The Basic Formula
Calculating concrete is straightforward — you need to find the volume of the area you're filling:
Volume = Length × Width × Depth
The tricky part? Converting to the right units and adding enough for waste.
Calculate It Instantly
Skip the math — use our concrete calculator:
How to Calculate for Common Projects
Slabs (Driveways, Patios, Garage Floors)
Most residential slabs are 4 inches thick. Here's how to calculate:
- Measure length and width in feet
- Convert thickness to feet (4 inches = 0.333 feet)
- Multiply: L × W × D = cubic feet
- Divide by 27 to get cubic yards
Example: A 20 × 10 foot patio, 4 inches thick:
- 20 × 10 × 0.333 = 66.6 cubic feet
- 66.6 ÷ 27 = 2.47 cubic yards
- Add 10% waste: 2.72 cubic yards → order 3 yards
Footings
Footings are typically deeper (12-24 inches) and narrower than slabs.
Example: A continuous footing 50 feet long, 2 feet wide, 1 foot deep:
- 50 × 2 × 1 = 100 cubic feet
- 100 ÷ 27 = 3.7 cubic yards
Columns and Posts
For round columns, use: Volume = π × r² × height
Example: A 12-inch diameter column, 8 feet tall:
- Radius = 6 inches = 0.5 feet
- π × 0.5² × 8 = 6.28 cubic feet
- For 4 columns: 25.13 cubic feet = 0.93 cubic yards
Stairs
Calculate each step as a separate rectangular box, then add them up.
Example: 4 steps, each 3 feet wide, 12 inches deep, 7 inches high:
- Step 1: 3 × 1 × 0.583 = 1.75 cu ft
- Step 2: 3 × 1 × 1.167 = 3.5 cu ft
- Step 3: 3 × 1 × 1.75 = 5.25 cu ft
- Step 4: 3 × 1 × 2.333 = 7.0 cu ft
- Total: 17.5 cubic feet = 0.65 cubic yards
How Much Extra Should You Order?
Always order more than your calculated amount:
| Situation | Waste Factor |
|---|---|
| Flat, formed slab | +5-10% |
| Uneven ground | +10-15% |
| Footings in dirt | +15-20% |
| First-time DIY | +10-15% |
Rule of thumb: Add 10% for standard projects. It's far cheaper to have a little extra than to be short and need a second delivery.
Bags vs. Ready-Mix Truck
| Factor | Pre-Mixed Bags | Ready-Mix Truck |
|---|---|---|
| Best for | Under 1 cubic yard | Over 1 cubic yard |
| Cost per yard | $100-150 | $120-180 |
| Labor | Heavy (80 lb bags!) | Minimal |
| Quality | Inconsistent mixing | Factory-mixed, consistent |
| Minimum order | 1 bag | Usually 1 yard |
| Delivery fee | None (pickup) | $50-100+ |
Break-even point: Around 1 cubic yard (about 45 bags of 80 lb mix). Beyond that, a truck is almost always better.
How Many Bags Do I Need?
| Bag Size | Covers (cu ft) | Bags per Cubic Yard |
|---|---|---|
| 40 lb | 0.30 | 90 |
| 60 lb | 0.45 | 60 |
| 80 lb | 0.60 | 45 |
Concrete Strength Guide
| PSI Rating | Use Case |
|---|---|
| 2,500 PSI | Sidewalks, patios |
| 3,000 PSI | Driveways, slabs |
| 3,500 PSI | Structural foundations |
| 4,000 PSI | Heavy traffic, commercial |
| 5,000+ PSI | Industrial, freeze-thaw areas |
For most residential projects, 3,000 PSI is the standard choice.
Tips for a Successful Pour
- Prep the ground — Remove soft soil, compact the base, add 4" of gravel for drainage
- Build forms — Use straight 2×4 lumber, staked every 2-3 feet
- Reinforce — Use rebar or wire mesh for slabs over 4" thick
- Pour all at once — Cold joints (where old concrete meets new) are weak points
- Finish properly — Screed, bull float, then let it set before final troweling
- Cure it — Keep concrete moist for 7 days (cover with plastic or use curing compound)
Key Takeaways
- Volume = Length × Width × Depth, converted to cubic yards (÷ 27)
- Always add 10% for waste (more for uneven ground)
- Use a ready-mix truck for anything over 1 cubic yard
- Standard residential concrete is 3,000 PSI
- Proper curing (7 days moist) is critical for strength
- When in doubt, order slightly more — running short mid-pour is a disaster