Concrete Bag Calculator

How concrete bag counts are calculated

Bag counts come from the slab's volume divided by how much concrete one bag actually makes:

Cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12
Bags = cubic feet (with waste added) ÷ yield per bag, rounded up

Yield per bag (the manufacturers' standard figures, printed on the bag): an 80 lb bag makes about 0.60 cu ft of concrete, a 60 lb bag about 0.45 cu ft, and a 40 lb bag about 0.30 cu ft. At those yields it takes about 45 of the 80 lb bags to make a full cubic yard. The waste allowance is added before dividing, since running half a bag short mid-pour is the outcome you're buying insurance against — a spilled wheelbarrow load, a low spot in the base, and slightly generous forms all come out of that allowance.

Which bag size? The concrete is the same; the choice is about your back and your mixer. 80 lb bags are the cheapest per cubic foot but are genuinely heavy to lift into a mixer all day. 60 lb bags are the common middle ground. 40 lb bags cost the most per volume but suit small mixers and solo work. Past roughly a cubic yard total (about 45 × 80 lb bags), price a ready-mix truck or a tow-behind mixer rental before committing to bags — the bag count and the mixing time both get unreasonable fast.

Thickness check: 4 inches is standard for patios, walkways, and shed pads; driveways and anything carrying vehicles usually want 5–6 inches plus reinforcement. Whatever the thickness, keep it consistent — the volume (and the bag count) scales directly with it.