How to work out bags of concrete
Find the volume in cubic feet, then divide by what one bag makes. Always round up to the next whole bag.
Bags of concrete for common pad sizes
| Pad at 4 in | 80 lb | 60 lb | 50 lb | 40 lb |
|---|---|---|---|---|
| 4 × 4 ft | 9 | 12 | 15 | 18 |
| 6 × 6 ft | 20 | 27 | 32 | 40 |
| 8 × 8 ft | 36 | 48 | 57 | 72 |
| 10 × 10 ft | 56 | 75 | 89 | 112 |
| 10 × 12 ft | 67 | 89 | 107 | 134 |
| 12 × 12 ft | 80 | 107 | 128 | 160 |
How much area does one bag cover?
| Thickness | 80 lb | 60 lb | 50 lb | 40 lb |
|---|---|---|---|---|
| 2 in | 3.6 sq ft | 2.7 sq ft | 2.2 sq ft | 1.8 sq ft |
| 3 in | 2.4 sq ft | 1.8 sq ft | 1.5 sq ft | 1.2 sq ft |
| 4 in | 1.8 sq ft | 1.4 sq ft | 1.1 sq ft | 0.9 sq ft |
| 6 in | 1.2 sq ft | 0.9 sq ft | 0.8 sq ft | 0.6 sq ft |
Which bag size should you buy?
80 lb bags usually cost the least per cubic foot and mean fewer bags to open. 60 and 50 lb bags are easier to lift into a wheelbarrow and carry across a yard. 40 lb bags suit small repairs and anyone who can't lift heavy loads. Check the yield printed on the bag, since some brands make slightly more or less.
Common questions
How many 80 lb bags of concrete are in a yard?
About 45. One 80 lb bag makes roughly 0.60 cubic feet, and a cubic yard is 27 cubic feet.
How many bags of concrete do I need for a 10×10 slab?
At 4 inches thick you need about 56 bags of 80 lb, or 75 bags of 60 lb, before adding extra for waste. At that size a ready-mix delivery is usually easier.
How many bags of concrete per fence post?
Usually 2 to 3 bags of 80 lb for a typical 6 ft fence post in a 10 in wide, 3 ft deep hole. Use the post hole calculator for your exact hole.
Is it cheaper to buy bags or ready-mix?
Bags are often cheaper for jobs under about a cubic yard because there's no delivery or short-load fee. For bigger pours, ready-mix usually costs less per yard and saves hours of mixing.