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CalcTheBuild

Concrete Slab & Foundation Calculator

Order the right amount of concrete the first time. Enter the length, width and thickness of your slab or footing to get the volume to order, the number of bags if you are mixing yourself, the rebar you need and the total cost.

ft
ft
in
%
$/cu yd

Delivered price of ready-mix concrete.

in

Set to 0 if no rebar is needed.

$/ft

Your estimate

Concrete to order
5.43 cu yd
Estimated total cost
$1,635.50
Net volume (no waste)
4.94 cu yd
Or 80 lb bags
245 bags
Or 60 lb bags
328 bags
Rebar length
924 ft
Concrete cost
$896.30
Rebar cost
$739.20

Bagged concrete is usually only economical for pours under about 1 cu yd (0.75 m³).

How to use this calculator

  1. Enter the length and width of the slab or footing.
  2. Enter the thickness: 4″ (100 mm) for patios and paths, 5–6″ (125–150 mm) for driveways and garage floors.
  3. If you are pouring several identical pads or footings, enter how many.
  4. Keep 5–10% extra for spillage, uneven sub-base and over-excavation.
  5. Set the rebar grid spacing, or 0 if you are using mesh or fibre-reinforced concrete.

How concrete volume is calculated

Volume = length × width × thickness. In imperial units, multiply feet × feet × (inches ÷ 12) to get cubic feet, then divide by 27 to get cubic yards. A 20 × 20 ft slab that is 4″ thick needs 4.94 cu yd before waste.

Rebar is estimated as a square grid: one set of bars runs the length of the slab and another runs across it, spaced at the distance you enter, plus 10% for laps and offcuts.

Pro tips

  • Ready-mix trucks usually charge a short-load fee under 3–4 cu yd (2–3 m³), so compare that with bagged concrete for small jobs.
  • Compact the sub-base well. A soft spot of just 1″ (25 mm) across a large slab adds a surprising amount of concrete.
  • Keep fresh concrete damp for at least 7 days for full strength.

Frequently asked questions

How many bags of concrete make a cubic yard?

About 45 bags of 80 lb concrete or 60 bags of 60 lb concrete make one cubic yard. An 80 lb bag yields roughly 0.6 cu ft.

How thick should a concrete slab be?

4 inches (100 mm) is standard for patios, sidewalks and shed bases. Driveways and garage floors should be 5–6 inches (125–150 mm), and heavier-duty slabs thicker still.

How much does a cubic yard of concrete cost?

Ready-mix concrete in the US typically costs $140–$200 per cubic yard delivered, before pumping and finishing. In the UK expect roughly £110–£150 per cubic metre.

Do I need rebar in a concrete slab?

Rebar or welded wire mesh is recommended for driveways, garage floors and any slab that carries load. Light patios on a well-compacted base are often poured without it.

These results are estimates for planning and budgeting. Actual quantities depend on site conditions, products and workmanship. Check structural and code requirements with a licensed professional before you build.