How do you calculate cubic yards for a concrete slab?

Multiply Length (ft) × Width (ft) × Thickness (ft) to get total cubic feet, then divide by 27 to convert to cubic yards. For example, a 10×10 ft slab that is 4 inches thick (0.333 ft) is 10 × 10 × 0.333 = 33.3 cu ft ÷ 27 = 1.23 cubic yards.

How many 80 lb bags of concrete make one cubic yard?

It takes 45 bags of 80 lb concrete mix to make one cubic yard. For 60 lb bags, you need 60 bags per cubic yard, and for 50 lb bags, you need 72 bags.

What is the standard thickness for a concrete driveway and patio?

Standard residential patios and walkways are typically 4 inches thick. Residential driveways are poured at 4 inches (light passenger cars) or 5 to 6 inches (heavy trucks/RVs) with #4 rebar grid reinforcement.

How much extra concrete should I order for waste?

We recommend adding 10% extra concrete to account for uneven grade, spillage, and formwork deflection. For pumping over long lines or irregular excavations, add 12% to 15%.

How much does ready-mix concrete cost per cubic yard in the USA?

Ready-mix concrete costs between $130 and $175 per cubic yard across the United States, depending on mix strength (3,000 PSI vs 4,000 PSI), regional quarry availability, and local supplier fuel surcharges.

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How to Calculate Concrete Volume: Formula & Instructions

Calculating concrete volume accurately is crucial for avoiding costly delivery short-load charges (often $75–$150 per truck) and preventing cold joints caused by pour interruptions. In the United States, ready-mix concrete is sold and delivered by the cubic yard (yd³), where one cubic yard equals 27 cubic feet.

The Basic Concrete Formula for Slabs & Rectangles

To calculate the concrete volume for any rectangular slab, walkway, patio, or foundation footing, use the following three-step calculation:

Volume (Cubic Yards) = [Length (ft) × Width (ft) × Thickness (ft)] ÷ 27

Tip: To convert thickness from inches to feet, divide the inch measurement by 12 (e.g., 4" ÷ 12 = 0.333 ft; 6" ÷ 12 = 0.50 ft).

Example Concrete Calculation

Suppose you are pouring a concrete patio that measures 24 feet long, 14 feet wide, and 4 inches deep (0.333 ft):

Calculate Area: 24 ft × 14 ft = 336 square feet
Calculate Cubic Feet: 336 sq ft × (4 ÷ 12) ft = 111.89 cubic feet
Convert to Cubic Yards: 111.89 ÷ 27 = 4.14 cubic yards
Add 10% Waste Buffer: 4.14 × 1.10 = 4.56 cubic yards (order 4.75 or 5.0 yards from ready-mix supplier).

Concrete Slab Reference Chart (Cubic Yards & 80-lb Bags)

Use this quick reference chart to estimate standard residential and commercial slab dimensions:

Slab Size (Feet)	Total Area (Sq Ft)	4" Thick (Cubic Yds)	6" Thick (Cubic Yds)	80-lb Bags (4" Slab)
8 ft × 8 ft	64 sq ft	0.79 yd³	1.19 yd³	36 bags
10 ft × 10 ft	100 sq ft	1.23 yd³	1.85 yd³	56 bags
12 ft × 12 ft	144 sq ft	1.78 yd³	2.67 yd³	80 bags
12 ft × 20 ft	240 sq ft	2.96 yd³	4.44 yd³	134 bags
20 ft × 20 ft (2-Car)	400 sq ft	4.94 yd³	7.41 yd³	222 bags
24 ft × 24 ft (Garage)	576 sq ft	7.11 yd³	10.67 yd³	320 bags
30 ft × 40 ft (Shop)	1,200 sq ft	14.81 yd³	22.22 yd³	667 bags

Ready-Mix Truck Delivery vs. Bagged Concrete

When should you order a ready-mix concrete truck instead of mixing pre-bagged concrete yourself with a mixer or wheelbarrow?

Under 1 Cubic Yard (Under 45–50 Bags): Pre-packaged bags (Quikrete or Sakrete 80lb/60lb) are cost-effective for fence posts, small air conditioner pads, mailbox footings, and step repairs.
Over 1 Cubic Yard (Over 50 Bags): Ready-mix concrete delivery is faster, structurally superior, and physically practical. Mixing 80 bags by hand takes 6+ hours of heavy labor and risks uneven hydration and cracking.

Why You Must Always Add a 5% to 10% Waste Margin

Excavated ground is never perfectly laser-flat. Subbase unevenness, deflection of wooden formwork under heavy hydrostatic pressure, spillage during chute discharge, and wheelbarrow residue will inevitably consume 5% to 10% more concrete than theoretical math predicts.

Concrete Calculator

1. Enter Dimensions

2. Required Concrete

Pre-Mixed Bag Breakdown (If Bagged):

Need a Certified Quantity Takeoff?

How to Calculate Concrete Volume: Formula & Instructions

The Basic Concrete Formula for Slabs & Rectangles

Example Concrete Calculation

Concrete Slab Reference Chart (Cubic Yards & 80-lb Bags)

Ready-Mix Truck Delivery vs. Bagged Concrete

Why You Must Always Add a 5% to 10% Waste Margin

Bidding a Commercial Concrete Project?

Concrete Estimating Q&A