How Many Bags of Concrete Do I Need?
Work out your volume in cubic feet, then divide by the bag yield: 0.6 ft³ for an 80 lb bag, 0.45 for a 60 lb bag, 0.3 for a 40 lb bag. Always round up.
A 10 × 10 pad at 4 inches is 33.3 cubic feet, which is 56 bags of 80 lb mix or 75 bags of 60 lb mix.
| Bag size | Yield per bag | Bags per cubic foot | Bags per cubic yard |
|---|---|---|---|
| 40 lb | 0.30 ft³ | 3.3 | 90 |
| 50 lb | 0.375 ft³ | 2.7 | 72 |
| 60 lb | 0.45 ft³ | 2.2 | 60 |
| 80 lb | 0.60 ft³ | 1.7 | 45 |
| 90 lb | 0.675 ft³ | 1.5 | 40 |
Yields are for standard pre-mixed concrete mix. High-strength and fast-setting products vary slightly — check the bag.
The two-step method
- Get cubic feet. Length (ft) × width (ft) × thickness (in ÷ 12). Skip the divide-by-27 step — bag yields are quoted in cubic feet, not yards.
- Divide by the yield. Cubic feet ÷ 0.6 for 80 lb bags, ÷ 0.45 for 60 lb bags. Round up to a whole bag.
Add a couple of spare bags on top. Partial bags are useless once opened and left in the rain, and running one bag short at the end of a pour is a genuine problem.
Worked examples
- Shed pad, 8 × 8 at 4 in: 21.3 ft³ → 36 bags of 80 lb, or 48 bags of 60 lb
- Patio, 10 × 12 at 4 in: 40 ft³ → 67 bags of 80 lb, or 89 bags of 60 lb
- Fence post, 10 in hole × 2 ft deep: 0.92 ft³ net of the post → 2 bags of 60 lb
- Mailbox post, 8 in hole × 2 ft deep: 0.53 ft³ → 1 bag of 80 lb
When bags stop making sense
Bags are the right call for post holes, small pads, and repairs. They stop being practical somewhere around three-quarters of a cubic yard.
One cubic yard is 45 bags of 80 lb mix — 3,600 lb of material to haul, open, and mix. At that point ready-mix is usually cheaper per yard and takes an hour instead of a weekend, even after the short-load fee. Compare against the price of a yard of concrete before you load the truck.
Mixing capacity matters too
A typical rented drum mixer handles two 80 lb bags at a time. Fifty-six bags for a 10 × 10 pad is 28 batches, and a slab has to be poured continuously to avoid cold joints. For anything above about 30 bags, line up help or switch to ready-mix.
Estimating notes
- Yields on the bag are approximate and assume correct water content — mixing wet inflates volume and weakens the mix.
- Fast-setting products for post holes are often poured dry into the hole, and coverage is quoted differently.
- Buy one or two spare bags; unopened bags stored dry keep for months.