Your Result Explained: Exact Takeoff Breakdown
A 12 ft × 12 ft concrete pad (144 square feet) is a classic size for a backyard gazebo, storage shed, or bistro patio. At a true 4-inch thickness (0.3333 feet), the exact geometric volume is 48.00 cubic feet, which divides by 27 to equal 1.78 net cubic yards.
Adding a prudent 5% subgrade allowance brings the required volume to 50.40 cubic feet (1.87 cubic yards). In pre-mixed bags, 50.40 cubic feet requires 84 bags of 80-lb concrete (exactly two full 42-bag retail pallets) or 112 bags of 60-lb concrete. Cured weight for the 1.87-yard slab is approximately 7,480 pounds (3.74 tons).
At 84 eighty-pound bags (6,720 lbs of dry mix), hand-mixing requires roughly 63 gallons of water and 42 batches in a two-bag portable mixer. Ordering 2.0 cubic yards from a ready-mix truck or volumetric mobile mixer saves an entire day of heavy lifting.
Worked Math: 12 ft × 12 ft Slab at 4 Inches Thick (+5% Overage)
Step-by-step volume, ready-mix yardage, and 80 lb / 60 lb bag calculation for a 144 sq ft slab.
- Slab Area = 12 ft × 12 ft = 144 sq ft.
- Thickness in Feet = 4 in ÷ 12 = 0.3333 ft.
- Net Volume = 144 sq ft × 0.3333 ft = 48.00 cubic feet (1.78 yd³).
- Net 80 lb Bags (0.60 ft³) = 48.00 ÷ 0.60 = 80 bags.
- With 5% Subgrade Overage = 48.00 × 1.05 = 50.40 cubic feet (1.87 yd³).
- Adjusted 80 lb Bags = 50.40 ÷ 0.60 = 84 bags (or 112 bags of 60 lb mix).
Bottom Line: Order 2.0 cubic yards of ready-mix concrete, or purchase 84 bags (2 pallets) of 80-lb concrete mix.
How This Scenario Is Calculated
If you form your 12×12 slab with standard 2×4 lumber resting flush on a flat gravel base, the actual board height is 3.5 inches rather than 4.0 inches. At 3.5 inches thick, net volume is 42.00 cubic feet (1.56 yd³, or 70 bags of 80-lb concrete).
You can test 3.5 inches vs. 4.0 inches directly in the calculator above by typing 3.5 into the Slab Thickness box.
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
Cut One Cross-Shaped Control Joint Pattern at 6 Feet
Because 12 feet slightly exceeds the 8-to-10-foot joint spacing rule for a 4-inch slab, tool or saw-cut two perpendicular control joints right down the 6-foot centerlines to divide the 12×12 pad into four neat 6×6 ft squares.
Pitch the Patio 1/8 to 1/4 Inch per Foot for Rain Runoff
Over a 12-foot span, set the outer form board 1.5 to 2.5 inches lower than the house-side form board so rainwater sheets off rapidly.
Mistake Trying to mix 84 bags in a wheelbarrow with a hoe
Recommended Fix: Concrete begins setting within 45 to 60 minutes in warm weather. Mixing 84 bags by hand takes far too long, creating weak cold joints between sections. Rent a towable mortar/concrete drum mixer and have a 3-person crew if pouring from bags.
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
How many 60 lb bags of concrete do I need for a 12x12 slab (4" thick)?
A 12×12 slab at 4 inches thick (48 net cubic feet) requires 107 sixty-pound bags net, or 112 sixty-pound bags with a 5% waste factor.
How much gravel base do I need under a 12x12 concrete slab?
A 4-inch compacted crusher-run sub-base across 144 sq ft requires 1.78 net cubic yards (about 2.5 to 2.75 tons of road base gravel).
How many pieces of rebar for a 12x12 slab on an 18-inch grid?
Spacing #3 or #4 rebar 18 inches on-center both ways across a 12×12 slab requires 16 twelve-foot lengths of rebar (8 running North-South and 8 running East-West).
How many bags of concrete for a 12x12 slab at 6 inches thick?
Increasing thickness to 6 inches raises net volume by 50% to 72 cubic feet (2.67 cubic yards), requiring 120 net bags (or 126 bags with 5% overage) of 80-lb concrete.
How long before I can walk or build a shed on a new 12x12 slab?
You can typically strip forms and walk carefully on the slab after 24 to 48 hours, begin framing light walls after 3 to 7 days, and apply full heavy loads after 28 days of curing.