Total Volume: 0.00 Cubic Feet
Premix Bags (80lb): 0 bags
Using the Concrete Calculator
Whether you are pouring a new driveway, setting fence posts, or building a retaining wall, using an accurate concrete calculator is essential for project planning. Ordering too little concrete leads to cold joints and structural weaknesses, while ordering too much is a waste of money and resources. This tool helps you calculate the exact volume of concrete needed in cubic yards and cubic feet.
To get started, simply choose your project shape from the dropdown menu and enter your dimensions. Most residential projects use the "Slab" setting for rectangular areas.
- Length & Width (Feet)
- Measure the horizontal dimensions of your area in decimal feet. For example, 10 feet 6 inches should be entered as 10.5.
- Thickness (Inches)
- Concrete slabs for walkways are typically 4 inches thick, while driveways often require 5 to 6 inches.
- Waste Margin (%)
- It is industry standard to add 5-10% to your total to account for spillage, uneven subgrade, or form deflection.
How Concrete Volume is Calculated
Concrete is traditionally sold by the cubic yard. To find this volume, you first calculate the cubic feet and then convert. The basic formula for a rectangular slab is:
Volume in Cubic Yards = [(Length × Width × Thickness / 12) / 27] + Waste %
- Thickness / 12: Converts inches to feet so all units are consistent.
- Division by 27: There are 27 cubic feet in one cubic yard (3′ x 3′ x 3′).
- Circular Formula: For holes/columns, use π × r² × height.
Concrete Calculation Example
Scenario: You want to pour a backyard patio that is 12 feet long, 15 feet wide, and 4 inches thick. You want to include a 10% waste factor.
Step-by-step solution:
- Convert thickness to feet: 4″ / 12 = 0.333 ft
- Calculate Cubic Feet: 12′ × 15′ × 0.333′ = 60 cubic feet
- Convert to Cubic Yards: 60 / 27 = 2.22 cubic yards
- Add 10% Waste: 2.22 × 1.10 = 2.44 Cubic Yards
- Premix Bags: If using 80lb bags (0.6 cu ft each), 60 / 0.6 = 100 bags (plus waste).
Common Questions
How many 80lb bags of concrete make a yard?
It takes approximately 45 bags of 80lb premixed concrete to equal one cubic yard. If you are using 60lb bags, you will need about 60 bags.
What is the standard thickness for a concrete driveway?
For standard passenger vehicles, a 4-inch thickness is common, but 5 inches provides significantly better durability. For heavy-duty trucks or RVs, 6 inches of reinforced concrete is recommended.
Why should I add a waste margin?
The ground is rarely perfectly flat. If your excavation is just a half-inch deeper than planned across a large area, you will run out of concrete before finishing. A 10% margin is the safest way to ensure a continuous pour.
When should I order a truck vs. using bags?
Generally, if your project requires more than 1 cubic yard (about 45-50 bags), it is more cost-effective and physically easier to order ready-mix concrete delivered by a truck. Projects smaller than 0.5 yards are ideal for bag mixing.