Concrete Volume Formulas
\(\text{Thickness (ft)} = \frac{\text{Thickness (in)}}{12}\) \(\text{Net Volume (cu ft)} = \text{Length (ft)} \times \text{Width (ft)} \times \text{Thickness (ft)}\) \(\text{Total Volume (cu yd)} = \frac{\text{Net Volume (cu ft)} \times (1 + \frac{\text{Waste \%}}{100})}{27}\)
\[\text{80 lb Bags} = \lceil \text{Total cu ft} \times 0.6 \rceil \quad | \quad \text{60 lb Bags} = \lceil \text{Total cu ft} \times 0.45 \rceil\]Concrete Volume & Bag Count Reference Table (4-Inch Slab Thickness)
| Slab Dimensions (Length × Width) | Net Volume (cu ft) | Total Volume (+10% Waste) | 80 lb Bags Needed | 60 lb Bags Needed | Recommended Delivery |
|---|---|---|---|---|---|
| 10 ft × 10 ft (100 sq ft) | 33.33 cu ft | 1.36 cu yds | 61 Bags | 82 Bags | Ready-Mix / Bags |
| 12 ft × 20 ft (240 sq ft) | 80.00 cu ft | 3.26 cu yds | 147 Bags | 196 Bags | Ready-Mix Truck |
| 20 ft × 20 ft (400 sq ft) | 133.33 cu ft | 5.43 cu yds | 245 Bags | 326 Bags | Ready-Mix Truck |
How to Use This Concrete Yardage Calculator
- Enter slab length and slab width in feet.
- Specify slab thickness in inches (standard patio = 4 in, driveway = 6 in).
- Set waste & overage factor (10% recommended).
- View cubic yards required, cubic feet volume, and exact 80lb/60lb pre-mix bag counts.
Frequently Asked Questions
How do you calculate cubic yards of concrete?
Multiply Length (ft) × Width (ft) × Thickness (ft) and divide by 27. (Thickness in feet = Inches / 12).
How many 80lb bags of concrete make a cubic yard?
It takes 45 bags of 80 lb pre-mix concrete to equal 1 cubic yard (27 cubic feet).
How many 60lb bags of concrete make a cubic yard?
It takes 60 bags of 60 lb pre-mix concrete to equal 1 cubic yard (27 cubic feet).
Why should you add a waste factor to concrete calculations?
Subgrades are rarely perfectly level, and forms flex under pressure. Adding a 5% to 10% waste factor prevents running short during a pour.
What thickness is recommended for concrete slabs?
Standard walkways and patios require 4 inches. Residential driveways require 5 to 6 inches, while heavy equipment pads require 6+ inches with rebar reinforcement.
When is ready-mix truck delivery cheaper than buying bags?
For projects requiring more than 1 cubic yard (approx. 45 bags of 80lb concrete), ready-mix truck delivery saves significant labor and often costs less overall.
Is my project data saved on external servers?
No. All calculation logic executes locally in your browser.