Work out exactly how much concrete a slab, footing or pad needs — in cubic yards and ready-mix
bags — before you order.
How to calculate concrete for a slab
Concrete is ordered by volume. Multiply the slab's length by its width by its thickness
(converted to feet), then convert cubic feet to cubic yards — the unit ready-mix suppliers
use:
volume (ft³) = length (ft) × width (ft) × thickness (in) ÷ 12
cubic yards = volume (ft³) ÷ 27
Then add a waste factor — 5–10% covers spillage, uneven subgrade and slight over-digging.
Running short mid-pour forces a cold joint, so round up, never down.
Bag yields
Pre-mixed concrete bags yield the following volumes when mixed with water:
| Bag size | Yield | Bags per cubic yard |
|---|
| 40 lb | 0.30 ft³ | 90 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
Concrete needed for common slab sizes (cubic yards)
| Slab size | 4″ thick | 6″ thick |
|---|
| 8×8 ft | 0.79 yd³ | 1.19 yd³ |
| 10×10 ft | 1.23 yd³ | 1.85 yd³ |
| 10×12 ft | 1.48 yd³ | 2.22 yd³ |
| 12×12 ft | 1.78 yd³ | 2.67 yd³ |
| 12×16 ft | 2.37 yd³ | 3.56 yd³ |
| 16×20 ft | 3.95 yd³ | 5.93 yd³ |
| 20×20 ft | 4.94 yd³ | 7.41 yd³ |
| 24×24 ft | 7.11 yd³ | 10.67 yd³ |
Figures exclude waste — add 5–10% when ordering.
Frequently asked questions
How much concrete do I need for a 10×10 slab?
A 10×10 ft slab at 4 inches thick needs 33.3 cubic feet, which is 1.23 cubic yards. With a 10% waste factor, order 1.36 cubic yards — or about 56 bags of 80 lb concrete mix.
How many 80 lb bags of concrete make a yard?
One 80 lb bag yields about 0.60 cubic feet of concrete, so you need roughly 45 bags of 80 lb mix for one cubic yard. For 60 lb bags it is about 60 bags, and for 40 lb bags about 90 bags.
How thick should a concrete slab be?
Standard patio and walkway slabs are 4 inches thick. Driveways carrying regular vehicles should be 5–6 inches. Shed bases are usually 4 inches, and slabs supporting heavy equipment 6 inches or more with reinforcement.
Should I order ready-mix or use bags?
As a rule of thumb, bags make sense up to about 1 cubic yard. Beyond that, ready-mix delivery is usually cheaper per yard and far less work — mixing a full yard by hand means lifting and mixing 45+ bags before any of it sets.
How much extra concrete should I order?
Add 5–10% for waste, spillage and uneven subgrade. For slabs poured on imperfectly graded ground, 10% is the safer figure — running short mid-pour creates a cold joint that weakens the slab.