How many bags of concrete for a slab? (40/60/80 lb)
Bagged concrete is sold by weight, but you need it by volume. This guide converts a slab into a bag count for 40, 60 and 80 lb bags, shows why the bag size matters, and helps you decide whether bags or a ready-mix truck is the smarter buy.
From volume to bags
First find the volume of your slab in cubic feet (length × width × thickness÷12 — see how much concrete do I need). Then divide by how many cubic feet a single bag yields, and round up, because you cannot buy a fraction of a bag:
Bags = ceil( cubic feet ÷ bag yield )
The catch is the yield. Bags are labeled by the dry weight of the mix, and heavier bags make more concrete. The standard labeled yields are:
| Bag size | Typical yield | Bags per cubic yard |
|---|---|---|
| 40 lb | ≈ 0.30 cu ft | ≈ 90 |
| 60 lb | ≈ 0.45 cu ft | ≈ 60 |
| 80 lb | ≈ 0.60 cu ft | ≈ 45 |
These are labeled typicals — always confirm the yield printed on your bag, because it varies slightly by mix and brand. The bags-of-concrete calculator shows all three sizes at once.
A worked example
Back to the 12 ft × 12 ft slab at 4 inches = 48 cubic feet. Divide by each yield and round up:
- 40 lb bags: 48 ÷ 0.30 = 160 → 160 bags.
- 60 lb bags: 48 ÷ 0.45 = 106.7 → 107 bags.
- 80 lb bags: 48 ÷ 0.60 = 80 → 80 bags.
Eighty 80-pound bags is a striking number — that is 6,400 pounds of material to haul, open, and mix one at a time. It is the clearest possible argument for ordering ready-mix on anything but a tiny pour.
Why the bag size matters
Notice the 48-cubic-foot slab took 160 small bags but only 80 large ones — exactly double, because an 80 lb bag yields twice what a 40 lb bag does. Fewer, heavier bags mean fewer trips, fewer bags to open and less chance of an inconsistent mix, so most people buy the largest bag they can comfortably lift (an 80 lb bag is heavy; some prefer 60 lb for handling). The total material is the same either way — the choice is about labor, not volume.
When bags beat ready-mix (and when they don't)
Bagged mix wins on small pours — setting a mailbox post, a couple of footings, a small landing — roughly under a cubic yard. Below that, ready-mix plants often add a short-load fee for undersized deliveries that erases the truck's price advantage. Above about a cubic yard, the math flips fast: our 48-cubic-foot slab is 1.78 yards, and mixing 80 bags by hand is a full, exhausting day, whereas a truck pours it in minutes. Use ready-mix vs bags to find the exact breakeven for your prices.
Buying tips
- Round up, then add a spare few. The ceil() already rounds up, but buy a couple extra bags for waste and returns — unopened bags are usually returnable.
- Check the yield on the bag. A "high-yield" or crack-resistant mix can differ from the 0.30/0.45/0.60 typicals; use the printed number.
- Mind the water. Bag yields assume the recommended water; over-watering weakens the concrete without adding usable volume.
- Have enough mixing capacity. Eighty bags is a rented mixer and a helper, not a wheelbarrow and a hoe.
Which bag mix to buy
"Concrete" on a bag is not one product. A standard ready-to-use concrete mix is the default for slabs, footings and posts. A high-early-strength or fast-setting mix sets in minutes and is handy for setting posts, but it gives you little working time. A high-strength (higher PSI) mix suits driveways and heavier loads. Crack-resistant and fiber-reinforced mixes add fibers for thin or exposed slabs. These products can yield slightly differently from the 0.30 / 0.45 / 0.60 cubic-foot typicals, which is exactly why you read the yield off the bag rather than assuming it. Matching the mix to the job matters as much as counting the bags: the right number of the wrong product is still the wrong order.
The water trap, storing and returns
Two practical points save money and grief. First, the printed yield assumes the labeled amount of water. It is tempting to add extra water to make the mix easier to work, but that weakens the concrete and does not increase usable volume — you get a soupier, weaker pour, not more of it. Mix to the label. Second, buy a few spare bags beyond the rounded-up count; most stores take back unopened, undamaged bags, so a couple of returns is cheaper insurance than a mid-pour run to the store (or a cold joint if the store is closed). Store bags off the ground and dry — concrete mix absorbs moisture and can harden in the bag, so buy it close to when you will pour, not weeks ahead.
The bottom line
Bags = cubic feet ÷ bag yield, rounded up. An 80 lb bag yields about 0.60 cubic feet, a 60 lb about 0.45, a 40 lb about 0.30 — so a 48-cubic-foot slab is 80, 107 or 160 bags respectively. If the count runs past a couple dozen bags, price a ready-mix delivery before you start lifting. These are material-quantity guides; confirm the yield on your own bag and order a little extra for spillage.