How Many Bags of Concrete Do I Need?
Short answer: one cubic yard of concrete takes about 45 bags of 80 lb mix, 60 bags of 60 lb or 90 bags of 40 lb. Below are bag counts for common slabs and fence posts, and the simple formula to work out any size.
Bags of concrete per cubic yard
Premixed concrete bags are sold by weight, but what matters is how much space the mixed concrete fills. Typical yields printed on the bags are:
| Bag size | Yield per bag | Bags per cubic yard (27 cu ft) |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 40 lb | 0.30 cu ft | 90 |
Check the yield printed on your brand. Fast-setting and high-strength mixes can yield a little differently.
The formula
Bags = cubic feet × 1.10 (waste) ÷ yield per bag, rounded up
The 10% waste allowance covers uneven ground, spills and the concrete that stays in the mixer. Always round the bag count up: a pour that runs out halfway leaves a cold joint.
How many bags for a 4 inch slab
A 4 inch thick slab is standard for patios, walkways and shed floors. With 10% waste added:
| Slab size | Cubic yards | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|
| 10 × 10 ft | 1.36 | 62 | 82 | 123 |
| 10 × 12 ft | 1.63 | 74 | 98 | 147 |
| 12 × 12 ft | 1.96 | 88 | 118 | 176 |
| 12 × 16 ft | 2.61 | 118 | 157 | 235 |
| 16 × 20 ft | 4.35 | 196 | 261 | 392 |
| 20 × 20 ft | 5.43 | 245 | 326 | 489 |
Going to 6 inches (common for driveways and hot tub pads) adds half again: a 10 × 10 ft slab at 6 inches takes about 2.04 cubic yards, or 92 bags of 80 lb mix.
Bags or ready-mix truck?
Above roughly 1.5 cubic yards, a ready-mix delivery is usually cheaper and much faster than mixing 70 or more bags by hand, and it gives a more even slab. Ask local suppliers about their minimum order and short-load fee; many deliver as little as one cubic yard for an extra charge. For small jobs, fence posts and steps, bags are the practical choice.
How many bags of concrete per fence post
Post holes are cylinders, so the volume is π × radius² × depth. The post itself takes up some of that space. For a 4×4 post (3.5 × 3.5 inches actual size) with 10% waste:
| Hole | Concrete after the post | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|
| 10 in wide × 24 in deep | 1.01 cu ft | 2 | 3 | 4 |
| 10 in wide × 36 in deep | 1.52 cu ft | 3 | 4 | 6 |
| 12 in wide × 36 in deep | 2.31 cu ft | 4 | 6 | 8 |
| 12 in wide × 42 in deep | 2.70 cu ft | 5 | 6 | 9 |
A common rule of thumb is a hole three times the width of the post and a third of the post's above-ground height deep, below the frost line where you live. For a whole fence, multiply by the number of posts, or use the fence calculator, which counts posts, rails, pickets and concrete together.
Worked example
You are pouring a 10 × 12 ft patio, 4 inches thick. 10 × 12 × 4 ÷ 12 = 40 cubic feet. Add 10% waste: 44 cubic feet, or 1.63 cubic yards. 44 ÷ 0.60 = 73.3, so you need 74 bags of 80 lb concrete. At around $6 to $7 a bag that is roughly $450 to $520 in bags, so it is worth pricing a ready-mix delivery too.
Common mistakes
- Mixing inches and feet. Thickness is usually measured in inches; divide by 12 before multiplying.
- Measuring the top of the form, not the ground. Low spots in the base can add 10% or more. Level and compact the base first, ideally with a few inches of gravel (see the gravel calculator).
- Forgetting the post displacement. It is small for one post, but it adds up across a long fence.
- Buying exactly enough. An extra bag or two costs little compared with a second trip in the middle of a pour.
Try the concrete calculator
Enter your own numbers below, or open the full concrete calculator with the formula and more examples.
Frequently asked questions
How many 80 lb bags of concrete make a yard?
About 45. Each 80 lb bag yields roughly 0.6 cubic feet, and a cubic yard is 27 cubic feet.
How many bags of concrete do I need for a 10x10 slab?
About 62 bags of 80 lb concrete for a 4 inch slab, including 10% waste. At 6 inches thick it is about 92 bags.
How many bags of concrete per fence post?
For a 4×4 post in a 10 inch wide, 24 inch deep hole, about 2 bags of 80 lb concrete. A 12 inch wide, 36 inch deep hole takes about 4 bags.
Is it cheaper to buy bags or ready-mix concrete?
For anything over about 1.5 cubic yards, ready-mix is usually cheaper and faster. Bags make sense for posts, steps and small pads.