Fire Brick Calculator

How many fire bricks do I need?

This fire brick calculator divides the area you are lining by 0.281 square feet, the size of the broad face of a standard 9 × 4.5 inch fire brick. It then rounds up to a whole brick. The formula is bricks = (length × height) ÷ 0.281, which is about 3.56 bricks per square foot when laid flat. A 4 ft wide, 3 ft tall firebox back wall needs 43 bricks.

Enter each surface’s length and height in feet, or the total area, and the price per brick. Tick the 10% box for cuts.

Where 0.281 comes from

A standard straight fire brick is 9 × 4.5 × 2.5 inches. Lay it flat with the 9 × 4.5 face toward the fire and it covers 9 × 4.5 = 40.5 square inches. Divide by 144 to get 0.281 sq ft. Fire brick is set with very thin refractory joints, so the calculator does not add a joint allowance the way it does for building brick.

Laying bricks flat gives a 2.5-inch-thick lining. Setting them on edge gives a 4.5-inch lining with more heat mass, but it takes about 1.8 times as many bricks.

Worked examples

1. Firebox back wall: 4 ft × 3 ft, $4 per brick

• Area: 4 × 3 = 12 sq ft
• Bricks: 12 ÷ 0.281 = 42.7, rounded up to 43 bricks, costing $172.00
• With 10% extra: 47.0, rounded up to 47 bricks, costing $188.00

2. Small fireplace firebox, all faces

• Back wall: 3 ft × 2 ft = 6 sq ft
• Two side walls: 2 × (1.5 ft × 2 ft) = 6 sq ft
• Hearth floor: 3 ft × 1.5 ft = 4.5 sq ft
• Total: 16.5 sq ft (enter 16.5 × 1)
• Bricks: 16.5 ÷ 0.281 = 58.7, rounded up to 59 bricks
• With 10% extra: 65 bricks, costing $260.00

Splayed side walls are trapezoids. Use the average of the front and back widths as the length.

Fire brick sizes and coverage

Brick	Size (in)	Face toward fire	Bricks per sq ft
Standard straight, flat	9 × 4.5 × 2.5	9 × 4.5	3.56
Standard straight, on edge	9 × 4.5 × 2.5	9 × 2.5	6.40
Split, flat	9 × 4.5 × 1.25	9 × 4.5	3.56

Type	Properties	Typical use
Dense fire brick	Heavy, holds and radiates heat, resists abrasion	Fireplaces, pizza oven floors, wood stoves
Insulating fire brick	Lightweight, insulates, softer surface	Kiln walls, behind a dense lining
Splits	Half-thickness dense brick	Re-lining a worn firebox where space is tight

Splits cover the same face area as full bricks, so the count is the same. Check your supplier’s actual dimensions, since some sizes differ slightly.

How to measure a firebox or oven

• Measure the inside faces that see the fire: back, sides, and floor. Treat each as a rectangle, a trapezoid, or a triangle.
• For a dome oven, estimate the dome as half of a sphere. The inner surface area is 2 × π × r², where r is the inside radius in feet. A 3 ft wide dome has r = 1.5 ft, so 2 × 3.1416 × 2.25 = 14.1 sq ft.
• Floor bricks are often laid flat on an insulating base. Count them separately from the walls.
• Leave out the flue and door openings, but keep the waste allowance for the cut pieces around them.

Common fire brick mistakes

• Using building brick or standard mortar inside the firebox. Both break down under repeated heat.
• Laying fire brick on bare concrete. Without insulation underneath, an oven floor loses heat into the slab and takes hours longer to heat up.
• Joints that are too thick. Thick refractory joints are a weak point. Keep them thin and tightly fitted.
• Not curing the first fires. New masonry and mortar hold moisture. Small fires for the first few days drive it out slowly and prevent cracking.

To estimate weight on a hearth or stand, use the brick weight calculator. For the outer shell, use the brick calculator and the mortar calculator. For a full repair job, see the chimney repair cost calculator.

Fireplaces, wood stoves, and their chimneys must meet local fire and building codes for clearances and hearth size. Have a licensed mason or chimney professional check the build.