How Many Bags of Concrete Do You Need?
One chart, one formula, and worked examples for the jobs people actually pour. Every number here uses the bag yields printed by the manufacturers themselves.
The yield chart
Quikrete and Sakrete both publish the same yields for their standard concrete mixes. A cubic yard is 27 cubic feet — everything follows from that.
| Bag size | Yield per bag | Bags per cubic yard | Bags per cubic foot | Dry weight per cubic yard |
|---|---|---|---|---|
| 80 lb | 0.60 cu ft | 45 | 1.7 | 3,600 lb |
| 60 lb | 0.45 cu ft | 60 | 2.2 | 3,600 lb |
| 50 lb | 0.375 cu ft | 72 | 2.7 | 3,600 lb |
| 40 lb | 0.30 cu ft | 90 | 3.3 | 3,600 lb |
Notice the pattern: the dry weight per yard is the same no matter the bag size. Smaller bags just mean more trips, more lifting, and usually more money per yard — their only advantage is that one person can handle them.
The formula
- Find your volume in cubic feet. For a slab: length (ft) × width (ft) × thickness (in) / 12.
- Divide by the bag yield. 0.60 for 80 lb, 0.45 for 60 lb, 0.375 for 50 lb, 0.30 for 40 lb.
- Round up. You cannot buy a partial bag.
- Add your waste allowance. 10% is the standard. Round up again.
Worked examples
A 10 × 10 ft patio, 4 inches thick
Volume: 10 × 10 × (4/12) = 33.33 cu ft (1.23 cu yd). In 80 lb bags: 33.33 / 0.60 = 55.6 → 56 bags exact, 62 with 10% waste. In 60 lb bags: 75 exact, 83 to order. This job is right at the edge of what most people will mix by hand in a day.
A fence post hole, 12 inches wide and 36 inches deep
Volume per hole: π × (0.5 ft)² × 3 ft = 2.36 cu ft. In 80 lb bags: 2.36 / 0.60 = 3.93 → 4 bags per hole. Ten holes: 40 bags exact, 44 to order.
A small stoop: 4 steps, 4 ft wide
Steps with 12-inch treads and 7-inch risers: 4 × 1 × 0.5833 × 10 = 23.33 cu ft (0.86 cu yd). In 80 lb bags: 39 exact, 43 to order.
Run your own numbers: the bag calculator does all three steps above instantly, forwards and in reverse.
Bags or a truck? The honest crossover
Mixing bags stops making sense around 1 cubic yard — 45 eighty-pound bags, each lifted, opened, mixed, and placed before the first ones start to set. Below half a yard, bags win easily. Between half a yard and a yard, it depends on your back, your help, and the short-load fee at your local plant (minimum orders and short-load fees under roughly 3–5 yards are typical). Above a yard, price the truck before you buy the bags.
Frequently asked questions
How many bags of concrete for a 10×10 slab?
At 4 inches thick: 56 bags of 80 lb mix exactly, or 62 bags with the standard 10% waste allowance. In 60 lb bags: 75 exact, 83 to order. Or skip the bags: 1.23 cu yd exact, 1.36 cu yd to order from a truck.
How many bags are in a cubic yard?
45 bags of 80 lb, 60 bags of 60 lb, 72 bags of 50 lb, or 90 bags of 40 lb — all from the same 27 cu ft, using the manufacturers' published yields.
Can I return unopened bags?
Most home centers accept returns of unopened, dry bags with a receipt. Keep bags off the ground and under cover until you decide — a bag that has drawn moisture and gone lumpy is yours.