How Many Bags of Concrete Do You Need?
Mixing concrete from bags is fine until the job gets big, then it’s a workout you’ll regret. The trick is knowing your cubic footage, what each bag actually yields, and the point where you should stop bagging and call a ready-mix truck.
What each bag yields
Concrete mix is sold by the dry bag, but you order against finished volume. Here’s what the common sizes give you once mixed:
| Bag size | Yield (cubic feet) | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 50 lb | 0.375 cu ft | 72 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
So a cubic yard (27 cubic feet) takes about 45 of the 80-pound bags. That’s roughly 3,600 pounds of mix to haul, open, and dump, which is exactly why the bag-versus-truck question matters past a certain size.
Figure your volume
Concrete volume is length times width times thickness, all in feet. The trap is the thickness: a 4-inch slab is 0.33 feet thick, not 4. Always convert inches to feet (divide by 12) before multiplying.
- Slab: length × width × thickness.
- Footing: width × depth × length.
- Round post hole or column: 3.14 × radius² × depth. A 10-inch-diameter hole has a radius of about 0.42 ft.
Four jobs, worked out
| Project | Size | Volume | 80-lb bags |
|---|---|---|---|
| Patio slab | 10 × 10 ft, 4 in | ~1.2 cu yd | ~56 |
| Footing | 16 in × 8 in × 20 ft | ~0.66 cu yd | ~30 |
| Fence post | 10 in hole, 2 ft deep | ~1.1 cu ft | ~2 per post |
| Sonotube column | 12 in, 4 ft deep | ~3.1 cu ft | ~6 |
The patio slab at 1.2 yards (about 56 bags) is right on the line. Past it, ready-mix usually wins on both price and your back.
When to stop bagging
Rough rule: under about a cubic yard, bags make sense, especially if a truck can’t reach the pour. Over a yard or two, ready-mix is cheaper per yard and far less labor, as long as the truck can reach the form or you can barrow it quickly before it sets. Most ready-mix suppliers have a minimum load and a “short load” fee under a few yards, so get a quote both ways near the breakeven.
Concrete, mortar, and cement are not the same
Concrete mix has stone aggregate and is what you pour for slabs, footings, and posts. Mortar is the mix that bonds brick and block (no large stone). Cement is the powder binder inside both. For a slab or post, you want concrete mix, not mortar or plain cement.
Don’t forget reinforcement
Most slabs and footings that carry load want steel. It controls cracking and adds tensile strength. Size the grid with the rebar calculator, and if you’re laying block on that footing, the concrete block calculator counts the block and the mortar.
Mixing and curing basics
Follow the water ratio on the bag; too much water weakens the mix. Concrete reaches most of its strength in the first week but keeps curing for 28 days. Keep a fresh slab damp for the first few days in hot weather so it doesn’t dry too fast and crack.
Frequently asked questions
How many 80 lb bags of concrete make a yard?
About 45. Each 80-pound bag yields roughly 0.6 cubic feet, and a yard is 27 cubic feet.
How many bags of concrete for a fence post?
Usually one to two 80-pound bags per post, depending on hole size. A 10-inch hole 2 feet deep takes about two.
Is it cheaper to mix bags or order ready-mix?
Bags win for small jobs under about a yard. Ready-mix wins on price and labor above that, once you account for the short-load fee.
How much water per bag of concrete?
Follow the bag, but it’s roughly 6 pints (3 quarts) for an 80-pound bag. Mix to a workable, not soupy, consistency.