How to Calculate Concrete for a Slab
Learn the industry-standard formula for calculating cubic yards of concrete needed for slabs, patios, driveways, and footings. This guide covers thickness recommendations, cost estimates, and common mistakes to avoid.
Quick Answer: The Concrete Formula
For a 10×10 foot slab at 4 inches thick: (10 × 10 × 4 ÷ 12) ÷ 27 = 1.23 cubic yards. Add 7.5% for waste = 1.32 cubic yards to order.
In This Guide
How do you calculate cubic yards of concrete?
Cubic yards of concrete are calculated using the formula: (Length × Width × Thickness/12) ÷ 27. Length and width are measured in feet, thickness in inches, and the result is in cubic yards. This is the industry-standard formula used by professional contractors and ready-mix suppliers.
Breaking Down the Formula
Length × Width = Square Feet
Multiply your slab dimensions to get the area in square feet
Thickness ÷ 12 = Thickness in Feet
Convert thickness from inches to feet by dividing by 12
Square Feet × Thickness in Feet = Cubic Feet
Multiply area by depth to get volume in cubic feet
Cubic Feet ÷ 27 = Cubic Yards
There are 27 cubic feet in 1 cubic yard (3×3×3)
Always Add a Safety Margin
Add 7.5-10% extra concrete to account for spillage, uneven ground, and form boards. Running short mid-pour is far more expensive than ordering slightly extra.
How thick should a concrete slab be?
Concrete slab thickness depends on the application and expected load. Using the correct thickness ensures structural integrity and prevents cracking. Thicker slabs cost more but provide greater durability and load-bearing capacity.
| Application | Recommended Thickness | Reinforcement |
|---|---|---|
| Patios & Walkways | 4 inches | Wire mesh or fiber |
| Residential Driveways | 5-6 inches | Wire mesh or rebar |
| Garage Floors | 5-6 inches | Wire mesh or rebar |
| Commercial/Heavy Loads | 6-8+ inches | Rebar grid required |
| Footings & Foundations | 8-12+ inches | Rebar per engineer specs |
Concrete Calculation Examples
Example 1: 10×10 Patio (4 inches thick)
With 7.5% margin: 1.23 × 1.075 = 1.32 cubic yards
Order 1.5 cubic yards (ready-mix minimums often apply) or 60 bags of 80-lb mix for DIY
Example 2: 20×20 Driveway (5 inches thick)
With 7.5% margin: 6.17 × 1.075 = 6.63 cubic yards
Order 7 cubic yards of ready-mix concrete (DIY bagging not practical at this volume)
Example 3: 12×24 Garage Floor (6 inches thick)
With 7.5% margin: 5.33 × 1.075 = 5.73 cubic yards
Order 6 cubic yards of ready-mix concrete with 4000 PSI for vehicle loads
When should I use bagged concrete vs ready-mix?
Bagged concrete (80-lb bags) is practical for small projects under 0.5 cubic yards. Ready-mix delivery is more cost-effective and ensures consistent quality for larger projects. One cubic yard requires 45 bags of 80-lb mix—that's over 3,600 lbs to mix by hand.
| Project Size | Recommendation | Approximate Cost |
|---|---|---|
| Under 0.5 cubic yards | 80-lb bags (DIY mixing) | ~$115 (23 bags × $5) |
| 0.5 - 1 cubic yard | Either option viable | Bags: ~$225 | Ready: ~$150-200 |
| 1+ cubic yards | Ready-mix delivery | $125-150 per cubic yard |
Pro Tip
Ready-mix trucks often have a minimum order (typically 1 cubic yard) plus a "short load" fee for small orders. Call local suppliers for exact pricing—it's often still cheaper than bags for anything over 0.75 yards.
How much does concrete cost?
Ready-mix concrete costs $125-150 per cubic yard for material delivered. Total installed costs including labor average $7-10 per square foot for a standard 4-inch slab. Prices vary by region, concrete strength (PSI), and site accessibility.
| Project | Size | Material Only | Installed |
|---|---|---|---|
| Small Patio | 10×10 ft | $165-200 | $700-1,000 |
| Medium Patio | 15×15 ft | $350-425 | $1,575-2,250 |
| 2-Car Driveway | 20×20 ft | $750-950 | $2,800-4,000 |
| Garage Floor | 12×24 ft | $700-900 | $2,000-2,900 |
Common Concrete Calculation Mistakes
❌ Not adding waste factor
Always add 7.5-10% extra concrete. Uneven ground, form boards, and spillage consume more material than expected. A short pour is disastrous and expensive to fix.
❌ Ordering exact calculated amount
Round up to the nearest quarter or half yard when ordering ready-mix. Having 0.25 yards extra is far better than being 0.25 yards short.
❌ Using wrong thickness for application
A 4-inch patio slab will crack under vehicle traffic. Always use 5-6 inches for driveways and any area with vehicle loads.
❌ Forgetting about irregular shapes
For L-shaped or irregular slabs, divide into rectangles and calculate each separately, then add the results together.
❌ Mixing too much bagged concrete at once
Concrete sets in 60-90 minutes. Only mix what you can pour and finish within that window. Have help available for larger DIY projects.
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