CalcConcrete Footing Volume Estimator
Last updated: 2026-08-24
How to Use This Calculator
Enter the length and width of your footing in feet, and the depth in inches. Then specify the rebar spacing in inches, which is the distance between parallel reinforcement bars. Click calculate to see the total volume of concrete needed in cubic yards, the number of 40lb bags required, the total rebar length needed for a two-layer grid, and an estimated cost.
Formula and Methodology
The concrete volume is calculated by multiplying length × width × depth (after converting depth to feet). The result is divided by 27 to get cubic yards. The bag count is based on 40lb bags, which cover about 0.0185 cubic yards each. For rebar, we assume two layers (top and bottom) with bars running in both directions. The number of bars in each direction is calculated by dividing the opposite dimension (in inches) by the spacing, adding one, then multiplying by the length of that direction. This gives the total linear feet of rebar. The cost is an average of $130 per cubic yard, including material and labor, though prices vary by region.
Practical Examples
Say you have a footing that is 20 feet long, 3 feet wide, and 12 inches deep. The volume is 20 × 3 × 1 = 60 cubic feet, which is 2.22 cubic yards. That would require about 120 bags of 40lb concrete. With rebar spaced at 12 inches, you'll have 7 bars along the length (since 3 ft = 36 inches / 12 + 1) and 21 bars along the width (20 ft = 240 inches / 12 + 1). Total rebar length = (7 × 20 + 21 × 3) × 2 = 406 feet. The cost would be about $289.
Tips and Best Practices
Always order a little extra concrete (5-10%) to account for spillage and uneven surfaces. Use a ready-mix truck for large footings, as it's more efficient than mixing bags. For rebar, ensure proper clearance from the edges using chairs or spacers. Check local building codes for minimum depth and rebar requirements based on frost line and soil type. Consider using a fiber-reinforced concrete mix to reduce cracking. When in doubt, consult a structural engineer.
FAQ
How many bags of concrete do I need for my footing?
The calculator estimates the number of 40lb bags. For a 12-inch thick footing, you typically need about 54 bags per cubic yard. If you use 60lb bags, divide the number by 1.5; for 80lb bags, divide by 2. Adjust accordingly.
What is the recommended rebar spacing for a concrete footing?
Typical rebar spacing for residential footings is 12 to 18 inches on center, with two layers (top and bottom). However, always consult your local building codes and a structural engineer, as spacing depends on soil conditions and load requirements.
How do I convert cubic feet to cubic yards?
One cubic yard equals 27 cubic feet. So divide the total cubic feet by 27. For example, 54 cubic feet / 27 = 2 cubic yards.
Can I use this calculator for a slab instead of a footing?
Yes, the formula is essentially the same for any rectangular concrete volume. However, for slabs, you might not need rebar, or you may use a different spacing. The calculator still gives you the volume and related estimates.