The math every bag question comes down to: an 80 lb bag yields 0.60 cubic feet of concrete, a 60 lb bag yields 0.45, and a 50 lb bag 0.375. A cubic yard is 27 cubic feet — so it takes about 45 eighty-pound bags to make one yard. Everything below is that math applied to real projects.
Bags of concrete per cubic yard
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 50 lb | 0.375 cu ft | 72 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
How many bags for a slab
Formula: length × width × (thickness ÷ 12) = cubic feet, ÷ 0.60 = 80 lb bags. Add 10% for waste. Common answers:
| Slab (4″ thick) | 80 lb bags |
|---|---|
| 4 × 4 pad | 9 |
| 6 × 6 pad | 20 |
| 8 × 8 (hot tub) | 36 |
| 10 × 10 | 56 |
| 12 × 12 patio | 80 |
Skip the hand math — our concrete slab calculator gives bags, yards, and cost for any size and state at once.
How many bags per fence post
A standard fence post in an 8″ hole, 24–30″ deep: 1.5 to 2 bags (80 lb) per post. Bigger 12″ holes for gate posts: 3–4 bags. For deck footings in tubes, use the sonotube calculator — a 10″ tube takes about one 80 lb bag per foot of depth.
60 lb vs 80 lb bags: which to buy
Same concrete, different backs. The 80 lb bag is 8–12% cheaper per cubic foot; the 60 lb bag is what your spine prefers on bag number thirty. Rule of thumb: 80s if you’re mixing with a machine or have help; 60s if you’re solo with a wheelbarrow. (For reference, a full pallet holds 42 bags of 80 lb or 56 of 60 lb — useful when comparing delivery quotes.)
When bags stop making sense
Past roughly 1.5 cubic yards (about 68 bags), ready-mix delivery beats bags on price, consistency, and your weekend — even after the short-load fee. Check the crossover for your state with the delivery cost calculator.