10×10 patio · 4"
1.5 cu yd
$1,005Step-by-step formula
How to Calculate Concrete Slab Cost
Calculating concrete slab cost comes down to four numbers: area, thickness, material price per cubic yard, and labor rate per square foot. The formula is straightforward — but most people get the units wrong.
Below, we walk through the exact calculation with three real examples, then give you a calculator that handles the math automatically. For installed price ranges by region and project type, see our concrete slab cost per square foot guide.
The Concrete Slab Cost Formula
The Formula
Total Cost = (Area × Thickness ÷ 324) × Concrete Price + (Area × Labor Rate) + Site Prep & Overhead
What Each Part Means
| Variable | What It Is | Typical Value |
|---|---|---|
| Area | Length × Width, in square feet | Your slab dimensions |
| Thickness | Slab thickness in inches | 4" (patio), 5" (driveway), 6" (garage) |
| ÷ 324 | Turns sq ft × inches into cubic yards | Fixed constant |
| Concrete Price | Delivered concrete per cubic yard | $125–$175/cu yd |
| Labor Rate | Labor per square foot | $3–$7/sq ft |
| Site Prep & Overhead | Gravel, rebar, permit, contractor margin | ~15–25% of subtotal |
The "÷ 324" is where most people trip up. Here is why it works: 1 cubic yard = 27 cubic feet. A 1-inch-thick slab covering 1 square foot uses 1/12 of a cubic foot. To get cubic yards, divide sq ft × inches by (27 × 12) = 324. Or just use the concrete slab cost calculator above — it handles the units automatically.
Example 1 — 10×10 ft Patio Slab (4" Thick)
Step 1: Calculate Area
Area = 10 ft × 10 ft = 100 sq ft
Step 2: Find Cubic Yards
Concrete needed = (100 × 4) ÷ 324 = 400 ÷ 324 = 1.23 cubic yards. Round up to 1.5 cubic yards (always add 10–15% for spillage and uneven base).
Step 3: Material Cost
1.5 cu yd × $150/cu yd (national average) = $225
Step 4: Labor Cost
100 sq ft × $5/sq ft (mid-range labor rate) = $500
Step 5: Site Prep & Overhead
Gravel base: ~$80 | Rebar/wire mesh: ~$100 | Permit: ~$100. Overhead subtotal: ~$280
Step 6: Total
$225 + $500 + $280 = $1,005
Price range: low end (Southeast, no rebar, basic finish) ~$600. High end (Northeast, rebar, stamped finish) ~$1,500.
Example 2 — 20×20 ft Driveway Slab (5" Thick)
Step 1: Calculate Area
Area = 20 ft × 20 ft = 400 sq ft
Step 2: Find Cubic Yards
Concrete needed = (400 × 5) ÷ 324 = 2,000 ÷ 324 = 6.17 cubic yards. Round up to 7 cubic yards.
Step 3: Material Cost
7 cu yd × $150/cu yd = $1,050
Step 4: Labor Cost
400 sq ft × $5/sq ft = $2,000
Step 5: Site Prep & Overhead
Gravel base (5" depth for driveway): ~$250 | Rebar (required for driveway): ~$250 | Permit: ~$150. Overhead subtotal: ~$650
Step 6: Total
$1,050 + $2,000 + $650 = $3,700
Price range: low end ~$2,800 | high end ~$5,200.
Driveways require 5" minimum thickness and rebar reinforcement — that is building code in most jurisdictions, not optional. This is why driveway slabs cost more per square foot than patios even though the concrete price per yard is the same.
Example 3 — 30×40 ft Workshop Floor (6" Thick)
Step 1: Calculate Area
Area = 30 ft × 40 ft = 1,200 sq ft
Step 2: Find Cubic Yards
Concrete needed = (1,200 × 6) ÷ 324 = 7,200 ÷ 324 = 22.22 cubic yards. Round up to 24 cubic yards (larger pours need more overage).
Step 3: Material Cost
24 cu yd × $140/cu yd (volume discount on large orders) = $3,360
Step 4: Labor Cost
1,200 sq ft × $4/sq ft (lower labor rate for large jobs) = $4,800
Step 5: Site Prep & Overhead
Gravel base (6" depth): ~$600 | Rebar + vapor barrier: ~$500 | Concrete pump (truck can't reach a 40-ft span from one side): ~$600 | Permit: ~$200. Overhead subtotal: ~$1,900
Step 6: Total
$3,360 + $4,800 + $1,900 = $10,060
Price range: low end ~$7,200 | high end ~$14,400.
Key insight: at 1,200 sq ft, the material cost and labor cost are roughly equal. For smaller slabs, labor dominates. For very large commercial pours, material becomes the bigger line item.
20×20 driveway · 5"
7 cu yd
$3,70030×40 workshop · 6"
24 cu yd
$10,0603 Common Calculation Mistakes
Mistake #1: Forgetting to change inches to feet for cubic yards
The formula uses "÷ 324" — not "÷ 27." If you forget that thickness is in inches, you will under-order concrete by a factor of 4 to 6. For a 20×20 slab at 4", the wrong math gives you 0.6 cubic yards instead of 6.2. That is a concrete truck showing up with 90% less than you need.
Mistake #2: Using the wrong thickness for your project
A patio at 4" uses ~5 cubic yards for 400 sq ft. The same area at 6" uses ~7.4 cubic yards — nearly 50% more concrete. If you calculate at 4" but the contractor pours at 6" (common for driveways and garages), your budget is off by hundreds of dollars.
Mistake #3: Forgetting site prep
The formula above includes site prep. If you only calculate concrete + labor, you will under-budget by 15–25%. Gravel base, rebar, permits, and pump fees are real costs that show up on every contractor invoice. See our concrete slab cost per square foot guide for a full breakdown of what is included.
When to Calculate Yourself vs Use a Calculator
Calculate by hand when:
- You are getting contractor quotes and want to verify their math
- You are comparing DIY material costs across suppliers
- You want to understand what drives the price (so you can negotiate)
Use the calculator when:
- You want an instant estimate without doing arithmetic
- You need to compare multiple sizes or thicknesses quickly
- You want a breakdown that includes material, optional labor, and delivery from the numbers you enter
The concrete slab cost calculator below uses the same formula — it just handles the units. Enter your local price per cubic yard for a project-specific estimate.
Quick Reference — Pre-Calculated Costs
Common sizes at 4 inches, $150/cu yd plus $5/sq ft labor
| Size | Area | Concrete (cu yd) | Material | Labor | Est. Total |
|---|---|---|---|---|---|
| 10×10 | 100 | 1.5 | $225 | $500 | $1,005 |
| 12×12 | 144 | 2.2 | $330 | $720 | $1,430 |
| 16×16 | 256 | 3.8 | $570 | $1,280 | $2,490 |
| 20×20 | 400 | 6.2 | $930 | $2,000 | $3,950 |
| 20×30 | 600 | 9.3 | $1,395 | $3,000 | $5,995 |
| 30×40 | 1,200 | 18.5 | $2,588 | $6,000 | $11,388 |
| 40×40 | 1,600 | 24.7 | $3,458 | $8,000 | $15,158 |
Includes 15% overage on concrete and a site prep estimate. See our concrete slab cost by size guide for full price ranges by thickness and project type.
Frequently Asked Questions
How do I calculate how much concrete I need for a slab?
Multiply length × width (in feet) × thickness (in inches), then divide by 324. That gives you cubic yards. Add 10–15% for spillage and uneven base. Example: a 20×20 ft slab at 4" needs (400 × 4) ÷ 324 = 4.94 cubic yards → order 5.5 cubic yards. Use our calculator to skip the math.
How do you calculate labor cost for a concrete slab?
Multiply the slab area (sq ft) by the local labor rate ($3–$7/sq ft for most US regions). Labor typically accounts for 40–50% of total project cost. For more accurate regional rates, see our concrete slab cost per square foot guide.
What is the formula for concrete slab cost?
Total Cost = (Length × Width × Thickness ÷ 324) × Concrete Price Per Cubic Yard + (Area × Labor Rate Per Sq Ft) + Site Prep & Overhead. See the worked examples above for a 10×10, 20×20, and 30×40 slab.
How much does 1 cubic yard of concrete cover?
One cubic yard of concrete covers 81 sq ft at 4" thick, 65 sq ft at 5" thick, or 54 sq ft at 6" thick. At the national average of $125–$175/cu yd delivered, that is $1.50–$3.25 per square foot in material cost alone — labor and site prep are extra.
Skip the Math — Get Your Instant Estimate
You now know the formula. But comparing multiple sizes, thicknesses, and project types by hand is tedious. Enter your dimensions in our concrete slab cost calculator and get a complete cost breakdown in seconds — materials, labor, site prep, and total. Free, no signup.
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