How Many Bags of Concrete Do I Need
Skip the arithmetic for the common jobs. Here are the answers, then the method for everything else.
Common answers: a fence post hole (12 in x 3 ft) takes 5 eighty-pound bags; a 10x10 slab at 4 in takes 62; a 12-inch sonotube 4 ft deep takes 6. The universal method is cubic feet divided by 0.6 per 80 lb bag, plus 10 percent waste.
Fence posts
A typical post hole 12 inches across and 3 feet deep is 2.36 cubic feet. With 10 percent waste that is 2.6 cubic feet, or 5 eighty-pound bags (4.3 rounded up). For a 20-post fence, that is 100 bags: a full pallet, and worth pricing as a small truck load if the posts are in a line the truck can reach.
Slabs and patios
A 10 by 10 foot patio at 4 inches: 33.3 cubic feet, 56 eighty-pound bags with waste. A 12 by 16 foot patio at 4 inches: 64 cubic feet, 118 bags. At 4 inches, every 100 square feet is roughly 60 bags with waste. Bump to 5 inches for driveways and multiply by 1.25.
Footings and columns
A 20-foot footing 12 inches wide and 12 inches deep: 20 cubic feet, 37 bags with waste. A 12-inch diameter sonotube 4 feet deep: 3.14 cubic feet, 6 bags. Deck footings add up fast: six sonotubes is 36 bags, a two-person day with a mixer.
The universal method
For anything not listed: volume in cubic feet, times 1.1 for waste, divided by 0.6 for 80 lb bags (0.45 for 60 lb, 0.3 for 40 lb), round up. When the answer passes 60 to 90 bags, get a ready-mix quote before you buy.
Skip the arithmetic
Have odd dimensions? The calculator handles slabs, footings, and round columns.
Project bag-count questions
How many bags of concrete for a fence post?
A standard 12-inch diameter hole 3 feet deep needs about 5 eighty-pound bags, including a 10 percent waste buffer.
How many bags for a 12x12 slab?
A 12 by 12 foot slab at 4 inches is 48 cubic feet, which is about 107 eighty-pound bags with 10 percent waste. That is nearly 2 yards: price a truck.
How many bags per deck footing?
One 12-inch diameter sonotube 4 feet deep takes about 6 eighty-pound bags including waste.
Should I add extra bags?
Yes. Add 5 to 10 percent for spillage and uneven subgrade. A short pour leaves a cold joint; a leftover bag is cheap insurance.