Concrete Slab Calculator
This concrete slab calculator works out how much concrete you need to pour a slab — in cubic yards, cubic feet, and 40, 60, or 80 lb bags. Enter the length, width, and thickness, choose a waste allowance, and you have an instant material estimate for your patio, garage, or shed slab.
How to use this calculator
- Enter the slab length and width and pick their unit.
- Enter the thickness — 4 inches for patios and walkways, 5–6 inches for driveways.
- Choose a waste allowance (10% is a safe default).
- Read the cubic yards for ready-mix delivery, or the bag count if you are mixing by hand.
The formula
A slab is a simple rectangular box, so the volume is length times width times thickness. Keep every measurement in feet, then divide by 27 to convert cubic feet into cubic yards:
Cubic yards = (Length ft × Width ft × Thickness ft) ÷ 27
Worked example. A 12 ft × 10 ft slab poured 4 inches thick: thickness is 4 ÷ 12 = 0.333 ft, so the volume is 12 × 10 × 0.333 = 40 cubic feet. Divide by 27 and you get 1.48 cubic yards, or about 67 × 80 lb bags. Add 10% waste and you would order about 1.63 cubic yards of ready-mix.
Frequently asked questions
How much concrete do I need for a slab?
Multiply the slab’s length, width, and thickness (all in feet) to get cubic feet, then divide by 27 to get cubic yards. For a 12 ft × 10 ft slab at 4 inches thick that’s about 1.5 cubic yards before waste.
How many bags of concrete are in a cubic yard?
It takes roughly 45 × 80 lb bags, 60 × 60 lb bags, or 90 × 40 lb bags to make one cubic yard. Bags are practical for small slabs; order ready-mix once you pass about half a yard.
How thick should a concrete slab be?
Four inches is standard for patios, walkways, and shed bases. Use 5–6 inches for driveways or anywhere vehicles will park, and add rebar or mesh for crack control.
Should I add extra concrete for waste?
Yes. Add about 5–10% to allow for spillage, uneven subgrade, and over-excavation. Running short mid-pour creates a cold joint, so it is better to slightly over-order.
Does this calculator work in metric units?
Yes. The results show both cubic yards and cubic meters, so you can use whichever your supplier quotes in.