How to Calculate Concrete Yardage: Formula & Examples
A step-by-step master walkthrough for figuring out cubic yards of concrete for slabs, patios, and footings, including conversion math, waste buffers, and batch plant ordering rules.
Cubic Yards = (Length × Width × Thickness in Inches) ÷ 324
To determine concrete volume in cubic yards, multiply slab length (ft) by width (ft) by thickness (in), divide by 324, and add a 10% safety buffer for ground variation and form bowing.
Interactive Concrete Yardage Calculator
Concrete Volume & Coverage Calculator
Convert dimensions, cubic yards, and bags into precise surface coverage and order quantities.
Need live cost estimates and reinforcement takeoff?
Use our full Concrete Calculator to estimate delivered ready-mix pricing, bagged concrete alternatives, and subbase gravel.
Step-by-Step Mathematical Derivation: From Measurements to Cubic Yards
In the United States and Canada, ready-mix concrete is ordered and batched in cubic yards (abbreviated as cu. yd. or yd³). However, job-site measurements are typically recorded in a combination of units: length in feet, width in feet, and slab thickness in inches.
To calculate the volume accurately, you must convert all three dimensions into the same unit before finding total volume:
The 3-Step Standard Method
- Convert Thickness from Inches to Feet: Divide the depth in inches by 12.
Thickness in Feet = Thickness in Inches ÷ 12 (e.g., 4" ÷ 12 = 0.3333 ft; 6" ÷ 12 = 0.50 ft). - Calculate Cubic Feet: Multiply Length (ft) × Width (ft) × Thickness in Feet.
Cubic Feet = Length (ft) × Width (ft) × (Thickness in Inches ÷ 12) - Convert Cubic Feet to Cubic Yards: Divide total cubic feet by 27 (because 1 cubic yard = 3 ft × 3 ft × 3 ft = 27 cubic feet).
Cubic Yards = Cubic Feet ÷ 27
The Direct Contractor Shortcut: The "324 Rule"
Instead of performing two separate divisions, contractors use the consolidated constant 324 (12 × 27 = 324):
For comprehensive coverage rates at different pour depths, review our reference on how much a yard of concrete covers.
Worked Calculation Examples for Common Slab Sizes
Review these three practical examples showing the exact mathematical volume and recommended purchase quantity factoring in safety buffers:
Example 1: Backyard Patio (10 ft × 10 ft @ 4 inches thick)
- Nominal Math: (10 × 10 × 4) ÷ 324 = 400 ÷ 324 = 1.235 cu. yds (33.33 cu. ft.)
- With 10% Waste Buffer: 1.235 × 1.10 = 1.358 cu. yds (36.67 cu. ft.)
- Ready-Mix Order: 1.5 cu. yds (or 62 eighty-pound bags). See our complete breakdown on concrete needed for a 10×10 slab.
Example 2: Two-Car Garage Floor / Pad (20 ft × 20 ft @ 5 inches thick)
- Nominal Math: (20 × 20 × 5) ÷ 324 = 2,000 ÷ 324 = 6.173 cu. yds (166.67 cu. ft.)
- With 10% Waste Buffer: 6.173 × 1.10 = 6.790 cu. yds (183.33 cu. ft.)
- Ready-Mix Order: 7.0 cu. yds from the batch plant.
Example 3: Residential Sidewalk / Walkway (4 ft × 50 ft @ 4 inches thick)
- Nominal Math: (4 × 50 × 4) ÷ 324 = 800 ÷ 324 = 2.469 cu. yds (66.67 cu. ft.)
- With 10% Waste Buffer: 2.469 × 1.10 = 2.716 cu. yds (73.33 cu. ft.)
- Ready-Mix Order: 3.0 cu. yds (or 123 eighty-pound bags).
Concrete Yardage Takeoff Reference Table
This reference table summarizes square footage, nominal yardage, order volume with a 10% buffer, and retail bag equivalents at a standard 4-inch slab thickness:
| Slab Dimensions | Area (sq ft) | Nominal Volume | +10% Order Volume | 80 lb Bags (+10%) | 60 lb Bags (+10%) |
|---|---|---|---|---|---|
| 8 × 8 ft | 64 sq ft | 0.79 yd³ | 0.87 yd³ | 40 bags | 53 bags |
| 10 × 10 ft | 100 sq ft | 1.23 yd³ | 1.36 yd³ | 62 bags | 82 bags |
| 10 × 12 ft | 120 sq ft | 1.48 yd³ | 1.63 yd³ | 74 bags | 98 bags |
| 12 × 12 ft | 144 sq ft | 1.78 yd³ | 1.96 yd³ | 88 bags | 118 bags |
| 12 × 16 ft | 192 sq ft | 2.37 yd³ | 2.61 yd³ | 118 bags | 157 bags |
| 16 × 20 ft | 320 sq ft | 3.95 yd³ | 4.35 yd³ | 196 bags | 261 bags |
| 20 × 20 ft | 400 sq ft | 4.94 yd³ | 5.43 yd³ | 245 bags | 326 bags |
| 24 × 24 ft | 576 sq ft | 7.11 yd³ | 7.82 yd³ | 352 bags | 470 bags |
* Bag counts assume standard manufacturer yields of 0.60 cu ft per 80 lb bag and 0.45 cu ft per 60 lb bag. For unit area requirements, check our guide on concrete required per square foot.
How to Calculate Yardage for Curved & Irregular Shapes
Not every pour is a simple rectangle. When measuring non-rectangular slabs, use these geometric formulas:
Circular Slabs & Fire Pit Pads
Area = π × Radius²
Example: A 12-ft diameter circular patio (6 ft radius) at 4" depth = (3.1416 × 36 × 4) ÷ 324 = 452.39 ÷ 324 = 1.40 cu. yds.
Cylindrical Pier Footings (Sonotubes)
Volume = π × Radius² × Depth in feet
Example: A 12-inch diameter tube (0.5 ft radius) dug 4 ft deep = (3.1416 × 0.25 × 4) ÷ 27 = 3.14 ÷ 27 = 0.116 cu. yds (5.2 eighty-pound bags).
Why You Must Never Order Exact Mathematical Yardage
The most common rookie mistake in concrete estimation is ordering the exact calculated volume without an installation allowance. Ground excavations are never laser-flat. If a 4-inch slab excavation varies by just 1/2 inch in depth across a 20×20 pad, that minor subgrade depression adds over 0.6 cubic yards of extra volume.
Key Factors That Consume Extra Concrete:
- Subgrade Irregularities: Crushed gravel compaction settles under the hydraulic weight of fresh concrete.
- Formwork Bowing: Wooden 2×4 and 2×6 forms flex outward under hydrostatic pressure.
- Chute & Pump Residuals: 0.25 to 0.5 cubic yards remains coated on the truck chute, mixer drum, or inside pump lines.
- Spillage & Screed Waste: Edge overfilling and screed runoff during leveling.
Running short during a pour causes weak cold joints and emergency delivery surcharges. For details on pricing and truck surcharges, see our 2026 concrete cost per yard guide and how many bags of concrete do I need guide.
Frequently Asked Questions About Concrete Yardage
What is the formula for calculating concrete yardage?
The standard contractor formula for rectangular flatwork is: Cubic Yards = (Length in feet × Width in feet × Thickness in inches) ÷ 324. For example, a 10 ft × 12 ft patio at 4 inches thick requires (10 × 12 × 4) ÷ 324 = 480 ÷ 324 = 1.48 cubic yards before adding a waste buffer.
Why is the concrete formula divided by 324?
The divisor 324 combines two unit conversions: 12 inches in a linear foot and 27 cubic feet in a cubic yard (12 × 27 = 324). This allows you to multiply length (ft), width (ft), and thickness (inches) directly without converting thickness to a decimal fraction first.
How much extra concrete should I order for waste?
Contractors recommend ordering a 10% safety allowance for ground flatwork slabs and 5% for well-formed indoor pours. Uneven subgrades, minor grade depressions, formwork deflection under wet hydraulic pressure, and mix residue in the pump or truck chute consume extra material.
How do I calculate concrete yardage for irregular shapes?
Divide the irregular area into standard geometric shapes (rectangles, triangles, circles). Calculate the cubic yardage for each sub-section individually, sum the results, and then apply your 10% safety buffer to the total.
In what increments do ready-mix suppliers deliver concrete?
Most commercial batch plants sell ready-mix concrete in 0.25 to 0.50 cubic yard increments. If your calculated requirement with waste is 3.2 cubic yards, round up to 3.5 cubic yards when placing your order to prevent shortfalls.