How Many Bags of Concrete Do I Need?
Concrete bag estimation depends on the volume of your project and the bag size you choose. Here's the step-by-step calculation.
Step 1: Calculate Volume
For a rectangular slab, footing, or pad:
Volume (cu ft) = length × width × (depth ÷ 12)
A 10 × 10 ft slab at 4" depth: 10 × 10 × (4 ÷ 12) = 33.33 cu ft.
Step 2: Add Waste Factor
Add 10% for spillage, overpour, and uneven subgrade:
Volume with waste = volume × 1.10
33.33 × 1.10 = 36.67 cu ft.
Step 3: Choose Bag Weight
Concrete bags come in three standard sizes:
| Bag Weight | Yield (cu ft) | Best For |
|---|---|---|
| 40 lb | 0.30 | Small repairs, post holes |
| 60 lb | 0.45 | Post holes, small slabs |
| 80 lb | 0.60 | Larger pours, most cost-effective |
Bag yield = bag weight ÷ 133.4 (an empirical constant derived from manufacturer specs).
Step 4: Calculate Bags
Bags = ceil(volume with waste ÷ bag yield)
With 80 lb bags: ceil(36.67 ÷ 0.60) = 62 bags.
With 60 lb bags: ceil(36.67 ÷ 0.45) = 82 bags.
When to Switch to Ready-Mix
For pours over 1 cubic yard (27 cu ft), consider ready-mix delivery:
- 1 cu yd = ~46 bags (80 lb) — at $5/bag = $230 in bags alone
- Ready-mix delivery: $120-200/cu yd (includes delivery, no mixing)
- Ready-mix saves significant labor — no mixing 46 bags by hand
Cost Estimation
The Concrete Bag Calculator lets you enter your cost per bag for a budget estimate. No fabricated prices — you enter the actual price from your local supplier.
Related Resources
- Concrete Bag Calculator — bag-count estimation with cost
- Concrete Calculator — general-purpose (cubic yards + bags)
- Concrete Bag Yield Chart — quick reference for bag yields