Firebrick lining calculator.

Worked example

A 28 in wide, 32 in high firebox with 16 in deep sides and a lined floor, 1/8 in joints and 10% cut waste. One brick plus joint covers (9 + 0.125) × (4.5 + 0.125) = 42.2 sq in. Back: 896 / 42.2 = 21.2, so 22 bricks. Each side: 512 / 42.2 = 12.1, so 13 bricks, 26 for both. Floor: 448 / 42.2 = 10.6, so 11. Total 59, plus 10% = 64.9, rounded up to 65 bricks. Mortar: 65 / 45 = 1.44 gallons.

Method, sources and as-of date

Formulas: bricks per surface = ceil(surface area / ((9 + j) × (4.5 + j))); total = ceil(sum of surfaces × (1 + waste)); mortar gallons = total bricks / 45. The page also shows a geometric joint-volume figure, ((9 + j)(4.5 + j) − 9 × 4.5) × thickness per brick, as a lower bound; the maker's figure is higher because it covers brick faces too.

Sources (checked 2026-09-30): brick size 9 × 4-1/2 × 1-1/4 in from the Rutland Firebox Bricks page; mortar coverage of 1 gallon per 45 firebricks of 9 × 4-1/2 in, joints 1/4 in or less and 1/16 in minimum, from the Rutland Refractory Cement page. Other makers differ. A 2-1/2 in thick brick is not listed on those pages, so enter its thickness yourself from your brick's spec; the mortar figure is then only a guide.

Safety: this is a materials estimate only. Fireplace and oven construction, clearances, flue and chimney design are governed by the manufacturer's instructions and local building code. Have a qualified mason or inspector confirm the build. Domed ovens and curved or tapered fireboxes are not modelled; dry lay the bricks to confirm the count.

FAQ

What size is a split firebrick?

Rutland lists its split firebox bricks at 9 × 4-1/2 × 1-1/4 in, which is why this tool uses a 9 × 4-1/2 in face.

How much refractory mortar do I need?

The maker states 1 gallon covers 45 bricks of that face size at joints of 1/4 in or less. The estimate scales your brick count by that figure.

Why add cut waste?

Bricks are cut at corners and edges. 10% is a planning default, not a sourced figure; change it to suit your layout.