Slab Calculator — Thickness Presets for Every Job

Pick your project type to set the right thickness, then enter the size. Thickness is the number people get wrong most often — too thin cracks, too thick wastes money.

Your slab

Project type (sets thickness)

Length (feet)

Width (feet)

Thickness (inches) Set by the presets above, or type your own.

Waste / overage allowance Always order a little extra. You cannot send back a shortage mid-pour.

Prices — national-average defaults, edit to your local prices

80 lb bag ($)

60 lb bag ($)

50 lb bag ($)

40 lb bag ($)

Ready-mix ($ per cu yd) National average; roughly $140–$170 in most areas.

Delivery fee ($)

Defaults are planning averages, not quotes. Your supplier's prices are the real ones — type them in.

Concrete to order (with your waste allowance)

0 cu yd

80 lb bags

60 lb bags

50 lb bags

40 lb bags

Mix it yourself (bags)

Ready-mix truck

Estimates only, for planning. Preset thicknesses are common practice, not a substitute for local code or an engineer's spec — frost depth, soil, and loads all vary. See the slab thickness guide for the reasoning.

Worked example: a 12 × 12 ft patio at 4 inches: 12 × 12 × (4/12) = 48 cu ft = 1.78 cu yd — 80 bags of 80 lb exactly, 88 bags with 10% waste. That is well past the bag-mixing crossover, so the truck price in the results is the one to beat.

Thickness quick reference

Project	Typical thickness	Notes
Walkway, patio, shed floor	4″	Foot traffic and light loads only.
Driveway — cars and pickups	5″	6″ at the apron where it meets the street.
Driveway — trucks, RVs, trailers	6″	Reinforcement strongly recommended.
Garage floor	5–6″	Check local code; vapor barrier underneath.
Heavy equipment / shop pad	6–8″	Engineered jobs may spec more, plus rebar grids.

Frequently asked questions

Is a 4-inch slab strong enough for a driveway?

Millions of driveways are 4 inches and hold up — on a well-compacted base with good drainage. Five inches adds only 25% more concrete but noticeably more crack resistance under car and truck loads, which is why many contractors default to 5. The base and the joints matter more than the last inch.

Does thickness change the bag count much?

Directly and proportionally. Going from 4 to 5 inches adds exactly 25% more volume, bags, and cost. That is why "just make it 6 inches to be safe" gets expensive on a big slab — run both numbers here before you decide.

What goes under the slab?

Typically 2–4 inches of compacted gravel (crusher run or 3/4″ stone), compacted in lifts. It levels the pour, drains water, and reduces frost movement. Wire mesh or rebar is cheap insurance on driveways and garage floors. More in the thickness guide.