How much concrete is a 30×40 slab?
A 30-by-40-foot slab covers 1200 square feet. At the standard 4-inch thickness that's about 14.81 cubic yards of concrete before waste, or roughly 16.3 yards once you add a 10% allowance. At about 16.3 cubic yards you are into multi-truck territory, so line up site access and timing before you schedule delivery. The calculator above is pre-filled to these dimensions; change the thickness or price to match your job.
Concrete for a 30×40 slab by thickness
| Thickness | Concrete to order | 80 lb bags | Material cost |
|---|---|---|---|
| 4″ (standard) | 16.30 yd³ | 734 | $2,607 |
| 5″ | 20.37 yd³ | 917 | $3,259 |
| 6″ | 24.44 yd³ | 1,100 | $3,911 |
Material cost at $160/yd³ ready-mix, before delivery fees. Cost data updated June 2026.
Common questions
How many yards of concrete for a 30x40 slab?
A 30x40 slab at 4 inches thick needs about 14.81 cubic yards before waste. Add 5-10% extra so you don't come up short. Change the thickness in the calculator above to see how it affects the total.
How many bags of concrete for a 30x40 slab?
At 4 inches thick that's roughly 734 eighty-pound bags with a 10% allowance. Past about 1 cubic yard (around 45 bags), ready-mix delivery is usually cheaper and far less work than mixing bags by hand.
What does a 30x40 slab cost?
Material alone is a small share of the total. Installed, slabs this size run a wide range depending on thickness, finish, site prep, and region. Use the calculator's installed-cost range as a ballpark, then compare local quotes for a real price.