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 deciding how much extra to allow for waste.
Calculate It Instantly
Skip the math — use our concrete calculator:
Approximate QUIKRETE Concrete Mix 1101 yields. Each bag size is an alternative; check the yield on your product before ordering.
Material only. Delivery, labor, reinforcement and formwork are not included.
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
- With a 10% allowance: 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?
How much extra to order depends on your site and forms. These ranges are common starting points, not fixed rules:
| Situation | Waste Factor |
|---|---|
| Flat, formed slab | +5-10% |
| Uneven ground | +10-15% |
| Footings in dirt | +15-20% |
| First-time DIY | +10-15% |
Weighing it up: Ordering too little can leave you short mid-pour, while ordering too much leaves concrete to dispose of. The calculator's 10% allowance is optional — treat it as a starting point and adjust it for your project.
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?
These estimates use the approximate yields for QUIKRETE Concrete Mix 1101. Check the yield on your chosen product and round the required bag count up. Each bag size below is an alternative.
| Bag Size | Covers (cu ft) | Bags per Cubic Yard |
|---|---|---|
| 40 lb | 0.30 | 90 |
| 60 lb | 0.45 | 60 |
| 80 lb | 0.60 | 45 |
For a 10 ft × 10 ft slab at 4 inches thick, this gives 112 bags of 40 lb, 75 bags of 60 lb, or 56 bags of 80 lb before waste. With a 10% allowance, the counts become 123, 82, or 62 respectively.
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)
- Choose a waste allowance that fits your project; 10% is a starting point, not a universal rule
- 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
- Settle your allowance before ordering — too little can leave you short mid-pour, too much means leftover concrete to dispose of
Doing a larger renovation? Pair this with the paint calculator and tile calculator — same "measure → add waste factor → order" logic, different materials.