Garage Floor Calculator
Works out the concrete for a garage floor. Like a driveway it carries a vehicle, so it is a 150 mm slab — but unlike a driveway it is enclosed, which changes what you do about damp and joints.
Includes the waste allowance — this is the figure to order.
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The formula
volume = length × width × thickness × (1 + waste)
A slab is a rectangular prism, so the volume is the three dimensions multiplied together. The only judgement in it is the waste allowance — concrete cannot be topped up once it starts curing, so under-ordering is far more expensive than over-ordering.
What to watch out for
What separates a garage floor from an outdoor slab:
- Damp-proof membrane is not optional. An enclosed slab with no DPM will sweat, and anything stored on it will rust.
- Point loads, not just weight. A trolley jack or an axle stand puts a car's weight through a few square centimetres. That is what 150 mm and reinforcement are for.
- Fall to the door. A gentle slope out means water off the car drains rather than sitting under it.
- Finish matters. A power-floated surface is easy to sweep and takes a floor paint; a brushed finish is grippier but holds dirt.
Frequently Asked Questions
How thick should a garage floor be?
150 mm (6 in) for a normal domestic garage. 200 mm (8 in) if you plan to use a two-post lift, store a large vehicle, or run heavy machinery.
How much concrete for a single garage floor?
A typical 6 × 3 m single garage at 150 mm needs 2.7 m³ before waste — about 3 m³ ordered, or 3.9 cubic yards. A comfortable ready-mix delivery.
Does a garage floor need reinforcement?
Yes — mesh in the middle third or a fibre-reinforced mix. Vehicles apply concentrated point loads through jacks and stands, which is where unreinforced slabs fail.
What strength concrete for a garage floor?
C25/30 (or 3,000–4,000 psi) is the usual specification. Ready-mix suppliers will quote by strength class; bagged general-purpose mix is typically around this once mixed to the stated water ratio.