Ad Space — Your advertisement here
Home / Concrete Calculator

Concrete Calculator

Calculate exactly how much concrete you need for any project—driveways, patios, foundations, and more.

Calculate Your Concrete Needs

Section 1
ft
ft
in
Standard thickness: 4-6 inches for slabs

Ready to Buy Materials?

Shop for quality materials on Amazon. As an Amazon Associate, we earn from qualifying purchases.

Concrete Mix Concrete Tools

As an Amazon Associate, we earn from qualifying purchases.



How to Calculate Concrete

Calculating concrete needs is straightforward once you understand the basic formulas for different shapes. Here are the standard methods for common project types:

Rectangular Slabs (Driveways, Patios, Floors)

Cubic Yards = (Length × Width × Thickness) ÷ 324

For example: A 20 ft long, 10 ft wide driveway with 4 inches of concrete requires (20 × 10 × 0.333) ÷ 27 = 2.47 cubic yards

Footings and Walls (Foundations, Posts, Perimeter)

Cubic Yards = (Length × Width × Height) ÷ 27

Measure the perimeter length, width of the footing, and depth below ground. A 100 ft perimeter, 2 ft wide, 3 ft deep footing requires (100 × 2 × 3) ÷ 27 = 22.2 cubic yards

Round Columns and Holes

Cubic Yards = (π × r² × Height) ÷ 27

For a 1 ft diameter post hole, 4 ft deep: (3.14159 × 0.5² × 4) ÷ 27 = 0.12 cubic yards

Stairs

Concrete stairs volume is the width multiplied by half the rise-run product multiplied by the number of steps, then divided by 27 for cubic yards. This accounts for the tapered wedge shape of each step.

How Many Bags of Concrete Do I Need?

Bagged concrete is perfect for small to medium projects and post holes. One cubic yard equals approximately:

  • 27 bags of 40lb concrete (most economical per bag)
  • 18 bags of 60lb concrete (moderate option)
  • 13.5 bags of 80lb concrete (most common and convenient)

Common Project Scenarios

Small Patio (10x10)

Area: 100 sq ft
Thickness: 4 inches
~0.74 cy = 10 bags (80lb)

Sidewalk (100ft)

Length: 100 ft
Width: 3 ft, 4" thick
~3.7 cy = 50 bags (80lb)

Driveway (20x20)

Area: 400 sq ft
Thickness: 4 inches
~4.94 cy = 67 bags (80lb)

Post Holes (4 posts)

Diameter: 1 ft each
Depth: 3 ft each
~0.5 cy = 7 bags (80lb)

Patio Stairs

10 steps, 3 ft wide
Rise: 7", Run: 11"
~1.2 cy = 16 bags (80lb)

Foundation Footing

100 ft perimeter
2 ft wide, 3 ft deep
~22.2 cy = 300 bags (80lb)

Choosing the Right Bag Size

  • 40lb bags — Best value per bag, but heaviest to carry individually. Good for mixing in small batches.
  • 60lb bags — Good middle ground between cost and weight. Requires modest mixing effort.
  • 80lb bags — Most popular choice. One bag yields about 0.6 cubic feet when mixed.

Concrete Calculator Formula Explained

Our calculator uses industry-standard formulas and automatically includes a 10% waste overage. Here's how the math works for each project type:

Step-by-Step Calculation Example

Project: 20 ft × 15 ft driveway, 4 inches thick

  1. Convert thickness to feet: 4 inches ÷ 12 = 0.333 feet
  2. Calculate volume: 20 × 15 × 0.333 = 99.9 cubic feet
  3. Convert to cubic yards: 99.9 ÷ 27 = 3.7 cubic yards
  4. Add 10% overage: 3.7 × 1.10 = 4.07 cubic yards
  5. Calculate bags (80lb): 4.07 ÷ 0.037 = 110 bags
  6. Estimate cost: 4.07 × $150 = $610.50 average

Why Add 10% Overage?

The 10% overage accounts for measurement variations, uneven surfaces, spillage, and settling during the pour. It's always better to have slightly extra than to run short mid-project.

Types of Concrete

Different projects call for different concrete mixes. Here's a comparison of common types:

Type Strength (PSI) Best For Cost Per Yard Setting Time
Standard Concrete 4,000 Driveways, patios, foundations $125-$150 24-48 hours
High-Strength 5,000+ Commercial, heavy loads, thin sections $150-$175 24-36 hours
Fast-Setting 3,500 Quick projects, cold weather, repairs $140-$165 6-8 hours
Fiber-Reinforced 4,000 Crack prevention, decorative finishes $160-$190 24-48 hours
Stamped/Colored 4,000 Decorative patios, driveways, accents $175-$225 24-48 hours

Tips for Ordering Concrete

Ready-Mix vs. Bagged Concrete

  • Ready-Mix (Truck Delivered) — Professional quality, consistent mix, ideal for projects over 5 cubic yards. Requires proper site access and weather planning. Costs $125-$175 per yard.
  • Bagged Concrete — Perfect for small repairs, post holes, and projects under 2 cubic yards. Mix on-site with water. Total cost higher per yard but no delivery charges.

How to Order Ready-Mix

  1. Calculate your cubic yardage plus 10% overage
  2. Choose your concrete type (standard, high-strength, etc.)
  3. Consider air entrainment for freeze-thaw climates
  4. Confirm site access for delivery truck (minimum 14 feet wide)
  5. Schedule pour during suitable weather (50-85°F is ideal)
  6. Have a crew ready to finish within 1-2 hours of delivery
  7. Plan for proper curing time (minimum 7 days, preferably 28 days)

Curing Tips for Long-Lasting Concrete

  • Keep it moist — Water concrete for 7 days after pouring to prevent cracking
  • Protect from elements — Cover with plastic or burlap in hot or cold weather
  • Avoid traffic — Don't drive or walk on concrete for at least 7 days, preferably 28
  • Control temperature — Extreme cold below freezing can damage uncured concrete
  • Use sealant — Apply sealant after 30 days for added protection

Money-Saving Tips

  • Order ready-mix for large projects (better price per yard)
  • Schedule pours off-peak (weekdays are often cheaper)
  • Prepare your site thoroughly to avoid delays and overage
  • Bundle multiple sections to avoid minimum charges
  • Do your own finishing work if you have experience