Concrete Bag Calculator — 40, 60 and 80 lb Bags

Bag counts come from one conversion. A cubic yard is 27 cubic feet, and you divide that by the yield of the bag. An 80 lb bag yields about 0.60 cubic feet, so a yard takes 45 bags. A 60 lb bag yields 0.45, which makes 60 bags a yard, and a 40 lb bag yields 0.30, which makes 90 bags. Lighter bags are easier to carry and cost more to buy and stack per yard, so the choice is about your back as much as the budget.

You need a volume before you can count bags, and that is the step people skip. If the job is not a simple box, the main concrete calculator turns slabs, stairs and columns into cubic feet, which you can paste into the direct volume option above. Then round up, because a part bag cannot be bought.

Here is a full worked example. A 10 × 12 ft slab at 4 in thick is 10 × 12 × 0.333 = 40 cubic feet. Divided by 0.60 that is 66.7, so 67 bags of 80 lb before waste. Add 10% for spillage and an uneven base and you need 44 ÷ 0.60 = 73.3, so 74 bags. That is about 5,920 lb of dry mix to move.

The reverse question matters just as often. Say there are 20 bags in the garage after a finished job. At 0.60 cubic feet each they make 12 cubic feet, or 0.44 cubic yards, enough for a 6 ft × 6 ft pad at 4 in thick. The Bags to volume tab does that sum for any product, so you can decide whether what you own covers the next small job before you drive to the store.

Post holes, fence posts and form tubes are the classic bag job, because each hole needs well under a cubic foot. A 10 in hole 24 in deep holds about a cubic foot once the post is subtracted, so five posts take 8 bags, and the form tube calculator adds tube counts and cutting for round piers. The footing calculator for piers and deck posts covers deeper holes with a belled base, and fast-setting bags suit posts because the mix hardens in minutes rather than days.

Now the break-even. Below roughly 1 cubic yard bags win on convenience: no delivery, no minimum order, no truck waiting. Above it, ready-mix wins on price and strength. At $6.50 a bag, a yard of bagged concrete costs about $292 against roughly $140 delivered, so ready-mix is cheaper per yard at almost any size. What keeps bags in the race is the short-load fee, a flat charge that suppliers add under a minimum order. That fee is a step, not a slope: the ready-mix price jumps by about $100 the moment you order a small load, then drops away at the minimum. With the defaults, the lines cross at about 0.66 cubic yards, or 30 bags. To see how that fee feeds into a whole project, the slab cost calculator prices every line item.

Mixing your own concrete also tends to be weaker in practice, even when the bag is rated for strength. Water is added by eye, and every batch ends up with a slightly different ratio, so strength varies across the slab. Cold joints are the second problem. Each batch starts to set while the next is being mixed, and if the gap is too long the new concrete does not bond to the old, leaving a plane of weakness. A truck delivers one continuous, uniform batch.

Water is where bag mixing most often goes wrong. An 80 lb bag needs roughly 3 quarts, a full yard about 34 gallons. Adding more to make the mix easier to work feels helpful, but every extra splash costs strength, raises shrinkage and invites cracks. Aim for a stiff, wet-oatmeal consistency and let the mixer, not the water, do the work.

Finally, pallets. A pallet of 80 lb bags is roughly 42 bags and weighs over 3,300 lb, which is a delivery rather than a car trip. Pallet counts vary by supplier, but if your bag total passes a pallet the store will often deliver for a modest fee. At that point it is worth asking what ready-mix would cost for the same job.