Most residential slabs are poured 4 inches thick: patios, walkways, shed floors, garage floors and driveways for ordinary cars. Go to 5 or 6 inches where heavy vehicles, RVs or trailers will park. The International Residential Code sets 3 1/2 inches as the minimum for a concrete floor slab, so 4 inches gives you a little margin.
| Project | Typical thickness | Notes |
|---|---|---|
| Walkway or garden path | 4 in | Foot traffic only |
| Patio | 4 in | Furniture, grills, people |
| Shed floor | 4 in | Thicken the edges for heavier sheds |
| Hot tub pad | 4 to 6 in | Follow the hot tub maker's spec; a filled tub is heavy |
| Driveway, cars | 4 in | 5 in is a common upgrade |
| Driveway, trucks, RVs or trailers | 5 to 6 in | Heavier loads need more thickness and reinforcement |
| Garage floor | 4 in | The IRC minimum for floor slabs is 3 1/2 in |
How much concrete each thickness takes
Thickness drives the volume directly: every extra inch on a 4 inch slab is 25% more concrete. Per 100 square feet (a 10 × 10 ft area), with no waste added:
| Thickness | Cubic feet | Cubic yards | 80 lb bags |
|---|---|---|---|
| 3.5 in | 29.17 | 1.08 | 49 |
| 4 in | 33.33 | 1.23 | 56 |
| 5 in | 41.67 | 1.54 | 70 |
| 6 in | 50 | 1.85 | 84 |
| 8 in | 66.67 | 2.47 | 112 |
Run your own dimensions in the concrete calculator, which adds a waste allowance and gives bag counts for 40, 50, 60 and 80 lb bags.
What an extra inch costs on a driveway
A single-car driveway 40 ft long and 12 ft wide, with 10% added for waste:
- 4 inches: 6.52 cubic yards
- 5 inches: 8.15 cubic yards, 1.63 yd³ more
- 6 inches: 9.78 cubic yards, 3.26 yd³ more than 4 inches
At this size you’d order ready-mix by the truck, and ready-mix is usually priced per cubic yard, so the jump from 4 to 5 inches adds a quarter to the concrete bill. Whether it’s worth it depends on what parks there. For cars on well-compacted base, 4 inches is standard. For a pickup towing a camper, the extra inch is cheap insurance.
Shed and hot tub pads
A 10 × 12 ft shed pad at 4 inches takes 1.63 cubic yards with 10% waste, or 74 bags of 80 lb mix. That’s a lot of bags to mix by hand; past about one cubic yard, many people find a short-load ready-mix delivery or a rented mixer worth the money.
For a hot tub, check the installation manual before you pour. Makers state the slab thickness and flatness they need, and a filled tub with people in it can weigh several tons.
What goes under the slab
The slab is only as good as what’s under it. The usual stack, from the bottom up:
- Undisturbed or compacted soil. Remove topsoil, roots and soft spots. Never pour on loose fill.
- Base course. The IRC calls for a 4 inch base of clean graded sand, gravel or crushed stone under floor slabs, except on well-drained, sandy soils. Many builders use 4 inches of compacted crushed stone under patios too.
- Vapor retarder, for slabs under living or storage space such as a garage or shed, to keep ground moisture out.
- Reinforcement. Wire mesh or rebar on chairs, so it ends up in the middle of the slab rather than lying on the ground.
- The slab, with control joints cut or tooled at regular spacing so cracks happen in the joints.
Thicken the edges
Slab edges carry more stress than the middle, especially under a shed wall or where a driveway meets the street. A common detail is a thickened edge: the outer 8 to 12 inches of the slab dug deeper, often to 8 inches or more. If you plan one, add its volume separately. A 12 in wide, 4 in deeper edge around a 10 × 12 ft pad adds about 44 ft × 1 ft × 0.33 ft = 14.7 cu ft, roughly half a cubic yard.
Mistakes to avoid
- Measuring thickness at the forms only. If the ground dips in the middle, the slab is thicker there and you’ll run short. Check the depth across the whole area with a string line.
- Ordering with no waste. Uneven subgrade and spillage eat 5 to 10%. Running out mid-pour leaves a cold joint.
- Skipping the base on clay. Clay swells and shrinks. A slab poured straight on it cracks sooner.
- Going thin to save money. A 3 inch patio uses 25% less concrete than a 4 inch one, but it cracks far more easily and is below the code minimum for floor slabs.