How to Calculate Concrete for Your Project
Step-by-step guide with formulas and examples for slabs, footings, columns, and walls.
Need a quick calculation?
Use our free concrete calculator for instant results.
Whether you're pouring a patio, building a foundation, or setting fence posts, knowing how to calculate concrete accurately is essential. Order too little, and your project stalls. Order too much, and you waste money. This guide will teach you the exact formulas professionals use to get it right every time.
Understanding Concrete Measurements
In the United States, concrete is measured and sold in cubic yards. One cubic yard equals 27 cubic feet. Understanding this conversion is the foundation of all concrete calculations.
Key Conversions
- 1 cubic yard = 27 cubic feet
- 1 cubic yard = 46,656 cubic inches
- 60 lb bag ≈ 0.45 cubic feet (covers ~5.4 sq ft at 4" thick)
- 80 lb bag ≈ 0.60 cubic feet (covers ~7.2 sq ft at 4" thick)
When you order concrete from a supplier, they'll ask for cubic yards. When you buy bags at a home improvement store, you'll need to know how many bags. This guide covers both scenarios.
How to Calculate Concrete for a Slab
A concrete slab is the most common project type—patios, driveways, shed floors, and walkways all use this calculation.
Volume (cubic feet) = Length × Width × (Thickness ÷ 12)
Divide thickness by 12 to convert inches to feet.
Example: 10′ × 12′ Patio, 4″ Thick
- Convert thickness to feet: 4″ ÷ 12 = 0.33 ft
- Calculate volume (cubic feet): 10 × 12 × 0.33 = 40 cu ft
- Convert to cubic yards: 40 ÷ 27 = 1.48 yd³
- Add 10% for waste: 1.48 × 1.10 = 1.63 cubic yards
For this patio, you'd order 1.63 cubic yards of concrete, or round up to 2 cubic yards to be safe.
How to Calculate Concrete for Footings
Footings are the foundations that support structures. They're typically deeper than slabs and often have a square or rectangular cross-section.
Volume (cubic feet) = Length × Width × Depth
Example: Deck Post Footings
Building a deck with 6 posts, each footing 2′ × 2′ × 1′ deep.
- Volume per footing: 2 × 2 × 1 = 4 cubic feet
- Total for 6 footings: 4 × 6 = 24 cubic feet
- Convert to cubic yards: 24 ÷ 27 = 0.89 cubic yards
How to Calculate Concrete for Round Columns
Round columns require a different formula because of their circular cross-section.
Volume = π × (Diameter ÷ 2)² × Height
π ≈ 3.14159
Example: 12″ Diameter Column, 8′ Tall
- Convert diameter to feet: 12″ ÷ 12 = 1 ft
- Calculate radius: 1 ÷ 2 = 0.5 ft
- Volume: 3.14159 × (0.5)² × 8 = 6.28 cubic feet
- Convert to cubic yards: 6.28 ÷ 27 = 0.23 cubic yards
Converting to Bags of Concrete
For smaller projects, you might prefer using premixed bags instead of ordering concrete by the yard. Here's how to convert.
60 lb bags = Volume (cu ft) ÷ 0.45 | 80 lb bags = Volume (cu ft) ÷ 0.60
Round up to the nearest whole bag.
Example: 40 Cubic Foot Project
| Bag Size | Coverage per Bag | Bags Needed |
|---|---|---|
| 60 lb | 0.45 cu ft | 40 ÷ 0.45 = 89 bags |
| 80 lb | 0.60 cu ft | 40 ÷ 0.60 = 67 bags |
Which should you choose? 80 lb bags are more economical (fewer bags needed), but 60 lb bags are easier to carry and mix, especially if you're working alone.
Estimating Concrete Costs
Concrete prices vary significantly by location and market conditions. Here's what to expect:
| Type | Price Range | Best For |
|---|---|---|
| Ready-mix (delivered) | $100–$200 / cu yd | Large projects (1+ yd³) |
| 60 lb bag | $4–$6 / bag | Small projects, DIY |
| 80 lb bag | $5–$7 / bag | Small projects, DIY |
Additional Costs to Consider
- Delivery fees (often $50–$100+ for small loads)
- Short load fees (orders under 6–8 cubic yards)
- Reinforcement (rebar, wire mesh)
- Forms and forming materials
- Labor if hiring a contractor
Pro Tips for Ordering Concrete
Always Add 5–10% Extra
Account for spillage, uneven ground, and over-excavation. Better to have extra than come up short.
Measure Twice, Order Once
A 6″ slab mistakenly calculated as 4″ will be 50% under-ordered. Double-check everything.
Consider the Subgrade
If the ground isn't perfectly level, measure depth at multiple points and use the average.
Order Early
Ready-mix plants get busy, especially in summer. Schedule delivery at least a few days ahead.
Common Mistakes to Avoid
Forgetting to Convert Units
Mixing inches and feet without converting is the #1 error. Always convert all measurements to the same unit first.
Not Accounting for Waste
Real-world conditions are never perfect. Always add a buffer for spillage and subgrade variations.
Calculating for Perfect Conditions
Drawings show ideal dimensions, but excavations may be wider or deeper. Measure the actual hole, not the plan.
Conclusion
Calculating concrete doesn't have to be complicated. Whether you're pouring a simple patio slab or a complex foundation, the formulas in this guide will help you get accurate estimates every time.
Remember: measure carefully, convert units properly, and always add a buffer for waste. When in doubt, use our free concrete calculator to double-check your math.
Ready to Calculate?
Use our free concrete calculator for instant, accurate results on any device.
Open Concrete Calculator