Concrete Slab Calculator
Estimate how much concrete you need to pour a slab — patio, garage, shed, or driveway pad. Get cubic yards, pre-mix bag counts, and an optional cost.
Results
- Concrete needed
- 1.78 cu yd
- With 10% waste
- 1.96 cu yd
- Cubic feet
- 48 cu ft
- Cubic meters
- 1.36 m³
- 80 lb bags
- 89 bags
- 60 lb bags
- 118 bags
- 40 lb bags
- 177 bags
Estimates only. Round up and buy a little extra to account for waste, spills, and breakage.
Pouring a patio, shed floor, garage pad, or driveway slab? This concrete slab calculator gives you the exact cubic yards plus 40, 60, and 80 lb bag counts so you can decide between bags and a ready-mix truck.
Enter the slab's length, width, and thickness, add a waste factor for uneven subgrade, and pour as many slabs as you like in one estimate.
Also used as a slab concrete calculator, patio slab calculator, and garage pad concrete estimator.
How to use the concrete slab calculator
- Enter the length in ft.
- Enter the width in ft.
- Enter the thickness in in.
- Enter the number of slabs.
- Enter the waste factor in %.
- Optionally add a price to estimate the total project cost.
- Read your quantities instantly — no sign-up, and you can print the estimate to take to the supplier.
Worked example
Inputs
- Length
- 12 ft
- Width
- 12 ft
- Thickness
- 4 in
- Number of slabs
- 1
- Waste factor
- 10 %
Results
- Concrete needed
- 1.78 cu yd
- With 10% waste
- 1.96 cu yd
- Cubic feet
- 48 cu ft
- Cubic meters
- 1.36 m³
- 80 lb bags
- 89 bags
- 60 lb bags
- 118 bags
- 40 lb bags
- 177 bags
Using the example values above, the concrete slab calculator returns 1.78 cu yd (concrete needed). Change any field to match your own project and the numbers update instantly.
How thick should a concrete slab be?
A 4-inch slab is standard for patios, walkways, and shed floors. Step up to 5–6 inches for driveways, garage floors, and anything carrying vehicle loads, and add wire mesh or rebar for crack control.
Always pour over a compacted gravel base. The base controls drainage and settling far more than an extra inch of concrete.
How much extra should I order?
Always order a little extra. A 5% waste factor suits simple, square projects; 10% is right for most jobs; bump it to 15–20% for irregular shapes, diagonal patterns, or uneven ground. Running short almost always costs more than the leftover material.
Concrete bag yields
| Bag size | Yield (cu ft) | Bags per cu yd |
|---|---|---|
| 40 lb | 0.30 | 90 |
| 50 lb | 0.375 | 72 |
| 60 lb | 0.45 | 60 |
| 80 lb | 0.60 | 45 |
Formula
Volume (cu ft) = length × width × thickness ÷ 12, × number of slabs. Cubic yards = cu ft ÷ 27. Add waste %.
Frequently asked questions
- How much concrete do I need for a 12x12 slab?
- A 12×12 ft slab at 4 inches thick is about 1.78 cubic yards, or roughly 80 bags of 80 lb mix, before waste.
- How thick should a concrete slab be?
- 4 inches is standard for patios and walkways; use 5–6 inches for driveways and anything carrying vehicles.
- How many bags of concrete for a 10x10 slab?
- A 10×10 ft slab at 4 inches is about 1.23 cubic yards — roughly 56 bags of 80 lb, 75 of 60 lb, or 112 of 40 lb mix. At this size a ready-mix truck is usually cheaper.
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