Short answer: A 10 ft by 10 ft slab 4 inches thick needs about 56 bags of 80 lb concrete mix, before any waste. An 80 lb bag makes about 0.60 cubic feet, a 60 lb bag about 0.45 and a 40 lb bag about 0.30.
| Slab size | Cubic feet | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|
| 4 x 4 ft | 5.3 | 9 | 12 | 18 |
| 5 x 5 ft | 8.3 | 14 | 19 | 28 |
| 8 x 8 ft | 21.3 | 36 | 48 | 72 |
| 10 x 10 ft | 33.3 | 56 | 75 | 112 |
| 12 x 12 ft | 48 | 80 | 107 | 160 |
How the bag count is worked out
First find the volume of the slab in cubic feet by multiplying length by width by thickness in feet. A 4 inch slab is one third of a foot thick. Then divide the volume by the yield of one bag, and round up.
Yields are approximate and differ a little between brands, so check the label on the bag you buy.
How much extra to order
Most people add about 10% for uneven ground, spills and a slightly thicker pour. Running short in the middle of a pour is the worst outcome, so it is better to finish with a spare bag than without one.
Worked example
A slab 10 ft by 10 ft and 4 inches thick holds 33.3 cubic feet. With 10% extra that is 36.7 cubic feet, which takes 62 bags of 80 lb mix, or 1.36 cubic yards if you order ready-mix instead.
Use the Concrete CalculatorEnter your own measurements and get an exact amount.
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
About 45 bags, since a cubic yard is 27 cubic feet and each bag makes about 0.60 cubic feet. A 60 lb bag takes 60 bags and a 40 lb bag takes 90.
How much does one 80 lb bag cover?
About 1.8 square feet at 4 inches thick, since 0.60 cubic feet spread one third of a foot deep covers 1.8 square feet.
When should I order ready-mix instead of bags?
Bags are practical up to a few cubic feet. For larger pours the mixing work becomes heavy, and a ready-mix delivery in cubic yards is usually easier.
Related calculators
Figures are typical planning values. Material sizes, densities and local requirements vary, so confirm with your supplier before ordering.
Last updated October 11, 2026.