How Much Concrete for a 12×12 Slab at 6 Inches?
A 12×12 ft concrete slab at 6 inches thick is common for garage floors and driveway aprons that see heavy vehicle traffic. A ready-mix truck is the practical choice. Here are the pre-calculated amounts with 10% waste included.
Bag size
You'll need
2.93 cubic yards
Cubic feet
79.20
80 lb bags
132
Weight
5.94 tons
How This Calculator Works
How we calculated this page
12 ft × 12 ft × 6 in (converted to feet) = 72.000 cu ft base → ×1.10 waste = 79.200 cu ft → ÷27 = 2.93 cu yd
The standard formula for a concrete slab — and the same math this calculator uses for all pour shapes:
Slab / Footing Volume (cu ft) = Length × Width × Depth (all in feet) Round column Volume (cu ft) = π × (Diameter ÷ 2)² × Height Convert to cubic yards Cu yds = Volume (cu ft) ÷ 27 Apply waste Final cu yds = Cu yds × (1 + Waste% ÷ 100) Bag count Bags = ⌈ Final cu ft ÷ yield per bag ⌉
One cubic yard equals exactly 27 cubic feet. A standard 80 lb bag yields approximately 0.60 cubic feet of finished concrete; a 60 lb bag yields 0.45 cu ft; a 40 lb bag yields 0.30 cu ft. These are industry-standard values used by Quikrete and Sakrete.
Bag counts always round up because you can't purchase a fraction of a bag. Concrete weight is estimated at 150 lb per cubic foot — the standard for normal-weight concrete.
Common Project Sizes
Base quantities without waste factor. Add 10% for a typical residential pour.
| Slab Size (4" thick) | Cubic Yards | Cubic Feet | 80 lb Bags |
|---|---|---|---|
| 4 × 4 ft | 0.20 | 5.3 | 9 |
| 4 × 8 ft | 0.40 | 10.7 | 18 |
| 6 × 6 ft | 0.44 | 12.0 | 20 |
| 8 × 8 ft | 0.79 | 21.3 | 36 |
| 10 × 10 ft | 1.23 | 33.3 | 56 |
| 12 × 12 ft | 1.78 | 48.0 | 80 |
| 16 × 16 ft | 3.16 | 85.3 | 143 |
| 20 × 20 ft | 4.94 | 133.3 | 223 |
How to Use This Calculator
The calculator above is pre-filled with the dimensions for a 12×12 ft slab at 6" thick. You can adjust any field — length, width, thickness, quantity, waste percentage, or bag size — and the results will update instantly.
Select Footing or Column mode at the top if your project shape differs. Switch units on any field by clicking the ft / in / m / cm buttons.
Use the Quantity field if you're pouring multiple identical shapes. The Waste % default of 10% is standard for residential slabs — increase to 15% for irregular forms or first-time pours.
Frequently Asked Questions
How many bags of concrete do I need for a 12×12 slab?
A 12×12 ft slab at 6 inches thick requires approximately 132 bags of 80 lb concrete mix (with the standard 10% waste factor). Switch to 60 lb bags and you'll need about 176, or 40 lb bags for about 264.
How many cubic yards of concrete for a 12×12 slab?
A 12×12 ft slab at 6" thick needs 2.93 cubic yards of concrete (including the standard 10% waste factor). Without waste, the base volume is 2.67 cubic yards.
How much extra concrete should I order?
Order 10% more than your calculated volume for most pours. Increase to 15% for larger slabs or if your forms are irregular. The 10% default in this calculator accounts for typical spillage, over-pour at edges, and uneven subgrades. Running short means a cold joint — always worse than having a small amount left over.
Should I use bags or order ready-mix concrete?
For pours under about 1 cubic yard, bags from a hardware store are practical. For larger pours, a ready-mix truck is far more economical and saves hours of mixing. The breakeven is typically around 0.5–1 cubic yard.
How many 80 lb bags of concrete are in a cubic yard?
You need approximately 45 bags of 80 lb concrete mix per cubic yard. An 80 lb bag yields about 0.60 cubic feet of finished concrete, and one cubic yard equals 27 cubic feet: 27 ÷ 0.60 = 45 bags exactly.
What does the waste percentage account for?
The waste factor covers spillage during pouring, over-pour at form edges, uneven subgrade that adds volume, and concrete left in the mixer or chute. For a clean slab on level ground, 10% is standard.
Related Calculators
Concrete Calculator
Calculate concrete for slabs, footings, and columns. Get cubic yards, bag count, and weight.
Gravel Calculator
Calculate gravel, crushed stone, and pea gravel by weight or volume for any area.
Mulch Calculator
Find out how many bags or cubic yards of mulch you need for any bed or landscape.
Fence Calculator
Estimate posts, panels, concrete, and pickets for wood, chain link, or vinyl fencing.
Flooring Calculator
Calculate square footage and box count for hardwood, laminate, vinyl, tile, or carpet.