CALCULATION METHOD
How much concrete do I need for a slab?
Multiply the slab area by its finished thickness to get volume. Convert cubic feet to cubic yards, then add your chosen order allowance. The calculator rounds bagged concrete up to whole bags because partial bags cannot be purchased.
Rectangle = length × width × thickness
Circle = π × (diameter ÷ 2)² × thickness
Cubic yards = cubic feet ÷ 27
Order volume = net volume × (1 + allowance)
Bags = order cubic feet ÷ yield per bag, rounded up
10 × 12 concrete slab example
A 10 ft × 12 ft slab at 4 inches thick contains 40 cubic feet, or about 1.48 cubic yards, before allowance. With 10% added, the planning order is about 1.63 cubic yards—approximately 74 bags when each 80 lb bag yields 0.60 cubic feet.
Concrete bag yield chart
These nominal yields follow QUIKRETE’s slab-calculation guidance. Actual yield depends on the exact product and mixing, so the product label should control your order.
| Bag size | Nominal yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
Ready-mix or concrete bags?
The calculator shows both ordering paths without assuming one is cheaper. Use the ready-mix yardage to request a delivered quote, including minimum-load and delivery charges. Use the whole-bag count with the exact store price and product yield when comparing bagged material.
Rectangle versus circular slab
Choose Rectangle / square for patios, walkways, garage floors, and pads with straight forms. Choose Circular slab for a round pad and enter its finished diameter. For irregular pours, divide the area into separate simple shapes and add their order volumes.
QUIKRETE Concrete Mix data sheet for 40/60/80 lb yields and nominal unit weight, plus the QUIKRETE concrete calculator. Accessed September 11, 2026.