How Many Bags of Concrete Do I Need? (Chart + Examples)
The short answer: one 80 lb bag of concrete fills about 0.6 cubic feet. Work out the volume of what you're pouring in cubic feet, divide by 0.6, and add 10% for waste. Or skip the math and use the concrete calculator.
Bag yields
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 50 lb | 0.375 cu ft | 72 |
| 40 lb | 0.30 cu ft | 90 |
Step 1: Find the volume
For a square or rectangular pour, multiply length × width in feet, then multiply by the thickness in inches divided by 12.
volume (cu ft) = length × width × (thickness ÷ 12)
For a round hole or column, use π × radius² × depth, with the radius in feet. A 10 inch wide hole has a radius of 5 inches, which is 0.417 ft.
Step 2: Divide by the bag yield
Divide the cubic feet by 0.6 for 80 lb bags, 0.45 for 60 lb bags or 0.3 for 40 lb bags. Round up.
Step 3: Add 10%
Subgrade is never perfectly flat, forms flex and some concrete stays in the wheelbarrow. 10% extra is a safe margin; returning one unopened bag is easier than running to the store mid-pour.
Worked examples
A 10 × 10 ft patio, 4 inches thick
10 × 10 × (4 ÷ 12) = 33.3 cu ft. Divided by 0.6 = 55.6 bags. With 10% extra: 62 bags of 80 lb (about 1.36 cubic yards). At that size, a small ready-mix delivery or a mix-and-tow trailer is worth pricing.
A 3 × 3 ft AC pad, 4 inches thick
3 × 3 × 0.333 = 3 cu ft. Divided by 0.6 = 5 bags, so 6 bags of 80 lb with a margin.
Fence posts: 4×4 post in a 10 inch hole, 3 ft deep
The hole holds π × 0.417² × 3 = 1.64 cu ft. The post takes up 0.26 cu ft, leaving 1.38 cu ft. That's 2.3 bags of 80 lb, so plan on 3 bags per post, or about 5 bags for every 2 posts if you're careful.
When to stop buying bags
Above roughly one cubic yard (45 bags of 80 lb), mixing by hand becomes a long, heavy day, and the concrete you mixed first starts to set before the last batch is in. Ready-mix trucks deliver from about 1 yard up, though small loads usually carry a short-load fee.