Your Result Explained: Exact Takeoff Breakdown
A 20 ft × 20 ft slab (400 square feet) is the standard footprint for a detached two-car garage, workshop, or pavilion. At a 4-inch floor thickness, the net interior volume is 133.33 cubic feet, which equals 4.94 cubic yards. Adding a 10% allowance for subgrade tolerance and thickened edges brings the order to 146.67 cubic feet, or 5.43 cubic yards (approx. 10.86 tons of wet concrete).
If you are pouring a heavy-duty 6-inch slab for a full-size truck garage or two-post automotive lift, net volume increases by 50% to 200.00 cubic feet (7.41 net cubic yards, or 8.15 cubic yards with 10% overage). Notice that in 80-lb bags, even the 4-inch slab would require 245 bags (nearly 6 pallets)—making a ready-mix concrete truck mandatory.
A standard rear-discharge or front-discharge ready-mix transit mixer holds 8 to 10 cubic yards, so both a 4-inch (5.5 yd³) and a 6-inch (8.2 yd³) 20×20 garage slab can be delivered in a single truckload.
Worked Math: 20 ft × 20 ft Garage Floor at 4" vs. 6" Thickness
Comparing ready-mix cubic yardage for a 400 sq ft garage slab at 4 inches and 6 inches thick with 10% waste.
- Garage Floor Area = 20 ft × 20 ft = 400 sq ft.
- 4" Net Volume = 400 × (4 ÷ 12) = 133.33 cu ft ÷ 27 = 4.94 cubic yards.
- 4" Adjusted Volume (+10%) = 4.94 × 1.10 = 5.43 cubic yards (order 5.5 yd³).
- 6" Net Volume = 400 × (6 ÷ 12) = 200.00 cu ft ÷ 27 = 7.41 cubic yards.
- 6" Adjusted Volume (+10%) = 7.41 × 1.10 = 8.15 cubic yards (order 8.25–8.5 yd³).
Bottom Line: Order 5.5 cubic yards of ready-mix for a 4-inch 20×20 slab (plus any separate perimeter footing trench volume), or 8.25 cubic yards for a 6-inch slab.
How This Scenario Is Calculated
Remember to add the volume of monolithic turned-down perimeter footings if pouring a "floating" Alaskan slab! For example, an 80-linear-foot perimeter trench dug 12 inches wide and 8 inches deeper than the 4-inch floor adds 80 × 1.0 × (8 ÷ 12) = 53.33 cubic feet (1.98 extra cubic yards).
Adding that 1.98-yard perimeter beam to the 4.94-yard center slab equals 6.92 net cubic yards (order 7.5 cubic yards total).
Cubic Yards = [Area (sq ft) × (Thickness in inches ÷ 12) × (1 + Waste%)] ÷ 27 Bags = Ceiling(Adjusted Cubic Feet ÷ Bag Yield in ft³) Project-Specific Tips & Common Mistakes
Lay a 6-Mil or 10-Mil Polyethylene Vapor Barrier Under Garage Floors
Installing a continuous plastic vapor retarder over the compacted gravel base stops ground moisture from wicking up through the concrete and rusting tools or blistering epoxy floor coatings.
Saw-Cut Four Equal 10×10 Foot Quadrants
Snap chalk lines at the 10-foot midpoints in both directions and saw-cut 1-inch-deep control joints as soon as the concrete supports a walk-behind concrete saw without raveling.
Mistake Forgetting to add monolithic perimeter grade-beam volume
Recommended Fix: Always calculate the center 4-inch slab and the deepened perimeter footing trench as two separate rectangular sections using our Multi-Section tab.
Reference Coverage Table (Normal-Weight Portland Cement Concrete)
Compare how coverage changes across common depths for Normal-Weight Portland Cement Concrete (default planning density 4000 lbs/yd³):
| Depth | 1 Cubic Yard Covers | 1 Ton Covers | Yards / 100 sq ft | Tons / 100 sq ft | Bags (0.6 ft³) / 100 sq ft |
|---|---|---|---|---|---|
| 3.5 in | 92.6 sq ft | 46.3 sq ft | 1.08 yd³ | 2.16 tons | 49 bags |
| 4 in Active | 81 sq ft | 40.5 sq ft | 1.23 yd³ | 2.46 tons | 56 bags |
| 5 in | 64.8 sq ft | 32.4 sq ft | 1.54 yd³ | 3.08 tons | 70 bags |
| 6 in | 54 sq ft | 27 sq ft | 1.85 yd³ | 3.7 tons | 84 bags |
| 8 in | 40.5 sq ft | 20.3 sq ft | 2.47 yd³ | 4.94 tons | 112 bags |
Reference Source: ACI 302.1R / ACI 318 (145–150 lbs/ft³) & Quikrete No. 1101 Technical Data Sheet.
Frequently Asked Questions
Will 5.5 cubic yards of concrete fit in one ready-mix truck?
Yes. Standard full-size ready-mix concrete trucks carry 8 to 10 cubic yards per load, and avoids any short-load fee (which typically only applies under 4 or 5 yards).
How thick does concrete need to be for a 2-post car lift in a 20x20 garage?
Most two-post automotive lift manufacturers (such as BendPak and Rotary) require a minimum 4.25- to 6-inch-thick slab of 4,000 PSI concrete reinforced with rebar.
How many tons of gravel base do I need under a 20x20 slab?
A 4-inch compacted crusher-run base across 400 sq ft requires 4.94 net cubic yards (5.68 yards with 15% compaction), which equals approximately 8.0 tons of road base gravel.
How much concrete for a 24x24 two-car garage slab at 4 inches?
A 24×24 slab (576 sq ft) at 4 inches thick requires 7.11 net cubic yards (7.82 cubic yards with 10% overage).
Should I use bull float and power trowel on a 20x20 garage floor?
Yes—for an interior garage floor, follow bull floating with a walk-behind power trowel for a dense, smooth, sweepable finish, while leaving the exterior driveway apron broom-finished for traction.