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(701)814-6992
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6296 Donnelly Plaza
Ratkeville, Bahamas.
How Much Concrete Do I Need?
When pouring a new patio, setting fence posts, or building a driveway, figuring out exactly "how much concrete do I need" is the most crucial step of your project. Ordering too little means a disastrous mid-pour scramble, while ordering too much wastes hard-earned money.
Concrete is ordered by volume and measured in cubic yards. For smaller DIY projects, it is often purchased in 80lb or 60lb pre-mixed bags (measured in cubic feet). Use our free calculator below to get exact measurements for your project, then read on to understand the math behind your pour.
Project Dimensions
Free Tool
Total Concrete Needed
0
Cubic Yards
Includes 10% waste factor
Pre-Mixed Bag Equivalents
If you are mixing by hand instead of ordering a truck, here is how many bags you need:
- 80 lb Bags: 0 bags
- 60 lb Bags: 0 bags
- Total Volume in Cubic Feet: 0 cu ft
When pouring a new patio, setting fence posts, or building a driveway, figuring out exactly "how much concrete do I need" is the most crucial step of your project. Ordering too little means a disastrous mid-pour scramble, while ordering too much wastes hard-earned money.
Concrete is ordered by volume and measured in cubic yards. For smaller DIY projects, it is often purchased in 80lb or 60lb pre-mixed bags (measured in cubic feet).
1 Cubic Yard = 27 Cubic Feet
How to Calculate Concrete Volume
1. Slabs, Patios, and Driveways (Rectangular)
To find the amount of concrete needed for a flat slab, you need to measure the length and width in feet, and the thickness (depth) in inches.
The Slab Formula:
- Convert thickness from inches to feet (Thickness ÷ 12)
- Length (ft) × Width (ft) × Thickness (ft) = Cubic Feet
- Cubic Feet ÷ 27 = Cubic Yards
Example: A 10ft x 10ft patio that is 4 inches thick.
4 inches ÷ 12 = 0.33 feet.
10 ft × 10 ft × 0.33 ft = 33.3 cubic feet.
33.3 ÷ 27 = 1.23 cubic yards.
2. Post Holes, Footings, and Columns (Cylindrical)
Calculating concrete for a round hole requires finding the volume of a cylinder. Measure the diameter of the hole in inches, and the depth of the hole in feet.
The Cylinder Formula:
- Find the radius in feet: (Diameter in inches ÷ 12) ÷ 2
- Area = π (3.14) × Radius²
- Volume in Cubic Feet = Area × Depth (ft)
- Cubic Feet ÷ 27 = Cubic Yards
How Many Bags of Concrete Do I Need?
If you are mixing concrete yourself, you won't be ordering cubic yards from a truck—you'll be buying bags from the hardware store. Pre-mixed concrete generally comes in two common sizes. Here is their yield:
| Bag Size | Yield (Cubic Feet) | Bags to make 1 Cubic Yard |
|---|---|---|
| 80 lb bag | ~0.60 cu. ft. | 45 bags |
| 60 lb bag | ~0.45 cu. ft. | 60 bags |
| 50 lb bag (fast-setting) | ~0.37 cu. ft. | 73 bags |
Pro Tip: To find how many 80lb bags you need, take your total Cubic Feet and divide by 0.60.
Standard Concrete Mix Formulas
If you are mixing your own concrete from scratch rather than buying pre-mixed bags, you need to follow standard mix volume ratios (Cement : Sand : Gravel). Here are the most common formulas depending on your project strength requirements:
| Project Type | Target PSI | Mix Ratio (C : S : G) |
|---|---|---|
| Standard footings & walls | 3,000 PSI | 1 part cement : 3 parts sand : 3 parts gravel |
| Driveways & structural slabs | 4,000 PSI | 1 part cement : 2 parts sand : 3 parts gravel |
| High-strength heavy traffic | 5,000 PSI | 1 part cement : 2 parts sand : 2 parts gravel |
Pro Tips & Best Practices
- 💧 The Water Factor: Too much water weakens concrete significantly. Always add water gradually. The perfect mix should look like thick oatmeal and hold its shape when squeezed in your gloved hand without crumbling or dripping excessive water.
- ☀️ Weather Matters: Never pour concrete if freezing temperatures are expected within 48 hours. In extreme heat, keep the concrete damp while it cures to prevent surface cracking.
- ⏱️ Time Constraints: Once water hits the mix, the chemical reaction (hydration) begins. You generally have about 1 to 2 hours to pour and finish the concrete before it becomes unworkable.
Why You Must Include a Waste Factor
The ground is never perfectly flat, forms bow slightly under the pressure of wet concrete, and some material always gets left in the mixer or wheelbarrow. Always add a 10% waste factor to your calculations.
If you calculate exactly 1.0 cubic yard, order 1.1 yards (or add 10% more bags). Failing to account for waste is the #1 mistake DIYers make, resulting in a slab that comes up an inch short at the edges.
Frequently Asked Questions
It takes exactly 45 bags of 80lb concrete to make one cubic yard. (Because 1 cubic yard = 27 cubic feet, and one 80lb bag yields 0.60 cubic feet. 27 ÷ 0.60 = 45).
For standard foot traffic (patios, walkways, sidewalks), 4 inches is the standard thickness. For passenger car driveways, use a 4 to 5-inch thickness. For heavy trucks or RV parking, you should pour a 6-inch thick slab.
The general rule of thumb is the "1 Yard Rule." If your project requires 1 cubic yard (45 bags) or less, it's usually cheaper and easier to mix bags yourself. If your project is over 1 to 1.5 cubic yards, ordering a ready-mix delivery truck will save you immense physical labor and often costs less per yard.
To calculate steps, treat each individual stair step as its own rectangular slab. Measure the length, width (tread depth), and height (riser) of each step, calculate the volume for each, and add them all together.
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