How Many Bags of Concrete for a 12×12 Slab?
Exact bag counts for a 12 ft × 12 ft slab in 80, 60 and 40 lb premix bags at 3.5, 4, 5 and 6 inches thick — with the math shown so you can check it.
Quick answer
A 12×12 ft slab, 4 inches thick, is 48.0 ft³ (1.78 yd³) of concrete. That takes 80 bags of 80 lb premix, 107 bags of 60 lb, or 160 bags of 40 lb — before waste. With a 10% allowance, buy 88 × 80 lb bags.
Formula & worked math
Cubic yards = Volume(ft³) ÷ 27
Bags = Volume(ft³) ÷ bag yield(ft³), rounded up
- Area: 12 × 12 = 144 ft²
- Volume at 4 in: 144 × 4 ÷ 12 = 48.00 ft³ (÷ 27 = 1.778 yd³)
- 80 lb bags: 48.00 ÷ 0.60 = 80.00 → 80
- 60 lb bags: 48.00 ÷ 0.45 = 106.67 → 107
- 40 lb bags: 48.00 ÷ 0.30 = 160.00 → 160
- With 10% waste (80 lb): 80.00 × 1.10 = 88.00 → 88
Bags for a 12×12 slab at common thicknesses
Each bag cell shows exact need / with 10% waste, both rounded up.
| Thickness | ft³ | yd³ | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 3.5 in | 42.0 | 1.56 | 70 / 77 | 94 / 103 | 140 / 154 |
| 4 in | 48.0 | 1.78 | 80 / 88 | 107 / 118 | 160 / 176 |
| 5 in | 60.0 | 2.22 | 100 / 110 | 134 / 147 | 200 / 220 |
| 6 in | 72.0 | 2.67 | 120 / 132 | 160 / 176 | 240 / 264 |
Want a different size or your own waste percentage? Use the concrete bag calculator or get cubic yards for a ready-mix order from the concrete slab calculator.
Assumptions
- Bag yields are the figures printed on standard premix bags: 80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 40 lb ≈ 0.30 ft³. Check your brand's bag — some high-strength or fast-setting mixes differ.
- The slab is a flat rectangle of uniform thickness on a level, compacted base. Thickened edges, sloped subgrade or low spots add volume.
- Bag counts are always rounded up. The "+10%" column covers spillage, form bulge and an uneven base; 5–10% is a common allowance.
Bags or ready-mix for a 12×12?
At 1.78 yd³, this slab is above the point where many contractors switch from bags to a ready-mix truck (roughly 1 yd³, or about 45 × 80 lb bags). 88 bags of 80 lb weigh about 3.5 US tons — several trips for a pickup and a lot of hand-mixing, so a mixer rental and a helper are worth planning for. Many ready-mix suppliers have a minimum order or short-load fee, so price both before deciding; bagged concrete also needs to be placed fast enough that each batch blends into the last one.
FAQ
How many 80 lb bags of concrete do I need for a 12x12 slab?
At 4 inches thick, 80 bags exactly (48.0 ft³ ÷ 0.60), or 88 with a 10% waste allowance.
How many cubic yards is a 12x12 slab 4 inches thick?
144 × 4 ÷ 12 = 48.0 ft³, and 48.0 ÷ 27 = 1.78 cubic yards.
How many 60 lb bags for a 12x12 slab?
107 bags at 4 inches (48.00 ÷ 0.45 = 106.67), before waste. At 6 inches it is 160.
Last updated: October 12, 2026.