How to use this concrete slab calculator
Measure the length and width of the area you're pouring and choose a thickness. The calculator multiplies these into a volume, converts it to cubic yards (how ready-mix is sold) and cubic feet (how bags are rated), then adds your waste allowance. It also estimates a delivered cost from the price you enter.
Bags vs. ready-mix: which is cheaper?
Bagged concrete is convenient for small jobs, but the math turns against it quickly:
- Under ~½ cubic yard (a small pad, post footings, a step): bags are fine.
- ½ to 1 cubic yard: either works — weigh the cost of ~25–50 bags plus mixing labour against a small delivery.
- Over 1 cubic yard: order ready-mix. Mixing 50+ bags by hand is slow, and cold joints can form if the pour isn't continuous.
What drives the cost of a concrete slab in 2026
- Material: ready-mix runs roughly $125–$175 per cubic yard delivered, before labour.
- Thickness & rebar: going from 4″ to 6″ adds 50% more concrete; wire mesh or rebar adds material and labour.
- Short-load fees: most plants charge extra for deliveries under ~3–4 yards.
- Site prep: grading, gravel base, and forms often cost as much as the concrete itself.
Estimates are for planning only. Always confirm quantities and pricing with your ready-mix supplier or contractor before ordering.
Frequently asked questions
How many 80 lb bags make a cubic yard?
About 45 bags. A cubic yard is 27 cubic feet, and each 80 lb bag yields ~0.60 cubic feet, so 27 ÷ 0.60 ≈ 45 bags.
How thick should a concrete slab be?
4 inches for patios, sheds and walkways; 5–6 inches for driveways and anything carrying vehicle weight.
Should I add extra concrete?
Yes — add about 10% for waste, spillage and uneven subgrade. Running short mid-pour is far more expensive than a little excess.