How to use
- Choose Imperial or Metric, and Slab or footing or Round column.
- Enter the dimensions and how many of them you are pouring.
- Set the extra for waste (10% is common) and read the volume and bag counts.
Options explained
- Slab or footing
- Length × width × thickness. A typical patio or shed slab is 4 inches (10 cm) thick.
- Round column
- For post holes and sonotube columns — π × (diameter ÷ 2)² × height.
- Extra for waste
- Covers spillage, uneven ground and forms that bow out. 5–10% is usual.
Common slab sizes
Volumes for a flat slab, before waste. Bag counts include 10% extra and assume about 0.6 ft³ per 80 lb bag and 0.45 ft³ per 60 lb bag.
| Slab | Volume | 80 lb bags (+10%) | 60 lb bags (+10%) |
|---|---|---|---|
| 10 × 10 ft, 4 in | 1.23 yd³ (33.3 ft³) | 62 | 82 |
| 12 × 12 ft, 4 in | 1.78 yd³ (48.0 ft³) | 89 | 118 |
| 10 × 20 ft, 6 in | 3.70 yd³ (100 ft³) | 184 | 245 |
| 20 × 20 ft, 4 in | 4.94 yd³ (133 ft³) | 245 | 326 |
| 24 × 24 ft, 4 in (two-car garage) | 7.11 yd³ (192 ft³) | 353 | 470 |
Above roughly one cubic yard, a ready-mix truck is usually the better choice than mixing bags by hand.
Post holes and columns
Fence post. A 10 in hole 3 ft deep holds 1.64 ft³ — three 80 lb bags with 10% extra, or six 40 lb bags.
Deck footings. Four 12 in tubes, each 4 ft tall, need 12.6 ft³ in total: 24 bags of 80 lb premix with 10% extra.
Metric examples
A 3 × 3 m slab 10 cm thick is 0.90 m³ (0.99 m³ with 10% extra). A 4 × 5 m slab 12 cm thick is 2.40 m³ (2.64 m³ with 10% extra).
Frequently asked questions
How many 80 lb bags make a cubic yard?
A cubic yard is 27 ft³. At about 0.6 ft³ per 80 lb bag, that is 45 bags; with 60 lb bags (0.45 ft³) it is 60.
How much concrete for a 10 × 10 slab, 4 inches thick?
10 × 10 × 0.333 ft = 33.3 ft³, about 1.23 cubic yards — 56 bags of 80 lb premix before adding waste.
Should I order ready-mix or use bags?
Bags suit small jobs; for more than about one cubic yard, a ready-mix delivery is usually easier and often cheaper.
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