How to use the concrete calculator
- Pick the shape you're pouring: a slab or patio, a footing, a round column (such as a Sonotube form), or fence and deck post holes.
- Enter the dimensions. You can type lengths as
12,12.5,12' 6"or even metric like3.5m. - Keep 5–10% extra for spillage, uneven subgrade and form bulging. Running short in the middle of a pour is far worse than having a bag left over.
- Read off cubic yards (for ordering ready-mix) or the number of bags (for mixing yourself).
Concrete formula
Concrete is ordered by volume. For a rectangular slab or footing:
cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12
cubic yards = cubic feet ÷ 27
For a round column or hole:
cubic feet = π × (diameter (in) ÷ 24)² × height (ft)
Then divide cubic feet by the bag yield. A standard 80 lb bag of premixed concrete makes about 0.60 cubic feet, a 60 lb bag about 0.45 cubic feet and a 40 lb bag about 0.30 cubic feet.
Concrete needed for common slab sizes
| Slab | Cubic yards | With 10% extra | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 4 × 4 ft, 4 in thick | 0.20 | 0.22 | 10 | 14 |
| 8 × 8 ft, 4 in thick | 0.79 | 0.87 | 40 | 53 |
| 10 × 10 ft, 4 in thick | 1.23 | 1.36 | 62 | 82 |
| 10 × 12 ft, 4 in thick | 1.48 | 1.63 | 74 | 98 |
| 12 × 12 ft, 4 in thick | 1.78 | 1.96 | 89 | 118 |
| 16 × 16 ft, 4 in thick | 3.16 | 3.48 | 157 | 209 |
| 20 × 20 ft, 4 in thick | 4.94 | 5.43 | 245 | 326 |
| 24 × 24 ft, 4 in thick | 7.11 | 7.82 | 353 | 470 |
| 12 × 20 ft, 6 in thick | 4.44 | 4.89 | 220 | 294 |
| 20 × 20 ft, 6 in thick | 7.41 | 8.15 | 367 | 489 |
Bag counts include 10% extra. For anything over about 1 cubic yard, ready-mix delivery is usually cheaper and much less work than 45+ bags.
Bags or ready-mix?
Bagged concrete makes sense for post holes, small pads and repairs. One cubic yard is about 45 bags at 80 lb, which is roughly 3,600 lb to lift and mix. Above that, call local suppliers: most deliver ready-mix by the yard, though many charge a short-load fee under 3–4 yards. Some also rent "mix-and-tow" trailers for half-yard to one-yard jobs.
How thick should a concrete slab be?
- Walkways, patios, shed floors: 4 inches
- Driveways for cars: 4–5 inches; 6 inches for trucks, RVs or soft soil
- Garage floors: 4–6 inches
- Hot tub pads: 4–6 inches, check the manufacturer's spec
Local building codes take priority over these rules of thumb, especially for structural footings.
Frequently asked questions
How many 80 lb bags of concrete make a yard?
About 45 bags. One cubic yard is 27 cubic feet, and an 80 lb bag yields about 0.60 cubic feet, so 27 ÷ 0.6 = 45.
How many bags of concrete do I need for a 10x10 slab?
A 10 × 10 ft slab at 4 inches thick is 33.3 cubic feet (1.23 cubic yards). That is 56 bags at 80 lb before waste, or about 62 bags with 10% extra.
How much concrete do I need for a fence post?
For a 4×4 post in a 10 inch wide, 3 ft deep hole, you need about 1.4 cubic feet of concrete. Buy three 80 lb bags or four 60 lb bags per post. Use the Post holes option above for your exact sizes.
Should I add extra concrete for waste?
Yes. Add 5–10% to cover spillage, an uneven base and forms that bow outward. Add more if the ground is not graded flat.