Concrete for a 400 Sq Ft Slab
How much concrete for a 400 sq ft slab, by thickness.
A 400 sq ft slab at 4 inches thick needs about 4.94 cubic yards of concrete.
| Thickness | Concrete | 80 lb bags |
|---|---|---|
| 4" | 4.94 cu yd | 223 bags |
| 5" | 6.17 cu yd | 278 bags |
| 6" | 7.41 cu yd | 334 bags |
| 8" | 9.88 cu yd | 445 bags |
An 80 lb bag yields ~0.6 cu ft. Jobs over a cubic yard are usually cheaper as ready-mix. Order 5-10% extra.
Have exact dimensions? Use the full Concrete Calculator to enter length, width and depth.
Last reviewed: November 2026
Data sources: Standard volume and area formulas.
Disclaimer: Quantities are estimates from the area you entered; measure your actual space and add a waste allowance. Material and labor costs vary by region.
Frequently Asked Questions
A 400 sq ft slab needs about 4.94 cubic yards at 4 inches thick. Thicker slabs need proportionally more — see the table for 4 to 8 inches.
It depends on thickness. The table on this page shows the number of 80 lb bags for each slab thickness, though large slabs are usually cheaper as ready-mix.