Concrete Slab Calculator
Cubic yards and bags of concrete for a slab, patio, or pad.
4 in is standard for patios and walkways, 5 to 6 in for driveways.
You need
4.07 cubic yards
Waste included. Order this amount from a ready-mix supplier.
- Volume with waste
- 110.0 cu ft
- Volume before waste
- 100.0 cu ft
- 80 lb bags
- 184 bags
- 60 lb bags
- 245 bags
Estimates only. Verify quantities before ordering.
How this is calculated
Volume = length (ft) x width (ft) x thickness (in) / 12, giving cubic feet. Divide by 27 to get cubic yards, then add your waste percentage. Bag counts assume an 80 lb bag yields 0.60 cu ft and a 60 lb bag yields 0.45 cu ft.
Enter the length and width of your slab in feet and the thickness in inches, and this calculator returns the volume of concrete you need in cubic yards, plus the equivalent number of bagged concrete mixes. Ready-mix suppliers sell by the cubic yard, so that is the number to give them when you call.
A waste factor is built in because some concrete is always lost to uneven subgrade, spillage, and over-excavation. 10% is a sensible default for a typical slab on grade. If your base is well compacted and formed precisely, you can drop it to 5%. For rough or sloped ground, use 15%.
Concrete for common slab sizes
Every row includes a 10% waste factor. Order the cubic-yard figure from a ready-mix supplier, or use the bag count for hand mixing.
| Slab size | Cubic yards | 80 lb bags |
|---|---|---|
| 8 x 8 ft, 4 in (shed pad) | 0.87 cubic yards | 40 bags |
| 10 x 10 ft, 4 in (patio) | 1.36 cubic yards | 62 bags |
| 12 x 12 ft, 4 in | 1.96 cubic yards | 88 bags |
| 10 x 20 ft, 4 in (parking pad) | 2.72 cubic yards | 123 bags |
| 20 x 20 ft, 4 in | 5.43 cubic yards | 245 bags |
| 24 x 24 ft, 6 in (garage) | 11.73 cubic yards | 528 bags |
Common questions
How thick should a concrete slab be?
4 inches is the standard for patios, walkways, and shed pads. Driveways that carry cars should be 5 to 6 inches, and slabs that will support heavy vehicles or equipment are often 6 inches or more with reinforcement. Local code may set a minimum, so check before you pour.
How many 80 lb bags of concrete make a cubic yard?
An 80 lb bag yields about 0.60 cubic feet, so a full cubic yard (27 cubic feet) takes about 45 bags. That is why bagged concrete only makes sense for small pours: beyond roughly one cubic yard, ready-mix delivered by truck is usually cheaper and far less work.
Should I order ready-mix or buy bags?
As a rule of thumb, use bags for anything under about 1 cubic yard (a small pad or a few fence posts) and ready-mix for anything larger. Mixing 40 or more bags by hand is exhausting and makes it hard to finish the slab before the first batches start to set.
Do I need rebar or wire mesh in a slab?
For most residential slabs 4 inches thick, welded wire mesh or fiber-reinforced mix controls cracking well. Driveways and thicker structural slabs usually call for #3 or #4 rebar on a grid. Reinforcement does not stop concrete from cracking, but it holds the cracks tightly together.