Concrete calculator for a 10x10, 10x12, or 12x12 slab
The three slab sizes people pour most often are 10×10, 10×12, and 12×12 — patio floors, shed bases, and small garage slabs. Each one is straightforward to size: length × width × thickness, then add waste. The widget at the top of this page already covers that math for any length, width, or depth; the tables below lock in the three sizes so you can read the answer without touching a calculator.
10x10 slab — cubic yards and bags
A 10 ft × 10 ft patio slab at the standard 4 in thickness holds about 33 cubic feet of concrete, which is 1.22 cubic yards raw. Adding the residential 10 % waste factor and rounding up to the nearest half yard, you order 1.5 cubic yards. That pour takes roughly 61 bags of 80 lb concrete or 81 bags of 60 lb concrete. Skip the cookies-and-creme of bag math and use ready-mix if your local vendor charges a fair short-load fee — the cost calculator below compares the two side by side.
10x12 slab — cubic yards and bags
A 10 ft × 12 ft slab at 4 in thick is 39.6 cubic feet — about 1.47 cubic yards raw. The vendor rolls that up to 2 cubic yards once you apply the 10 % waste factor. That is roughly 73 × 80 lb bags or 97 × 60 lb bags. This size is the sweet spot for a small two-car carport or a shed that holds riding equipment.
12x12 slab — cubic yards and bags
A 12 ft × 12 ft slab at 4 in thick is 47.52 cubic feet — about 1.76 cubic yards raw — and rounds up to 2 cubic yards with waste. Expect around 88 × 80 lb bags or 117 × 60 lb bags. A 12×12 easily carries a hot tub, a pool pump pad, or a small workshop floor — and at that footprint most homeowners switch to ready-mix because the bag count crosses the short-load threshold.
How thick should a concrete slab be?
Thickness is the single biggest driver of cost and the easiest place to undersize. Residential rule of thumb:
- 4 in — patios, walkways, shed floors, AC pads. No rebar needed; welded wire mesh is enough.
- 5–6 in — garage floors and driveways that see passenger vehicles. Rebar on a 12 in or 16 in grid is standard.
- 6 in with rebar on a tighter grid (12 in o.c.) — anything that holds a heavy load: hot tubs, RV pads, equipment storage, small commercial floors.
The slab is only as good as what sits under it: a 4 in patio pour over uncompacted fill will crack in the first frost heave. Compact the sub-base, add 4 in of crushed gravel, and use a vapor barrier under interior slabs.
Turn the volume into a price
Once you have cubic yards and bag counts, the next question is cost. The concrete cost estimator compares ready-mix dollars per yard against 80 lb and 60 lb bag prices for the same pour and surfaces the per-yard break-even so you know when bags stop being cheaper.