Slab Thickness Guide
The common practice in US residential work: 4 inches for patios, walkways and shed bases; 5–6 inches for driveways and garage floors that carry vehicles. These are customary figures, not a specification — your local building code, soil and expected loads have the final say.
Why not just pour everything thicker? Thickness is the cheapest strength you can buy per inch, but it isn’t free: on a 20 × 20 ft slab, going from 4 to 6 inches takes the pour from 4.94 to 7.41 cubic yards — about 50% more concrete, before you dig a deeper base for it.
Typical thickness by use
| Use | Common thickness | Notes |
|---|---|---|
| Walkways & garden paths | 4 in | Foot traffic only; control joints every 4–6 ft are customary |
| Patios | 4 in | Furniture and foot traffic; 4–6 in gravel base beneath is typical |
| Shed & slab-on-grade bases | 4 in | Thickened edges are common where walls bear on the slab |
| Driveways (cars & light trucks) | 5–6 in | Vehicle loads concentrate at edges and joints; base compaction matters as much as thickness |
| Garage floors | 5–6 in | Often poured with the driveway spec; check local code for your area |
| Heavy vehicles / RV pads | 6 in+ | Treat as an engineered pour — loads and soil decide, not rules of thumb |
Figures reflect common residential practice in the US. Frost depth, expansive or soft soils, drainage and local amendments to building codes can all push a spec thicker. When a permit is involved, the permitted spec is the one to pour.
What thickness does to the yardage
| 20 × 20 ft slab | Cubic yards (exact) | vs 4 in |
|---|---|---|
| 4 in thick | 4.94 yd³ | — |
| 5 in thick | 6.17 yd³ | +25% |
| 6 in thick | 7.41 yd³ | +50% |
Yardage scales linearly with thickness — each extra inch on any slab adds exactly one quarter of the 4-inch volume. Run your own footprint:
Bag counts use the waste-adjusted volume and always round up — you can’t buy part of a bag. Ready-mix is usually ordered to the nearest quarter yard; your plant may add short-load or delivery fees.
Base and reinforcement, briefly
Most slab failures blamed on thickness are really base failures: uncompacted or organic soil settles, and the slab bridges the gap until it can’t. Common practice is 4–6 inches of compacted gravel under residential slabs, a vapour barrier where the slab is under a building, and reinforcement — rebar or welded wire mesh — placed in the middle third of the slab’s depth, not lying on the ground. Control joints, typically cut to a quarter of the slab’s depth, tell the inevitable shrinkage cracks where to go. Details vary by code and climate; this page is a planning guide, not a substitute for either.
Frequently asked questions
Is a 4 inch driveway acceptable?
Some are poured at 4 inches and survive on excellent bases with light cars — but 5–6 inches is the more common recommendation for vehicle loads, and many local codes or HOA specs call for it. The extra inch or two is cheap insurance compared with replacing a cracked slab.
Does thickness include the gravel base?
No. Thickness figures on this page are the concrete only. The gravel base sits below it and is ordered separately, in its own cubic yards.
How deep should I excavate?
Slab thickness plus base depth, plus enough extra to set your forms at finished grade. For a 4 in slab on a 4 in base, that’s roughly 8 in below finished surface — before topsoil stripping changes the picture.
What if my ground slopes?
Either grade it level first or measure thickness at the shallowest point and let the calculator’s waste allowance absorb the deeper side. Pouring to a sloped subgrade with a level top quietly adds volume.