Slab thickness is the single biggest decision in any flatwork project. Too thin and the slab cracks under load; too thick and you’re paying for concrete that does nothing. The good news: for residential work, the right answers are well established. Here’s how to pick, and what each extra inch really costs.
Recommended thickness by application
| Application | Thickness | Notes |
|---|---|---|
| Garden path, mower strip | 3 in (75 mm) | Foot traffic only |
| Patio, sidewalk | 4 in (100 mm) | The residential standard |
| Shed base | 4 in (100 mm) | 5 in for heavy workshop tools |
| Driveway (cars) | 4–5 in | 5 in if trucks or RVs will park |
| Garage floor | 4–6 in | 6 in for lifts or heavy vehicles |
| Workshop / small commercial | 6 in (150 mm) | With rebar or heavy mesh |
| Equipment pads, loading areas | 6–8 in | Engineered; use 8 in for trucks |
Thickness alone doesn’t carry the load — the sub-base does most of the work. A 4-inch slab on 4–6 inches of well-compacted gravel outperforms a 6-inch slab poured on soft, uncompacted soil. If your ground is expansive clay or fill, fix the base before adding concrete.
What an extra inch costs
Thickness scales volume linearly, and volume is most of your cost. The coverage rule makes this concrete (sorry):
Coverage per cubic yard = 324 ÷ thickness in inches
- 4 in: 81 ft² per yd³
- 5 in: 64.8 ft² per yd³
- 6 in: 54 ft² per yd³
For a 24 × 24 ft (576 ft²) garage slab:
- At 4 in: 576 ÷ 81 = 7.1 yd³
- At 6 in: 576 ÷ 54 = 10.7 yd³
At a typical ready-mix price of $120–$180 per cubic yard, going from 4 to 6 inches adds roughly $430–$650 in concrete alone — often worth it for a garage, rarely worth it for a patio. Compare scenarios quickly with the slab calculator, or jump straight to the preset 6-inch slab calculator and 8-inch slab calculator.
Thickness and reinforcement work together
Reinforcement doesn’t stop concrete cracking — it holds cracks tightly closed so they stay hairline and the slab keeps acting as one piece.
- 4-inch slabs: welded wire mesh or fiber reinforcement is common; rebar is optional for patios.
- 5–6-inch slabs: #3 or #4 rebar on an 18–24 in grid is typical for driveways and garages.
- 6+ inch structural pads: engineered rebar layout, often two-way #4 at 12–16 in centers.
Keep steel in the middle third of the slab depth — rebar lying on the ground does nothing. Chairs or dobies cost pennies. Estimate bar quantities with the rebar calculator.
Edges, footings and thickened sections
Slab edges that carry walls or posts should be thickened to 8–12 inches, typically twice the field thickness with a 45° transition. Monolithic “slab-and-footing” pours for sheds and garages combine a 4-inch field with a 12-inch turned-down edge. When you estimate volume, calculate the field slab and edge footings separately, then add them — the volume calculator handles multi-section pours.
Don’t shave the thickness to save money
A common temptation: pour 3.5 inches instead of 4 and stretch the concrete. It backfires twice. First, slab strength in bending scales roughly with the square of thickness, so a 12% reduction in depth cuts load capacity by over 20%. Second, forms and sub-bases are never perfect — a nominal 3.5-inch slab will have real spots at 3 inches, exactly where a car tire will find them.
If budget is tight, reduce the slab area, improve drainage, or stage the project — never the thickness under wheeled loads.
Quick decision path
- Foot traffic only → 4 inches on 4 inches of compacted gravel.
- Cars → 4 inches minimum, 5 if in doubt, with mesh or rebar.
- Trucks, lifts, machinery → 6 inches with rebar, engineered if loads are known.
- Bad soil → fix the base first; thickness can’t compensate for a soft foundation.
Settle the thickness, then let the concrete calculator turn your dimensions into an order quantity with waste included.