, depends on local rules.
Worked example
Inputs
- Shape
- Round
- Inside diameter / inside side
- 24 in
- Block face length
- 6.5 in
- Block depth
- 7 in
- Block height
- 3.5 in
- Wall height
- 14 in
- Gravel pad depth
- 6 in
- Pad larger than the pit by
- 6 in
- Spare blocks
- 0 %
Result
- Blocks per course
- 12
- Courses
- 4
- Blocks in total
- 48
- Built wall height
- 14 in
- Outside diameter / outside side
- 38 in (3.17 ft)
- Gravel for the pad
- 5.28 cu ft (0.196 cu yd)
Common mistakes
- Using the long, outer face of a wedge block. Wedge blocks are counted on the inside face; the outer face is longer and gives too few blocks.
- Measuring the outside of the ring as the diameter. The inside diameter sets how many blocks go around; the outside is the inside plus twice the block depth.
- Building on loose soil. Kit instructions start with a level, compacted pad of gravel or crushed stone, so the courses stay level.
- Leaving the inside unprotected. Keystone’s fire pit instructions recommend lining the inside with a steel shield or high-strength brick or paver units to protect the wall blocks from direct heat.
- Skipping the adhesive. Kit instructions glue each course to the one below with exterior construction adhesive and stagger the joints.
Assumptions and limits
- Round pit – blocks per course = ⌈ π × inside diameter ÷ block face length ⌉, the blocks touching on the inside circle.
- Square pit – the corners are laid pinwheel style, so each side is the inside length plus one block depth, and blocks per course = 4 × ⌈ (inside side + block depth) ÷ block length ⌉.
- Every course has the same number of blocks; joints are staggered by shifting, not by different blocks.
- Courses = wall height ÷ block height, rounded up. Blocks are rounded up after any spare allowance.
- Caps follow the centerline of the wall. The gravel pad is a circle or square as wide as the outside of the pit plus the extra you enter.
- Fire rings, liners, grates, adhesive and sand are not included.
- Material quantities only. The pit’s location, clearances, liner and blocks suitable for fire are safety questions for the product maker, local rules and your fire department.
Frequently asked questions
How many blocks do I need for a fire pit?
Divide the inside circumference (π × inside diameter) by the block’s inside face length and round up for one course, then multiply by the number of courses. A 24 in ring of wedge blocks with a 6½ in inner face takes 12 per course; four 3½ in courses make 14 in and 48 blocks.
How many blocks are in a fire pit kit?
Kits list their pieces. RumbleStone’s Round Fire Pit No. 02, 45.8 in wide and 14 in high, uses 48 trapezoids and 48 minis; a granite square fire pit kit uses 12 blocks per course for its 21 in size and 16 for its 30 in size. A kit’s list beats any calculation.
How deep should the gravel base be?
Kit instructions give it. Keystone’s square fire pit builds a 6 in leveling pad of crushed stone in two compacted 3 in layers; the granite kit’s pad is about 6 in larger than the pit. Enter your own depth if your instructions differ.
Can I use retaining wall blocks for a fire pit?
Ask the block maker. Concrete blocks can crack or spall from direct heat, which is why Keystone recommends a steel liner or high-strength brick or paver lining inside, and says not to use fire brick as the liner. This calculator only counts the blocks.
Why is the outside of the ring bigger than I planned?
The outside diameter is the inside diameter plus two block depths. With 7 in deep blocks, a 24 in inside ring is 38 in across the outside – plan the space and the pad for that.
Sources
- RumbleStone Weekend Projects – Specifications & Pallet Information — Pavestone (hosted by The Home Depot). Nominal dimensions: Trapezoid 6 1/2–10 1/4 in L × 7 in W × 3 1/2 in H, Large 10 1/2 × 7 × 3 1/2 in, Medium 7 × 7 × 3 1/2 in, Mini 7 × 3 1/2 × 1 3/4 in; Round Fire Pit No. 01 45.8 in W × 10.5 in H: 36 Mini + 36 Traps; No. 02 45.8 × 14 in: 48 + 48.
- RumbleStone Project Building Instructions – Round Fire Pit No. 02 — Pavestone (hosted by The Home Depot). 45.8 in W × 14 in H; number of pieces per project: 48 Mini, 48 Trap; "Prepare stable, level project area"; "Stack stones, adding construction adhesive between layers"; measurements approximate.
- Keystone Building Ideas – Square Fire Pit Installation Instructions (Stonegate Country Manor) — Keystone Hardscapes (hosted by Pavestone). 24 Large, 32 Medium and 24 Small units plus 18 caps; leveling pad: "Add 3” of crushed stone … Repeat one time to complete the 6” leveling pad"; inside of the pit should be lined with a steel shield or high-strength brick or paver units; "Do no use fire brick as a liner material".
- Granite Square Fire Pit Kit Installation Instructions — Natural Concrete Products (hosted by The Home Depot). Level gravel or rock on compacted soil, "The leveling pad will be approximately 6” larger than the fire pit kit"; "The 21” fire pit kit will have 12 blocks per course, the 30” fire pit kit will have 16 blocks per course"; stagger joints, glue each course with construction adhesive.
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 6, 2026