How much concrete do I need for a slab?
Multiply length × width × thickness, with the thickness in feet (4″ is 0.333 ft), to get cubic feet. Divide by 27 to get cubic yards, which is how ready-mix is sold. Then add 5–10% for an uneven base and spillage. A 10 × 10 ft slab at 4″ is 33.3 cu ft, or 1.23 cu yd before waste.
| Thickness | Cu ft per 100 sq ft | Cu yd per 100 sq ft | ≈ 80 lb bags |
|---|---|---|---|
| 4″ | 33.3 | 1.23 | 56 |
| 5″ | 41.7 | 1.54 | 69 |
| 6″ | 50.0 | 1.85 | 83 |
| 8″ | 66.7 | 2.47 | 111 |
Bag counts in the table don't include waste. One 80 lb bag makes about 0.6 cu ft, a 60 lb bag 0.45 and a 40 lb bag 0.3.
Bags or ready-mix?
Bags make sense up to about a cubic yard: 45 bags of 80 lb mix is already a long day of mixing. Beyond that, ready-mix is cheaper per yard, more consistent, and lets you pour the whole slab at once without cold joints. Small orders often carry a short-load fee, commonly $40–80 a yard below the supplier's minimum of around 8–10 yards. Ask for it when you get quotes.
What does a concrete slab cost?
Ready-mix runs roughly $125–175 per cubic yard in 2026, before delivery and short-load fees, and more in remote areas. A professionally poured 4″ slab with basic finishing typically costs around $4.50–8.50 per square foot, and a 6″ slab around $6–10. Reinforcement, stamped or colored finishes, difficult access and site prep all push the price up. These are national ballparks, so get two or three local quotes before you budget.
Gravel base and reinforcement
Most slabs sit on 4″ or more of compacted gravel, which drains water and gives a firm, even base. The calculator shows how much gravel that is. Driveways and garage floors usually get wire mesh or rebar. Patios and walkways often use fiber-reinforced mix or nothing at all. Check your local code and what the slab needs to carry.
More concrete calculators
Building footings for a deck? Use the Sonotube calculator. Building a wall? The concrete block calculator counts blocks and mortar.