Bricks, mortar, cement and sand for the masonry of a chimney breast or fireplace surround — breast width, height and projection in; net area, unit count and mortar materials out. The safety-critical scope stays where it belongs.
How to Use the Fireplace Masonry Calculator
A projecting chimney breast has three brick faces: the front you see (width × height) and the two returns that run back to the wall (projection × height each). Measure the width and height at the widest points of the face, and the projection as the distance the breast stands proud of the wall behind — usually 300–500 mm on older houses. With Two side returns selected the calculator adds all three faces automatically; choose Face only for a breast that sits flat on the wall plane, or for counting just the visible face of a party-wall breast.
Deduct the openings honestly. The fire opening itself is the big one — width × height of the recess — plus any airing cupboards, flue accesses or air bricks that pierce the breast. In older houses the breast often steps in as it rises, gathering the flues toward the stack: measure each storey's width, height and projection separately, run the calculator once per section, and add the results. The shape above the ceiling is rarely the shape at the floor.
The unit fields follow the same module method as the brick and block calculator: one brick plus one joint in each direction, so a 215 × 65 mm metric brick laid with a 10 mm joint occupies a 225 × 75 mm module and covers about 59 per m². Most breasts are at least bonded into the main wall; if yours is a genuine double-skin construction, set the skin accordingly — it doubles the units and the mortar. Five percent waste covers the cuts that always land around a fire opening; push it toward 10% for complicated old breasts being matched in new brick.
The mortar side of the results uses the same chain as the mortar calculator: wall area converts to mortar volume (0.026 m³ per m² of single-skin brickwork at a 10 mm joint), the wet volume multiplies by the 1.33 dry factor, and the ratio splits it into cement and sand. The ratio is for general masonry around the fireplace — 1:4 to 1:6 covers the common bands, and the mix ratios guide explains the differences. What it must never model is the firebox or the flue: refractory brick and refractory mortar are specialist materials, specified by the appliance and installed by a registered installer.
What the numbers do not include: the hearth base (a volume job — the concrete slab calculator does it at whatever thickness the specification calls for), the making-good render or plaster after flue work (the render calculator), lintels, and any flue or chimney components at all. For the full sequence — professional scope first, then measuring, then counting — see the fireplace and chimney project hub.
The Math Behind the Calculator
Three faces become a net area; the module method turns area into units; the mortar constants turn area into materials. Every constant is the same one used across the site's masonry tools.
face = width × height
returns = 2 × projection × height (if included)
net_area = face + returns − openings
per_m² = 1 ÷ ((unit_length + joint) × (unit_height + joint))
units = ⌈ net_area × per_m² × skins × (1 + waste% ÷ 100) ⌉
mortar = net_area × skins × 0.026 × (joint ÷ 10) × (1 + waste% ÷ 100)
dry_volume = mortar × 1.33
cement_bags = ⌈ (dry_volume ÷ parts) × 1440 ÷ 25 ⌉
sand_m³ = dry_volume × n ÷ parts
The constants are the standard ones: dry ingredients bulk about a third more than finished wet mortar (factor 1.33), loose cement weighs 1440 kg/m³, loose dry sand 1600 kg/m³, and a single skin of brickwork at a 10 mm joint consumes about 0.026 m³ of mortar per m² of wall — scaled by the joint field. Blockwork runs leaner at roughly 0.012 m³ per m², which the presets handle automatically.
Worked Example: A 1.5 × 2.4 m Projecting Breast
A chimney breast 1.5 m wide and 2.4 m high, projecting 400 mm from the wall, with a fire opening of 0.6 m² to deduct. Standard metric brick with 10 mm joints, single skin, mortar at 1:5, waste 5%. Enter width 1.5, height 2.4, projection 0.4, returns “two”, opening 0.6.
- Face: 1.5 × 2.4 = 3.60 m². Returns: 2 × 0.4 × 2.4 = 1.92 m². Gross: 5.52 m².
- Net area: 5.52 − 0.6 = 4.92 m².
- Bricks per m²: 1 ÷ (0.225 × 0.075) = 59.3 — the familiar “about sixty”.
- Bricks: 4.92 × 59.3 × 1.05 = 306.2 → 307 bricks.
- Mortar: 4.92 × 0.026 × 1.05 = 0.134 m³ wet → × 1.33 = 0.178 m³ dry.
- Cement (1:5, parts 6): 0.178 ÷ 6 = 0.0297 m³ → × 1440 = 42.8 kg → 2 bags of 25 kg.
- Sand: 0.178 × 5 ÷ 6 = 0.149 ≈ 0.15 m³ — roughly a quarter of a bulk bag.
Sanity-check the brick count by hand: 4.92 m² at sixty to the metre is about 295, and five percent for cutting brings it past three hundred — matches. If the same breast were being built double-skin, every quantity except the area would double: about 614 bricks and twice the mortar. And if the breast steps in above ceiling height, that gathered section is its own calculation with its own width and projection — add the two passes, never average them.
Assumptions & Limitations
Every number above rests on these constants. If your project differs, change the matching input — or read the linked guide for the full reasoning.
- General masonry only — flue sizing, hearth construction, combustion air and structural work are professional, regulated territory
- Firebox and flue linings use refractory materials this tool does not model — they follow the appliance specification
- Dry factor 1.33, cement 1440 kg/m³, sand 1600 kg/m³, mortar 0.026 m³ per m² single-skin brickwork at a 10 mm joint (0.012 for blockwork) — standard constants as used across the site's masonry calculators
- Units per m² from the module method: unit plus joint in each direction — identical to the brick and block calculator
- Quantities only — no lintels, hearth base (see the concrete slab calculator), render (see the render calculator), flue components or labour