How Much Concrete Do I Need? A Simple Formula With Examples
Whether you are pouring a patio slab, setting fence posts, or building a foundation, knowing how much concrete to order saves you time, money, and headaches.
Ordering too little concrete means a trip back to the store and a cold joint in your pour. Ordering too much means wasted money and having to dispose of excess material. Getting the quantity right starts with a simple volume calculation.
Use the Concrete Calculator to compute your needs instantly, or follow the formulas below to do it yourself.
The Basic Formula for Concrete Volume
Concrete is sold by volume, typically in cubic yards (in the US) or cubic meters (internationally). The fundamental formula for a rectangular slab is:
Volume = Length × Width × Thickness
All measurements must be in the same unit. For the formula to give you cubic feet, measure everything in feet.
Step 1: Measure in Feet
First, convert all your measurements to feet. If your thickness is in inches, divide by 12:
- 4 inches ÷ 12 = 0.33 feet
- 6 inches ÷ 12 = 0.50 feet
- 8 inches ÷ 12 = 0.67 feet
Step 2: Calculate Cubic Feet
Multiply length × width × thickness (all in feet) to get cubic feet.
Step 3: Convert to Cubic Yards
Concrete is usually ordered by the cubic yard. There are 27 cubic feet in 1 cubic yard:
Cubic yards = Cubic feet ÷ 27
Example 1: A Patio Slab
You are pouring a rectangular patio that is 12 ft long, 10 ft wide, and 4 inches thick.
- Thickness in feet = 4 ÷ 12 = 0.33 ft
- Cubic feet = 12 × 10 × 0.33 = 39.6 cu ft
- Cubic yards = 39.6 ÷ 27 = 1.47 cu yd
You would need about 1.5 cubic yards of concrete. To be safe, order 1.75 cubic yards to account for waste and spillage.
Example 2: A Driveway
You are pouring a driveway that is 20 ft long, 12 ft wide, and 6 inches thick.
- Thickness in feet = 6 ÷ 12 = 0.50 ft
- Cubic feet = 20 × 12 × 0.50 = 120 cu ft
- Cubic yards = 120 ÷ 27 = 4.44 cu yd
Order approximately 4.75 cubic yards to account for waste.
How to Calculate Concrete for a Round Column
For round columns or footings, use the formula for the volume of a cylinder:
Volume = π × r² × h
Where r is the radius (half the diameter) and h is the height, both in feet.
Example 3: A Circular Column
You need to pour a column with a diameter of 1.5 ft (radius = 0.75 ft) and a height of 8 ft.
- Area of the circle = π × (0.75)² = 3.1416 × 0.5625 = 1.77 sq ft
- Volume = 1.77 × 8 = 14.16 cu ft
- Cubic yards = 14.16 ÷ 27 = 0.52 cu yd
Estimating Bagged Concrete
For small projects, you can buy bagged concrete mix instead of ordering a truck. Standard bag sizes and their approximate yields:
- 40 lb bag = 0.30 cu ft
- 60 lb bag = 0.45 cu ft
- 80 lb bag = 0.60 cu ft
To find the number of bags, divide your total cubic feet by the yield per bag:
Example 4: Bagged Concrete for a Small Slab
Using the patio from Example 1 (39.6 cu ft), with 80 lb bags (0.60 cu ft each):
- Bags needed = 39.6 ÷ 0.60 = 66 bags
Adding 10% for waste: 66 × 1.10 = 73 bags. That is a lot of bags to mix by hand. For projects requiring more than about 20 bags, consider ordering ready-mix concrete instead.
Ready-Mix vs. Bagged Concrete
- Bagged concrete is best for small projects (under 1 cubic yard). It costs more per cubic yard but requires no delivery minimum.
- Ready-mix concrete is delivered by truck and is ideal for larger jobs. It is cheaper per cubic yard but typically has a minimum order of 3-4 cubic yards.
- Mixing time — mixing 73 bags by hand takes hours. A truck delivers and pours in minutes.
- Quality — ready-mix is more consistent because it is batched in a controlled environment.
Common Concrete Thickness Guidelines
- Patios and walkways: 4 inches
- Driveways (standard cars): 4-6 inches
- Driveways (heavy vehicles): 6-8 inches
- Garage floors: 4-6 inches
- Foundation walls: 8-12 inches
- Footings: 12-24 inches (check local building codes)
Use the Concrete Calculator to enter your project dimensions in any unit and get results in cubic feet, cubic yards, cubic meters, and number of bags. The calculator handles slabs, columns, footings, and tubes, so no matter what shape your project is, you will know exactly how much concrete to order.
Frequently Asked Questions
How do I calculate how much concrete I need?
Calculate the volume of the area you need to fill. For a slab, multiply length × width × thickness (all in feet). The result is cubic feet. Divide by 27 to get cubic yards. For example, a 10 ft × 10 ft slab that is 0.33 ft thick (4 inches) is 10 × 10 × 0.33 = 33 cu ft, which is about 1.22 cu yd.
How many 80 lb bags of concrete do I need per cubic foot?
One 80 lb bag of concrete mix yields approximately 0.6 cubic feet. To find how many bags you need, divide the total cubic feet by 0.6. For example, 33 cubic feet ÷ 0.6 = 55 bags. Always round up and order 5-10% extra to account for spillage and waste.
How many cubic feet are in a cubic yard of concrete?
There are 27 cubic feet in 1 cubic yard (3 ft × 3 ft × 3 ft = 27 cu ft). Concrete is typically ordered by the cubic yard, so divide your total cubic feet by 27 to get cubic yards.
What is the formula for calculating concrete for a round column?
Use the formula for the volume of a cylinder: π × r² × h. Where r is the radius (half the diameter) and h is the height, both in feet. For a column with a 1 ft diameter and 8 ft height: π × (0.5)² × 8 = π × 0.25 × 8 = 6.28 cu ft, which is about 0.23 cu yd.
How thick should a concrete slab be?
Patios and walkways: 4 inches. Driveways: 4-6 inches (for standard cars) or 6-8 inches (for heavy vehicles like RVs and trucks). Garage floors: 4-6 inches. Foundations: 8-12 inches or more, depending on the structure and local building codes. Always check local building codes before pouring.
Should I order ready-mix or bagged concrete?
For small projects under 1 cubic yard (about 27 cubic feet), bagged concrete is more practical and cost-effective. For larger projects, ready-mix delivered by truck is cheaper per cubic yard and ensures consistent quality. Ready-mix trucks typically have a minimum order of 3-4 cubic yards.