How many bags you need
A bag count is the volume of the pour divided by what one bag makes, rounded up. A slab 10 feet by 10 feet and 4 inches thick is 10 × 10 × 4/12 = 33.3 cubic feet; 10% for waste makes 36.7; an 80 lb bag of concrete mix makes 0.60 cubic feet, so 36.7 ÷ 0.60 = 61.1, and you buy 62. The same slab is 82 bags of 60 lb, or 123 bags of 40 lb. Count up, never to the nearest bag: 62.5 bags is 63, and the half bag you round away is the corner that does not get filled.
A cubic yard is 27 cubic feet, which is 45 bags of 80 lb. And because every size on the data sheet yields the same 0.0075 cubic feet per pound, a cubic yard is 3,600 lb of bags whatever size you buy — 45 × 80 lb, 60 × 60 lb or 90 × 40 lb all weigh the same on the trolley. Choosing a bag size changes how many times you lift, not how much you carry.
Bags per cubic yard, and for this slab
| Bag | Makes | Per cubic yard | 10×10 slab, 4 in, +10% | That order weighs |
|---|---|---|---|---|
| 90 lb | 0.675 ft³ ~ | 40 | 55 | 4,950 lb |
| 80 lb | 0.60 ft³ | 45 | 62 | 4,960 lb |
| 60 lb | 0.45 ft³ | 60 | 82 | 4,920 lb |
| 50 lb | 0.375 ft³ | 72 | 98 | 4,900 lb |
| 40 lb | 0.30 ft³ | 90 | 123 | 4,920 lb |
The yields are the ones on the Quikrete Concrete Mix 1101 data sheet, and every one of them is a box you can type over under Assumptions if your bag says something else. We are not connected with Quikrete or any other maker. Two rows come with a warning: the sheet lists no 90 lb concrete mix, so that yield is worked out from the others, and the 50 lb bag is on the sheet but we have not found one on a shelf. Those two sizes are offered on their own and never put inside a mixed order, because an order that needs a bag nobody stocks is not an order. A 50 lb bag is 72 to the cubic yard, not the 75 a large calculator still prints.
Which size to buy
Since the weight is the same either way, the size is a question about your back and your boot. Sixty-two 80 lb bags is 4,960 lb, about four trips in a full-size pickup at the 1,500 lb we assume for a payload — your truck's real figure is on the door-jamb sticker and there is a box for it. The same slab in 40 lb bags is 123 lifts instead of 62, and 123 bags to cut open and empty.
Money usually runs the other way: the bigger the bag, the less it costs per cubic foot, so the same slab usually costs more in 40 lb bags than in 80 lb ones — the cost column on the rows above says by how much. Every row is priced, including the cheapest mixture of sizes, which here saves a couple of dollars by sending you to three shelves rather than one. Those starting figures are dated reference prices and only the 80 lb one came from a store listing, so type what your store charges into the row you are reading and every total redoes itself.
Water, and mixing it
Concrete mix takes about 6 pints of water per 80 lb bag — three quarters of a gallon — and the smaller bags take their share of that by weight. For 62 bags that is about 46 gallons, which is on the shopping list so you can see it before you are standing in a dry yard. Start at the low end: the data sheet allows 6 to 9 pints, more water makes the mix easier to place and weaker when it sets, and you cannot take it back out. Mix only what you can place and finish before it stiffens, and keep the last bags dry under cover until you need them.
Fence posts take a different bag
A post is set with fast-setting mix, which comes in 50 lb bags of 0.375 cubic feet, goes into the hole dry, and takes about a gallon of water poured over it. A 4×4 post — 3½ inches across, really — in a 10 inch hole 24 inches deep, filled to 3½ inches below the ground, takes 0.79 cubic feet: 3 bags. Pick Fast-setting mix in the form above and the whole page switches to that bag. Holes deeper than 2 feet 6 inches are filled in layers rather than in one go.
When bags stop being the cheap way
Past about a cubic yard, price a truck before you load a trolley. This slab sits right at that line, and bags still come out slightly ahead of ready-mix on it, because a supplier bills a minimum order — often 2 cubic yards — and adds a short-load fee below five. The comparison under the shopping list puts both on one line with the prices in boxes, says which is cheaper and by how much, and works out from how many yards the truck takes over. Past that point you are buying a day of mixing as well as the concrete. The concrete calculator is the version of this page that adds up a whole job — slab, post holes and steps together — and prints whichever way of buying is cheaper.
Common questions
How many 80 lb bags of concrete are in a cubic yard?
Forty-five. A cubic yard is 27 cubic feet and an 80 lb bag makes 0.60, so 27 ÷ 0.60 = 45 exactly, weighing 3,600 lb.
How many bags of concrete for a 10x10 slab?
At 4 inches thick, 56 bags of 80 lb for the concrete itself and 62 once 10% is added for waste. At 6 inches, 92 bags. In 60 lb bags, 82 and 123 respectively.
How many bags do I need for a fence post?
Three bags of 50 lb fast-setting mix for a 4×4 in a 10 inch hole 24 inches deep. A wider hole climbs quickly: the concrete goes around the post, so the hole's diameter counts twice over.
Can I mix bag sizes in one pour?
Yes — it is the same mix in different sacks, and the cheapest row above often uses two or three sizes. The reason to buy one size is the shelf: stores run out of the popular bag, and a plan that needs exactly one 40 lb bag is a plan that can fail at the till.
What if I have bags left over?
Keep them dry and off the floor; a bag that has taken damp sets in the sack and is no use. The page shows what is left over on every row — about half a cubic foot on the 80 lb row here — so you can pick the order that wastes least rather than the one that rounds most.