Slab on Grade Calculator
Size a complete slab-on-grade pour in one place: concrete volume, gravel sub-base, rebar grid and total weight. Enter the slab dimensions and the tool builds the full material list.
Your slab
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A slab on grade is a concrete slab poured directly on the ground, over a compacted gravel base. To size one you need four things: the concrete volume (length × width × thickness ÷ 27), the gravel sub-base beneath it, the rebar grid inside it, and a waste allowance. A typical residential slab is 4 inches thick on 4 inches of compacted gravel, with #4 bar at 18 inches on center.
What “slab on grade” means
A slab on grade sits directly on the soil — “grade” is the ground level — rather than being suspended on a foundation or supported over a crawl space. It’s the standard approach for garage floors, shed bases, workshop floors, patios and many house foundations in warm climates. Because it rests on the ground, everything under it matters as much as the concrete itself: a well-compacted gravel base and proper reinforcement are what keep it from cracking and settling.
The three layers you’re ordering
- The concrete slab. Length × width × thickness (÷ 12 for inches, ÷ 27 for yards) gives the volume. Add 5–10% waste — no subgrade is perfectly level, and running short mid-pour leaves a cold joint.
- The gravel sub-base. Usually 4–6 inches of compacted crushed stone below the slab. It drains water away from the underside and spreads the load. Same footprint as the slab, so the volume calculation is identical at its own depth.
- The rebar grid. #4 bar at 12–18 inches on center, set mid-slab on chairs, kept 3 inches back from the edges. It carries the tension the concrete can’t.
Standard thickness by use
| Slab use | Thickness | Gravel base | Reinforcement |
|---|---|---|---|
| Patio, walkway | 4 in | 4 in | Mesh or #3 @ 18″ |
| Shed / workshop floor | 4 in | 4 in | #4 @ 18″ |
| Garage floor | 4–6 in | 4–6 in | #4 @ 12–18″ |
| Driveway | 5–6 in | 4–6 in | #4 @ 12″ |
| House slab foundation | 4–6 in + thickened edges | 4–6 in | #4–#5 grid |
Thickness is set by the load, not the area. A bigger patio doesn’t need a thicker slab — but a car parked on it does. Each extra inch is roughly 25% more concrete, so don’t over-thicken a slab that only carries foot traffic.
Don’t skimp on the base
The gravel sub-base is invisible the moment the concrete covers it, which is why it’s the first thing cut from a rushed job. It’s also what decides how long the slab lasts. A well-compacted 4-inch base under a 4-inch slab will outlast a slab poured an inch thicker straight onto soft ground. Size the base with the crushed stone calculator or the paver base calculator, and remember that gravel compacts — order for the compacted depth, not the loose depth.
Frequently asked questions
How thick should a slab on grade be?
4 inches for patios, walkways and shed floors; 5–6 inches where a vehicle sits. House foundations add thickened edges. Thickness follows the load, not the size of the slab.
How much gravel goes under a slab?
4–6 inches of compacted crushed stone, matching the slab footprint. It drains water and spreads the load. Order for the compacted depth, since gravel loses height when compacted.
How much concrete for a 24×24 slab?
A 24 × 24 ft slab at 4 inches is about 7.1 cubic yards, or roughly 7.8 with 10% waste. At 6 inches it climbs to about 10.7 yards before waste.
Do I need rebar in a slab on grade?
For anything carrying vehicle or structural load, yes — #4 bar at 12–18 inches. Light-duty patios can use fibre mesh or welded wire, but rebar is the safe default for a garage or driveway.
Should I use a vapor barrier?
Under any enclosed, heated, or floored slab, yes — 6-mil poly between the gravel and the concrete. It stops ground moisture wicking up through the slab into your floor.