How to Calculate Concrete Needs for Your Project
Concrete volume is calculated by multiplying length times width times thickness (depth), then converting to cubic yards by dividing by 27. One cubic yard of concrete equals 27 cubic feet and covers approximately 81 square feet at 4 inches thick. Getting the calculation right means you avoid costly shortages during a pour and minimize waste from over-ordering.
Quick Answer
- 1. Formula: Length x Width x Depth (in feet) / 27 = cubic yards.
- 2. 1 cubic yard covers 81 sq ft at 4" thick, 54 sq ft at 6" thick.
- 3. Always add 10-15% extra for waste, spillage, and uneven subgrade.
- 4. Ready-mix costs $125-$175/yard in 2025; bagged costs ~$250-$330/yard equivalent.
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Calculate Concrete FreeThe Basic Concrete Formula
Concrete is ordered and delivered in cubic yards. The basic formula to calculate how much you need is:
Volume (cubic yards) = Length (ft) x Width (ft) x Depth (ft) / 27
The division by 27 converts cubic feet to cubic yards (since 1 cubic yard = 3 x 3 x 3 = 27 cubic feet). The most common mistake is forgetting to convert the depth from inches to feet before multiplying. A 4-inch depth is 4/12 = 0.333 feet, not 4 feet.
How to Calculate Concrete for a Slab
Slabs are the most common concrete project, including patios, driveways, garage floors, and sidewalks. The calculation is straightforward since slabs are rectangular.
Worked Example: 20 x 24 Foot Patio, 4 Inches Thick
- Convert thickness to feet: 4 inches / 12 = 0.333 feet
- Calculate volume in cubic feet: 20 x 24 x 0.333 = 159.84 cubic feet
- Convert to cubic yards: 159.84 / 27 = 5.92 cubic yards
- Add 10% waste factor: 5.92 x 1.10 = 6.51 cubic yards
- Order: 6.5 to 7 cubic yards
Quick Reference: Coverage per Cubic Yard
| Slab Thickness | Square Feet per Cubic Yard |
|---|---|
| 3 inches | 108 sq ft |
| 4 inches | 81 sq ft |
| 5 inches | 65 sq ft |
| 6 inches | 54 sq ft |
| 8 inches | 40.5 sq ft |
| 12 inches | 27 sq ft |
Shortcut for 4-inch slabs: Divide your total square footage by 81 to get cubic yards. A 500 square foot driveway at 4 inches = 500 / 81 = approximately 6.2 cubic yards.
How to Calculate Concrete for Footings
Footings are the foundation elements that support walls, columns, and posts. They are typically deeper and narrower than slabs.
Continuous Footings (Wall Footings)
For a footing that runs along the perimeter of a structure:
Volume = Length (ft) x Width (ft) x Depth (ft) / 27
Example: A 100-foot perimeter footing that is 2 feet wide and 1 foot deep: 100 x 2 x 1 / 27 = 7.41 cubic yards. With 15% waste factor: approximately 8.5 cubic yards.
Column Footings (Pier Footings)
For square or rectangular pads under individual columns:
Calculate each footing's volume individually, then multiply by the number of footings. A 2 x 2 x 1 foot footing = 4 cubic feet. Ten such footings = 40 cubic feet / 27 = 1.48 cubic yards.
How to Calculate Concrete for Columns and Posts
Cylindrical columns and round post holes use a different formula because they are circular, not rectangular.
Volume (cubic feet) = pi x radius2 x height
Example: A 12-inch diameter post hole that is 3 feet deep: radius = 6 inches = 0.5 feet. Volume = 3.14159 x 0.52 x 3 = 3.14159 x 0.25 x 3 = 2.36 cubic feet per hole.
For 10 post holes: 2.36 x 10 = 23.6 cubic feet / 27 = 0.87 cubic yards. With waste: approximately 1 cubic yard.
Bagged Concrete: How Many Bags Do You Need?
For small projects, you may prefer bagged concrete (like Quikrete or Sakrete) over ready-mix delivery. Each bag size yields a different volume:
| Bag Size | Yield per Bag | Bags per Cubic Yard |
|---|---|---|
| 40 lb | 0.30 cubic feet | 90 bags |
| 60 lb | 0.45 cubic feet | 60 bags |
| 80 lb | 0.60 cubic feet | 45 bags |
Example: If you need 0.5 cubic yards (13.5 cubic feet) of concrete: 13.5 / 0.60 = approximately 23 bags of 80-pound concrete.
Concrete Cost Estimation
Knowing the volume lets you estimate project cost:
Ready-Mix Pricing (2025)
- Standard mix: $125-$175 per cubic yard
- Delivery fee: $0-$60 (often waived for orders over 5 yards)
- Short load fee: $40-$80 per yard under the minimum order (typically 1-3 yards)
- Saturday delivery: $50-$100 surcharge
Total Project Cost Example
A 20 x 24 foot patio requiring 7 cubic yards of ready-mix concrete: 7 x $150 = $1,050 for materials (concrete only, not including labor, forms, rebar, or finishing).
Common Concrete Calculation Mistakes
Forgetting to Convert Inches to Feet
This is the most common error. If your slab is 4 inches thick, you must divide by 12 to get 0.333 feet before using the volume formula. Using 4 instead of 0.333 will give you a result 12 times too large.
Skipping the Waste Factor
Never order exactly the calculated amount. Uneven subgrade, form bulges, spillage, and measurement errors mean you always need more than the theoretical volume. Add 10% minimum, and 15-20% for complex shapes or difficult site conditions.
Ignoring Site Conditions
If the subgrade is uneven (which it usually is), the actual slab thickness will vary. An area that dips 2 inches below grade over a 20-foot span can add 10% or more to your concrete needs compared to a perfectly flat base.
The Bottom Line
Calculating concrete comes down to one simple formula: length times width times depth, divided by 27 for cubic yards. Add 10 to 15 percent for waste, convert to bags if you are mixing by hand, and estimate your cost based on current local prices. Getting the math right saves you money and prevents the costly mistake of running short during a pour.
Our free concrete calculator handles all the math for slabs, footings, columns, and stairs, and gives you the exact number of bags or cubic yards needed for your project.
Frequently Asked Questions
How many bags of concrete do I need for a 10x10 slab?
A 10x10 foot slab at the standard 4-inch thickness requires approximately 1.23 cubic yards of concrete. Since one 80-pound bag of premixed concrete yields about 0.6 cubic feet, and 1.23 cubic yards equals 33.2 cubic feet, you would need approximately 56 bags of 80-pound concrete. However, for a project this size, ordering ready-mix concrete delivered by truck is typically more cost-effective and results in better quality than mixing individual bags.
How thick should a concrete slab be?
Standard thickness depends on the application. Sidewalks and patios are typically 4 inches thick. Residential driveways should be at least 4 inches thick, but 5 to 6 inches is recommended for heavier vehicles or areas with poor soil. Garage floors are typically 4 to 6 inches thick. Foundations and footings require 8 to 12 inches or more, depending on local building codes and structural load requirements. Always check local building codes before pouring, as minimum thicknesses vary by jurisdiction.
Should I order extra concrete?
Yes. Always order 10 to 15 percent more concrete than your calculated volume. Extra concrete accounts for uneven subgrade, spillage during pouring, variations in form dimensions, and slight inaccuracies in measurement. For projects with complex shapes, slopes, or irregular forms, consider ordering up to 20 percent extra. Running short during a pour is much more problematic than having a small amount left over, as cold joints (where new concrete meets hardened concrete) create weak points.
What is the difference between ready-mix and bagged concrete?
Ready-mix concrete is mixed at a batch plant and delivered by truck, typically in quantities of 1 cubic yard or more. It provides consistent quality, saves labor, and is more cost-effective for large projects. Bagged concrete (like Quikrete or Sakrete) comes premixed in 40, 60, or 80-pound bags that you mix on site with water. Bags are practical for small projects under 1 cubic yard, repairs, and posts. For a project requiring more than about 30 bags, ready-mix delivery is usually the better choice in terms of both cost and quality.
How much does concrete cost per cubic yard?
In the United States in 2025, ready-mix concrete costs approximately $125 to $175 per cubic yard, depending on your region, the mix design, delivery distance, and any additives. Specialty mixes (like high-strength or fiber-reinforced) cost more. Bagged concrete costs roughly $4 to $6 per 80-pound bag, which works out to about $250 to $330 per cubic yard when you factor in that you need approximately 55 bags per cubic yard. This makes bagged concrete significantly more expensive per yard than ready-mix for larger projects.
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