How much concrete do you need?
4″ is the standard minimum for a residential slab; driveways usually 5–6″.
Worked examples: three jobs, three different answers
A 10 ft by 10 ft patio slab at 4 inches is 33.3 cubic feet, or 1.23 cubic yards before waste. With the 5 percent cushion that is about 1.30 yards. In 80-lb bags at roughly 0.6 cubic feet each, that is about 58 bags, near $319 at $5.50 per bag. A ready-mix truck at $150 per yard is about $195 for the same volume, plus any short-load fee. This is exactly the crossover zone: bags are possible in a long morning, ready-mix is cheaper and finished in an hour.
A 20 ft by 24 ft garage slab at 4 inches is 5.93 cubic yards, about 6.22 yards with the cushion, or roughly 467 bags. Nobody should mix 467 bags. At this size the question is not bags versus truck, it is scheduling, pump or buggy access, and whether your forms and help are ready when the truck arrives.
Four sonotube piers, 12 inches across and 3 ft deep, need about 0.35 cubic yards in total, roughly 26 bags of 80-lb mix. That is a bag job: small, spread across four holes, and easy to mix one tube at a time without racing a truck clock.
How thick should it be?
| Sidewalk, patio, shed floor (foot traffic only) | 4 inches, the residential minimum |
| Driveway for cars and light trucks | 5 to 6 inches, with reinforcement and a compacted base |
| Garage floor | 4 to 6 inches depending on vehicle weight and soil |
| Footings | Depth set by your local frost line, width by load and code |
| Sonotube piers | Below the frost line, diameter matched to the post and load |
Thickness is not a place to save money by going thin. A slab that is an inch too thin cracks under loads it was meant to carry, and there is no cheap repair that restores its strength. If the budget is tight, make the slab smaller, not thinner.
The base matters as much as the volume
Concrete poured over soft, uncompacted soil settles unevenly and cracks even when the yardage math was perfect. Strip sod and loose topsoil, compact the subgrade, and add 2 to 4 inches of compacted gravel where drainage or frost is a concern. Check the full sequence in the slab prep checklist before delivery day. Also confirm slope: exterior slabs should fall about 1/8 to 1/4 inch per foot away from buildings so water does not pool against a foundation.
Common ordering mistakes
Ordering exactly the calculated volume is the first mistake; forms bulge and low spots eat the difference, which is why the calculator adds a cushion. The second is forgetting that ready-mix is sold in whole or partial truck increments with minimums and short-load fees, so a 1.3-yard pour may be billed very differently from the per-yard rate. The third is mixing bag counts between 60-lb and 80-lb bags mid-job. Pick one size, use its yield, and buy a few spare bags you can return unopened.
Content reviewed October 2026. Local codes set footing depth and reinforcement requirements; this calculator gives volume and cost estimates only.