Concrete Slab Calculator
Your estimate
- Before waste
- 1.78 yd³48 ft³ · 1.36 m³
- Slab area
- 144 ft²16 yd² · 13.38 m²
- Weight
- 7,022 lb3.51 tons · 3,185 kg
- Estimated cost
- $323
For patios, shed bases, garage floors and driveways — enter the slab size and thickness, get yards and bags.
Updated 2026-10-20 · Based on: bag yields printed by concrete-mix makers (0.30–0.60 ft³ per bag) and a 1.54 dry-volume factor.
How to use the Concrete Slab Calculator
- Measure the slab length and width (inside the forms).
- Choose the thickness: 4 in for patios and walks, 5–6 in for driveways and garages.
- Leave 10 % extra for waste, pick a bag size and your price per yard.
- Order the cubic yards shown, or buy the number of bags for small slabs.
Formula
Slab volume = length × width × thickness. A 12 × 12 ft slab 4 in thick is 12 × 12 × 0.333 = 48 ft³, which is 1.78 yd³ (48 ft³).
Bags = volume ÷ bag yield (0.60 ft³ per 80 lb bag).
Worked example
With the values preset in the calculator above:
- Length
- 12 ft
- Width
- 12 ft
- Thickness
- 4 in
- Extra for waste
- 10 %
- Bag size
- 80 lb bags
- Concrete needed (with waste)
- 1.96 yd³ (52.8 ft³)
- Before waste
- 1.78 yd³ (48 ft³)
- Slab area
- 144 ft² (16 yd²)
- Bags
- 88
- Weight
- 7,022 lb (3.51 tons)
How thick should a concrete slab be?
A 4 in (10 cm) slab is standard for patios, sidewalks and sheds. Driveways and garage floors that carry cars are usually 4–5 in, and 6 in for trucks or RVs. Every extra inch adds 25 % more concrete to a 4 in slab — the chart below shows the difference.
Slab on grade: what goes under the concrete
- 4 in (10 cm) of compacted gravel base — use the gravel calculator for tons.
- A vapour barrier under interior slabs.
- Wire mesh or rebar for driveways and garage floors.
- Control joints every 8–12 ft (2.5–3.5 m) to stop random cracks.
Concrete slab chart (4 in and 6 in)
| Length (ft) | Width (ft) | Thickness (in) | Before waste | Bags (80 lb bags) |
|---|---|---|---|---|
| 12 | 12 | 4 | 1.78 yd³ | 80 |
| 12 | 12 | 6 | 2.67 yd³ | 120 |
| 20 | 20 | 4 | 4.94 yd³ | 222 |
| 20 | 20 | 6 | 7.41 yd³ | 333 |
| 24 | 24 | 4 | 7.11 yd³ | 320 |
| 24 | 24 | 6 | 10.67 yd³ | 479 |
| 30 | 40 | 6 | 22.22 yd³ | 998 |
Related guides
Frequently asked questions
How much concrete do I need for a 20×20 slab?
At 4 in thick a 20 × 20 ft slab needs 4.94 yd³ (133.3 ft³); at 6 in it needs 7.41 yd³. Add 5–10 % when ordering.
How many bags of concrete for a 10×10 slab?
A 10 × 10 ft slab at 4 in takes about 61 bags of 80 lb concrete including 10 % extra.
Is it cheaper to use bags or ready-mix for a slab?
Below about 1 yd³ bags are usually cheaper and easier. Above that, ready-mix delivered by truck costs less per yard and gives a stronger, more even slab.
More calculators
Estimates only — check quantities with your supplier and local building code before you order.