How much concrete do you actually need — in yards for the truck, or bags for the wheelbarrow.
Rule of thumb: over 1 cubic yard, order ready-mix — mixing 40+ bags by hand is a punishment, not a project. Under a yard, bags are fine: an 80 lb bag yields about 0.60 cu ft, a 60 lb bag about 0.45 cu ft.
Always keep the 10% waste factor — uneven ground eats more concrete than you think, and a second truck trip costs far more than an extra bag.
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Concrete is sold by the cubic yard, so the job is to turn your slab or footing into cubic yards and add a little for waste. The volume is length × width × thickness, all in the same units. Because a cubic yard is 27 cubic feet, the shortcut is: cubic feet ÷ 27 = cubic yards. We also add a waste allowance (about 5–10%) for spillage, uneven subgrade, and forms that aren't perfectly square.
A 10 ft × 10 ft slab poured 4 inches thick is 100 sq ft × 0.33 ft = about 33 cubic feet, which is roughly 1.23 cubic yards — call it 1.35 with waste. If you're mixing bags instead of ordering a truck, an 80-lb bag yields about 0.6 cubic feet, so you'd need around 55–60 bags. Past roughly half a cubic yard, ready-mix delivery is usually easier and cheaper than hauling bags.
Bags or a ready-mix truck?
Small jobs (footings, post holes, a small pad) suit bags. Anything approaching a cubic yard or more is usually better as ready-mix.
How much extra should I order?
Ordering 5–10% extra is normal — running short mid-pour is far worse than a little leftover.
Does thickness really matter that much?
Yes — volume scales directly with thickness, so going from 4 to 6 inches adds 50% more concrete and cost.