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How to calculate concrete for any project
Concrete is sold by the cubic yard, and one cubic yard equals 27 cubic feet. That single conversion drives every concrete estimate. For a slab, multiply length × width × depth in feet, then divide by 27. A 10 × 12 ft patio at 4 inches deep needs (10 × 12 × 0.333) / 27 = 1.48 cubic yards. Suppliers prefer half-yard increments, so order 1.5 cubic yards.
\[\text{Cubic Yards} = \frac{\text{Length (ft)} \times \text{Width (ft)} \times \text{Depth (ft)}}{27}\]The depth conversion is where most mistakes happen. Slabs are spec’d in inches; the formula needs feet. Divide inches by 12: 4 inches = 0.333 ft, 6 inches = 0.5 ft, 8 inches = 0.667 ft.
Common slab depths
| Project | Standard depth |
|---|---|
| Walkway, patio | 4 in (0.333 ft) |
| Driveway (passenger car) | 4-5 in |
| Driveway (truck/RV) | 6 in |
| Garage floor | 4-6 in |
| Shed slab | 4 in |
| Pool deck | 4 in |
| Heavy equipment pad | 6-8 in |
A typical residential driveway at 5 inches uses 0.417 ft in the formula.
Worked examples
Patio slab
A 10 × 14 ft patio at 4 in deep:
\[\frac{10 \times 14 \times 0.333}{27} = \frac{46.7}{27} \approx 1.73 \text{ cubic yards}\]Order 1.75 or 2 yards. The extra covers spillage and minor depth variation.
Driveway slab
A 12 × 24 ft driveway at 5 in deep:
\[\frac{12 \times 24 \times 0.417}{27} = \frac{120}{27} \approx 4.45 \text{ cubic yards}\]Order 4.5 cubic yards. For mixer truck deliveries, anything above 4 yards is usually a single load.
Garage floor
A 24 × 24 ft two-car garage at 4 in deep:
\[\frac{24 \times 24 \times 0.333}{27} = \frac{192}{27} \approx 7.11 \text{ cubic yards}\]Order 7.5 cubic yards.
Round footings (sonotubes and posts)
Cylindrical concrete forms use the area of a circle:
\[\text{Cubic Yards} = \frac{\pi \times r^2 \times \text{Depth (ft)}}{27}\]Where r is the radius in feet (half the diameter).
Sonotube footing
For a 12-inch diameter sonotube (radius 0.5 ft) at 48 inches deep (4 ft):
\[\frac{\pi \times 0.5^2 \times 4}{27} = \frac{3.14}{27} \approx 0.116 \text{ cubic yards}\]A single sonotube is small enough that bagged concrete is usually more practical than ordering a delivery.
For 6 sonotubes of the same size: 6 × 0.116 = 0.7 cubic yards. Bag math: an 80 lb bag of concrete makes about 0.022 cubic yards, so 0.7 / 0.022 ≈ 32 bags. At that quantity, ordering a half-yard delivery is usually faster.
| Sonotube diameter | Concrete per linear foot |
|---|---|
| 8 in | 0.013 cubic yards |
| 10 in | 0.020 cubic yards |
| 12 in | 0.029 cubic yards |
| 14 in | 0.040 cubic yards |
| 16 in | 0.052 cubic yards |
| 18 in | 0.066 cubic yards |
| 24 in | 0.116 cubic yards |
For a 12 in sonotube at 4 ft deep: 4 × 0.029 = 0.116 cubic yards (matches the calculation above).
Footings for foundations and retaining walls
Strip footings are long, narrow slabs:
\[\text{Cubic Yards} = \frac{\text{Length (ft)} \times \text{Width (ft)} \times \text{Depth (ft)}}{27}\]For a 40 ft long footing, 16 inches wide (1.333 ft), 8 inches deep (0.667 ft):
\[\frac{40 \times 1.333 \times 0.667}{27} = \frac{35.6}{27} \approx 1.32 \text{ cubic yards}\]Footings below grade should add 10-15% for over-dig and irregular soil. So order 1.5 cubic yards for the example above.
Bagged concrete vs ready-mix
Bagged concrete is sold by weight; the bag tells you the cubic yards or cubic feet of finished concrete:
| Bag size | Cubic yards per bag | Cubic feet per bag |
|---|---|---|
| 40 lb | 0.011 | 0.30 |
| 50 lb | 0.014 | 0.375 |
| 60 lb | 0.017 | 0.45 |
| 80 lb | 0.022 | 0.60 |
| 90 lb | 0.025 | 0.675 |
To convert cubic yards to bags:
\[\text{Bags Needed} = \frac{\text{Cubic Yards}}{\text{Yards per Bag}}\]For 0.5 cubic yards using 80 lb bags: 0.5 / 0.022 ≈ 23 bags.
The break-even between bagged and ready-mix is roughly 1 cubic yard. Below that, bagged is usually cheaper after delivery and minimum-load fees. Above 1 yard, ready-mix is faster, more consistent, and often cheaper per cubic yard.
Waste factors and ordering practices
Concrete suppliers fill orders in half-yard increments (1.5, 2.0, 2.5, etc.). Round up:
- Calculated need: 1.45 yards → order 1.5
- Calculated need: 2.10 yards → order 2.5
Add waste:
- Slabs (well-formed, level subgrade): 5%
- Slabs (uneven subgrade or sloped forms): 10%
- Footings or below-grade work: 10-15%
- Pumping concrete (long hose): add 1 yard for line waste
Running short of concrete mid-pour is one of the worst outcomes. The truck will not wait while you order more. A short pour leaves a cold joint that weakens the slab. Slightly over-ordering is much better than running out.
Pricing
Ready-mix concrete typically costs $130-180 per cubic yard delivered, with these common surcharges:
- Short load fee (under 5 yards): $50-150
- Wait time over 5-10 minutes: $50-100 per hour
- Saturday delivery: $50-100
- Pumping (if needed): $500-1,500
For a 5 cubic yard pour at $150/yard with no extras: $750. The same pour at 4.5 yards might trigger a short-load fee that raises the per-yard cost.
Bagged concrete is roughly $5-10 per 80 lb bag (about $230-460 per cubic yard equivalent). The hidden cost is labor: mixing 23 bags of concrete by hand is several hours of work versus 30 minutes of pour time from a truck.
Quick reference
For a 4-inch slab, multiply square footage by 0.0123 to get cubic yards:
| Slab size | Sq ft | Cubic yards (4 in) |
|---|---|---|
| 8 × 10 | 80 | 1.0 |
| 10 × 12 | 120 | 1.5 |
| 10 × 20 | 200 | 2.5 |
| 12 × 24 | 288 | 3.5 |
| 16 × 20 | 320 | 4.0 |
| 20 × 24 | 480 | 6.0 |
| 24 × 24 | 576 | 7.1 |
For a 6-inch slab, multiply by 0.0185 instead.
The concrete calculator handles slabs, footings, and round forms with depth and waste built in.