This page answers one size only: a concrete slab 10 feet by 12 feet. The answer below is for that size only. To change the size, thickness, waste allowance, or bag size, use the Concrete Slab Calculator.
Concrete slab 10 by 12 ft: summary
| Item | Value |
|---|---|
| Slab length | 12 ft |
| Slab width | 10 ft |
| Area | 120 sq ft |
| Thickness used | 4 in |
| Volume before waste | 40 cu ft (1.48 cu yd) |
| Waste allowance | 10% |
| Order volume | 44 cu ft (1.63 cu yd) |
| Bags, 80 lb | 74 bags |
Worked example in full
Step 1 — Area: 12 ft × 10 ft = 120 square feet.
Step 2 — Thickness in feet: 4 in ÷ 12 = 0.333 feet.
Step 3 — Volume in cubic feet: 120 sq ft × 0.333 ft = 40 cubic feet.
Step 4 — Volume in cubic yards: 40 ÷ 27 = 1.48 cubic yards.
Step 5 — Waste allowance: 40 cu ft × 1.10 = 44 cubic feet, and 44 ÷ 27 = 1.63 cubic yards.
Step 6 — Bags: each 80 lb bag covers 0.60 cubic feet, so 44 ÷ 0.60 = 73.33, which rounds up to 74 bags of 80 lb.
Answer for this size only: a 10 by 12 ft slab at 4 inches thick needs 1.63 cubic yards, or 74 bags of 80 lb concrete, with a 10% waste allowance. This result was verified against the Concrete Slab Calculator using length 12 ft, width 10 ft, thickness 4 in, waste 10%, and an 80 lb bag size.
Frequently asked questions
Is 74 bags the answer for any slab?
No. It is the answer for a 10 by 12 ft slab at 4 inches thick with 10% waste and 80 lb bags only. A different size, thickness, waste allowance, or bag size gives a different answer.
What if I use 60 lb bags?
Change the bag size in the Concrete Slab Calculator. The volume stays 44 cubic feet with waste, but the bag count changes because each bag covers less volume.
What if my slab is thicker?
Repeat the steps above with your thickness in feet, or enter your size and thickness in the Concrete Slab Calculator.
Method note
This is a planning calculation for the stated size and thickness only. It assumes a flat rectangular slab at a uniform 4-inch thickness. Confirm thickness, reinforcement, curing, and local requirements with your supplier before ordering.