Floor Slab Calculator
Calculate concrete volume, gravel sub-base and wire mesh for a ground-floor slab on grade — patio, garage, shed or workshop.
What this calculates
Calculates slab area, concrete volume (with waste), gravel sub-base volume and estimated wire mesh area for a simple rectangular ground floor slab.
Formula used
Area = length × width. Concrete = area × thickness. Concrete with waste = concrete × (1 + waste %). Gravel = area × sub-base depth. Mesh = area × 1.08 (for laps and cuts).
Worked example
A slab of 5 m × 4 m × 10 cm: area = 20 m². Concrete = 2.0 m³. With 8% waste: 2.16 m³. Gravel (8 cm): 1.6 m³. Mesh: 21.6 m².
Typical slab thickness
Patios, sheds and light foot traffic: 10 cm (4 in). Garages: 12–15 cm (5–6 in). Driveways: 12–15 cm (5–6 in). Any vehicular or heavy load: consult a structural engineer.
When not to use this calculator
Elevated slabs, slabs on fill, slabs with post-tension, structural slabs and any slab carrying vehicles or heavy equipment require engineered design.
Frequently asked questions
How thick should a floor slab be?
10 cm (4 in) for light residential use. 12–15 cm for garages and driveways. Heavier loads require engineering.
Do I need a gravel sub-base?
Yes, strongly recommended. Typically 8–15 cm of compacted crushed stone improves drainage and prevents differential settlement.
Does a floor slab need wire mesh?
Recommended for crack control. It does not add structural capacity — for that you need rebar designed by an engineer.
How much concrete for a floor slab?
Length × width × thickness, add 8–10% waste. For a 5×4 m slab at 10 cm: 2.0 m³ + 8% = 2.16 m³.
Assumptions
- Wire mesh factor: 1.08 (8% extra for laps and cuts at edges).
- Gravel volume = area × sub-base depth, no compaction factor.
- Does not include polyethylene membrane, formwork, reinforcement bars or finishing.
- Single rectangular slab on grade only.
Slabs for vehicle traffic, ampliations, foundations or uncertain soil conditions must be reviewed by a structural engineer. Ground conditions (fill, clay, high water table) significantly affect slab design.
Common mistakes
- Not compacting the sub-grade before pouring.
- Forgetting to slope the slab for water drainage (minimum 1–2%).
- Pouring in extreme heat without curing compound or wet burlap.