How Many Bags of Concrete You Need
Count bags before you buy them. One formula covers every project size.
Bag count = total cubic feet / yield per bag, rounded up. An 80 lb bag yields 0.6 cu ft, a 60 lb bag 0.45, a 40 lb bag 0.3. Add 5 to 10 percent waste, and switch to ready-mix delivery around 1 to 2 cubic yards.
Bag yields, the numbers that matter
An 80 lb bag of premix yields about 0.6 cubic feet of cured concrete. A 60 lb bag yields about 0.45 cubic feet. A 40 lb bag yields about 0.3 cubic feet. These are manufacturer figures for standard premix; high-strength or fast-setting mixes land within a few percent of the same yields.
The formula: measure your project in cubic feet (length x width x thickness, all in feet), multiply by 1.05 to 1.10 for waste, then divide by the yield. Always round up: a partial bag is useless once the pour starts.
Worked examples
Fence post hole, 12 inches wide and 3 feet deep: 3.14 x 0.5 x 0.5 x 3 = 2.36 cubic feet, times 1.1 waste = 2.6 cubic feet, divided by 0.6 = 4.3, so 5 eighty-pound bags. Patio slab 12 by 12 feet at 4 inches: 48 cubic feet x 1.1 = 52.8 / 0.6 = 88 bags. That 88-bag patio is nearly 2 yards, which is exactly where you should price a truck.
When bags stop making sense
Mixing 88 bags by hand is roughly 7,000 pounds of lifting and a full day with a mixer. A ready-mix truck delivers 2.2 yards in minutes for a comparable or lower price once you pass about 1.5 yards, and the concrete is more consistent. The crossover moves with short-load fees, so get the truck quote before you commit to bags.
Buying tips
Buy all bags from the same pallet when finish color matters; dye lots vary slightly. Store bags off the ground under cover, because one rain ruins them. And stage the bags near the pour: every bag gets carried twice if you do not.
Skip the arithmetic
Enter your dimensions to get exact bag counts with waste included.
Concrete bag questions
How many 80 lb bags of concrete do I need per cubic foot?
One 80 lb bag yields about 0.6 cubic feet, so you need cubic feet / 0.6 bags, rounded up. That works out to roughly 1.7 bags per cubic foot before waste.
Is it cheaper to mix your own concrete?
For small jobs under a cubic yard, bags are cheapest. Above about 2 yards, ready-mix delivery typically costs less per yard and eliminates a day of mixing labor.
How many bags fit in a wheelbarrow load?
A standard wheelbarrow handles about two 80 lb bags per batch for hand mixing. More than that and you cannot turn the mix properly.
Do concrete bags go bad?
Yes. Any moisture, even humidity through a torn bag, starts the curing reaction. Store bags off the ground under cover and use them within a few months of purchase.