How Many Bags of Concrete Do I Need?
Bag yields, the cubic-yard formula, and worked examples for the projects homeowners actually build.
To calculate bags of concrete, divide the volume of the pour by the yield of one bag, then add 10–15% for waste. The hard part is getting the volume right, especially for post holes and footings.
The formula
Measure in feet. Convert inches by dividing by 12. A cubic yard is 27 cubic feet, so divide cubic feet by 27 if you are comparing with ready-mix pricing.
- Slab: length × width × thickness in feet.
- Footing: length × width in feet × depth in feet.
- Post hole: π × radius² × depth × number of holes.
- Column: π × radius² × height × number of columns.
Bag yields
| Bag size | Yield | 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 |
These are standard yields for general-purpose concrete mix. Fast-setting and high-strength mixes can differ, so check the bag before you buy a pallet.
Worked examples
10 × 10 slab, 4 inches thick
Volume = 10 × 10 × 0.333 = 33.3 cubic feet = 1.23 cubic yards. With 10% waste, 36.7 cubic feet. That is 62 bags of 80 lb, 82 bags of 60 lb, or 123 bags of 40 lb.
12-inch post hole, 30 inches deep
Radius = 6 inches = 0.5 ft. Volume = π × 0.25 × 2.5 = 1.96 cubic feet. With 10% waste, 2.16 cubic feet. That is 4 bags of 80 lb, 5 bags of 60 lb, or 8 bags of 40 lb.
4 × 8 sidewalk, 4 inches thick
Volume = 32 × 0.333 = 10.7 cubic feet. With 10% waste, 11.7 cubic feet. That is 20 bags of 80 lb, 26 bags of 60 lb, or 39 bags of 40 lb.
24 × 24 patio, 4 inches thick
Volume = 576 × 0.333 = 192 cubic feet = 7.11 cubic yards. With 10% waste, 211 cubic feet. That is 352 bags of 80 lb — a strong signal to price ready-mix instead.
Bags per cubic yard
| Project volume | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|
| 1/4 yard | 12 | 15 | 23 |
| 1/2 yard | 23 | 30 | 45 |
| 1 yard | 45 | 60 | 90 |
| 2 yards | 90 | 120 | 180 |
| 5 yards | 225 | 300 | 450 |
Bags vs. ready-mix
Bags make sense for small pours, remote sites, and projects you can finish in stages. Ready-mix makes sense when you need more than about 2 cubic yards, want consistent strength, and have truck access.
| Project size | 80 lb bags | Best option |
|---|---|---|
| Under 1/2 yard | Under 23 | Bags |
| 1/2–1 yard | 23–45 | Bags or small ready-mix |
| 1–3 yards | 45–135 | Compare both |
| Over 3 yards | 135+ | Ready-mix |
Buying tips
- Add 10–15% extra. Running out mid-pour creates a cold joint.
- Check the date code. Concrete mix has a shelf life; old bags can be lumpy.
- Store bags off the ground. Moisture ruins them before you open them.
- Rent a mixer for large pours. Mixing 60+ bags in a wheelbarrow is slow and inconsistent.
- Have a plan for the extra bag. Use it for a post, a step, or return it unopened.
Use the concrete bag calculator for your exact dimensions, then check cost per yard to compare with ready-mix.
Frequently asked questions
How many bags of concrete for a 10x10 slab?
A 10x10 slab at 4 inches thick needs about 1.23 cubic yards. That is 56 bags of 80 lb before waste, or 62 bags with a 10% allowance. In 60 lb bags, plan for 82; in 40 lb bags, 123.
How many 80 lb bags of concrete make a yard?
One 80 lb bag yields about 0.60 cubic feet, and a cubic yard is 27 cubic feet, so it takes about 45 bags of 80 lb to make a cubic yard.
How many bags of concrete for a 12-inch post hole?
A 12-inch diameter hole, 30 inches deep, holds about 1.96 cubic feet. That is about 4 bags of 80 lb with a 10% waste allowance. A 10-inch by 30-inch hole needs about 3 bags.
Is it cheaper to buy bags or ready-mix?
Bags are usually cheaper for small pours under about 1 cubic yard. Ready-mix is more economical at 2 cubic yards and up, but delivery, short-load, and minimum charges can erase the savings on small jobs.
How many bags are on a pallet of concrete?
Most 80 lb bags come 42 to 56 per pallet, depending on the brand and store. A pallet is roughly 1 to 1.25 cubic yards. Do not rely on the pallet count alone; check the bag yield.
Should I add extra bags for waste?
Yes. Add 10% for a flat slab on a well-prepared base, and 15% for uneven ground, post holes, or a first-time pour. Running short mid-pour is much worse than having one spare bag.