How much concrete for a 20x20 garage slab 6 inches thick?
Check it with your own numbers
Open the Concrete Calculator with these values
| Length | 20 ft |
| Width | 20 ft |
| Thickness | 6 in |
| Extra for waste | 10 % |
| Concrete needed | 8.15 yd³ |
| Cubic feet | 220 ft³ |
| 80 lb bags | 367 |
| 60 lb bags | 489 |
Volume
20 × 20 × 0.5 ft equals 200 cubic feet, or 7.41 yd³. Add 10% and you are at 8.15 yd³ (220 ft³). Order 8.5 yards. Running short on a garage pour is far worse than paying for half a yard extra, since the plant may not be able to get a second truck out before your first load sets.
Why 6 inches?
A 4-inch slab is fine for cars on good soil, and plenty of garages are built that way. Six inches buys you room for a truck, a car lift, a heavy workbench, or soil that is less than ideal. If you ever plan to bolt a two-post lift to the floor, most lift makers want at least 4.25 inches of 3,000 psi concrete, and many installers prefer more.
Bags are off the table
The calculator says 367 bags of 80 lb mix. That is not a DIY bag job. Get a ready-mix truck and ask for a 4,000 psi mix with air entrainment if you live where it freezes.
What adds yardage on a garage
- Thickened perimeter or grade beam. Many codes want a footing at the edge, often 12 inches wide and 12 to 18 inches deep. Around an 80 ft perimeter that adds 2.5 to 3.5 yd³.
- Apron. A 4 ft deep, 16 ft wide driveway apron at 6 inches is about another yard.
- Uneven base. Every half inch of low spot across the full 400 ft² is 0.6 yd³.
Reinforcement and joints
Use #4 rebar at 18 to 24 inches each way on chairs, or fiber mesh plus wire mesh if your inspector allows it. Lay out control joints at 10 ft, which splits the floor into four 10x10 panels. Saw-cut them within 12 hours of the pour.
Crew
An 8-yard pour moves fast. Plan on at least four people: two pulling and raking, one on the screed, one running the bull float. Rent a power trowel for the finish.
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