How many bags of concrete do I need?
Work out the volume of the pour in cubic feet, then divide by the volume one bag makes. Premixed concrete bags list their yield on the label; the standard figures are below. Always round up — a partly filled bag can't be saved once it's mixed.
| Bag size | Yield per bag | Bags per cubic yard | Bags per 10 × 10 ft slab, 4 in |
|---|---|---|---|
| 40 lb | 0.30 ft³ | 90 | 112 |
| 50 lb | 0.375 ft³ | 72 | 89 |
| 60 lb | 0.45 ft³ | 60 | 75 |
| 80 lb | 0.60 ft³ | 45 | 56 |
Bags of concrete per fence post
For post holes, the calculator works out the volume of each round hole and subtracts the space the post takes up. A typical 4 × 4 post (3½ in actual) in a 10 in hole 3 ft deep needs about 1.38 cubic feet of concrete — about 2.3 bags of 80 lb mix, so plan on 3 bags per post.
| Hole (diameter × depth) | Post | 80 lb bags per hole | 50 lb bags per hole |
|---|---|---|---|
| 8 in × 2 ft | 4 × 4 | 1 | 2 |
| 10 in × 2 ft | 4 × 4 | 2 | 3 |
| 10 in × 3 ft | 4 × 4 | 3 | 4 |
| 12 in × 3 ft | 4 × 4 | 4 | 6 |
| 12 in × 3 ft | 6 × 6 | 3 | 5 |
| 12 in × 4 ft | 6 × 6 | 4 | 7 |
Counts are rounded up to whole bags, with no waste allowance. Holes dug by hand are rarely straight-sided, so buy a spare bag or two.
Which bag size should you buy?
- 80 lb bags are the cheapest per cubic foot and the usual choice for slabs and many posts.
- 60 lb and 50 lb bags are easier to lift and carry, which matters on a long job or a steep site.
- 40 lb bags suit small repairs and setting a single post.
- Fast-setting mixes for posts often come in 50 lb bags with their own yield — check the label and adjust.
When to switch to ready-mix
Above about one cubic yard — 45 bags of 80 lb mix, over 3,600 lb to lift and mix — ready-mix delivered by truck is usually cheaper and far less work. Compare both with the concrete cost calculator, or get the cubic yards for any shape with the concrete calculator. Hauling the bags yourself? The concrete weight calculator shows what the pour weighs.
Frequently asked questions
How many 80 lb bags of concrete make a yard?
45 bags. An 80 lb bag yields about 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, a 10 × 10 ft slab is 33.3 cubic feet: 56 bags of 80 lb mix, 75 bags of 60 lb or 112 bags of 40 lb, before any waste allowance.
How many bags of concrete per fence post?
For a 4 × 4 post in a 10 inch hole 3 feet deep, about 2.3 bags of 80 lb mix — plan on 3. Use the Post holes option above for your exact hole size.
How many 50 lb bags of concrete make a yard?
About 72 bags, using a yield of 0.375 cubic feet per 50 lb bag. Check the label, as fast-setting mixes can differ.
How much does a bag of concrete cover?
An 80 lb bag covers about 1.8 square feet at 4 inches thick, or 2.4 square feet at 3 inches. A 60 lb bag covers about 1.35 square feet at 4 inches.