Concrete Footer Calculator: Size & Volume

A concrete footer calculator works out the volume of concrete you need for a footing, based on the footing’s length, width, and depth. You enter those three dimensions and the calculator returns the volume in cubic yards (or cubic meters), plus an estimate of how many 80-lb or 60-lb bags are required. This page explains the formula behind the calculation, how to measure your footer correctly, and what to do with the result.

Formula

The basic formula for a rectangular footer is:

Volume = Length × Width × Depth

Where:

• Length (L) – the total length of the footing, in feet (ft) or meters (m).
• Width (W) – the width of the footing, in feet or meters.
• Depth (D) – the thickness of the footing, typically in inches (in) or millimeters (mm). Convert to the same unit as length and width before multiplying.

If you measure in feet and inches, convert depth to feet by dividing by 12. The result is in cubic feet. To get cubic yards, divide by 27 (since 1 cubic yard = 27 cubic feet). For metric, measure in meters and get cubic meters directly.

Worked Example

Using the default values from the calculator above:

• Length = 20 ft
• Width = 1 ft
• Depth = 12 in = 1 ft

Step 1: Convert depth to feet: 12 in ÷ 12 = 1 ft

Step 2: Calculate volume in cubic feet: 20 × 1 × 1 = 20 ft³

Step 3: Convert to cubic yards: 20 ÷ 27 ≈ 0.74 yd³

So you need about 0.74 cubic yards of concrete. For bag count, an 80-lb bag yields roughly 0.6 cubic feet, so 20 ÷ 0.6 ≈ 33.3, meaning you’d need 34 bags. A 60-lb bag yields about 0.45 cubic feet, so 20 ÷ 0.45 ≈ 44.4, meaning 45 bags.

When to Use a Footer Calculator

Footers (also called footings) are the wide, shallow concrete bases that support foundation walls, columns, or posts. You need to calculate concrete volume for a footer when:

• You’re pouring a new foundation for a house, garage, or shed.
• You’re setting deck posts or porch columns on concrete footings.
• You’re installing a retaining wall or a masonry wall that requires a footing.
• You’re replacing an existing damaged footer.

The calculator gives you the exact amount of concrete to order or mix, helping you avoid waste or running short.

How to Measure Your Footer Correctly

Accurate measurements are critical. Here’s what to do:

1. Measure the length – For a continuous wall footer, measure the total length along the centerline, including any corners. For a single post footer, measure the side length of the square or the diameter if round.
2. Measure the width – The width is the shorter horizontal dimension. Typical footer widths range from 12 to 24 inches, depending on the load.
3. Measure the depth – The thickness of the concrete after it’s placed. Common depths are 8 to 12 inches, but always follow your local building code.
4. Convert all units to the same system – If you measure length in feet and depth in inches, convert depth to feet before multiplying.

If your footer is not a simple rectangle (e.g., stepped or L-shaped), break it into rectangular sections, calculate each separately, and add the volumes.

Common Mistakes and How to Avoid Them

• Unit mix-ups – Forgetting to convert inches to feet is the most common error. Always divide inches by 12.
• Overlooking the actual depth – The depth is the thickness of the concrete, not the depth of the trench. If the trench is deeper than the footer, you’ll need extra concrete for the pad.
• Forgetting to account for waste – Concrete settles, spills, and doesn’t always fill perfectly. Add 10% to your total volume to cover waste and uneven ground.
• Rounding too early – Round only at the final step. If you round the cubic feet too early, you might order too little or too much.

Bag vs. Ready-Mix: Which to Choose

For small footers (under about 1 cubic yard), bagged concrete is practical. You mix it yourself in a wheelbarrow or mixer. For larger footers, ready-mix concrete delivered by truck is more economical and consistent. Here’s a quick comparison:

• Bagged concrete – Good for small projects, easy to control, but labor-intensive. An 80-lb bag makes about 0.6 cubic feet; a 60-lb bag makes about 0.45 cubic feet.
• Ready-mix – Best for large footers, saves time, but you must order the right amount and schedule delivery. You’ll typically order by the cubic yard.

Use the calculator to get the volume, then decide based on the size of your project.

Limits of the Formula

The simple length × width × depth formula assumes a perfectly rectangular, uniform footer. Real-world footers may have variations, such as:

• Stepped footers on sloped ground – each step is a separate rectangle.
• Round or octagonal footers – use the appropriate area formula (Ï€r² for a circle).
• Footers with rebar or gravel – these displace some concrete, but the difference is usually negligible for ordering purposes.

Also, the calculator does not account for the concrete’s “slump� (how much it settles) or the need for a vapor barrier or gravel base. Those are separate considerations.

Safety and Code Considerations

Concrete work involves heavy materials and wet concrete, which can cause burns. Wear gloves, long sleeves, and eye protection. Follow local building codes for footer size and depth – they are based on frost depth and soil bearing capacity. The calculator gives you the volume, not the structural design. Always consult a structural engineer or your local building department for footing dimensions that are safe for your specific project.

Frequently Asked Questions

How deep should a concrete footer be?

The depth depends on your local frost line and the load. In cold climates, footers must extend below the frost line to prevent heaving. Typical depths range from 12 inches for a small shed to 24 inches or more for a two-story house. Check your local building code for the exact requirement.

How many 80-lb bags of concrete do I need for a 20×1×1 footer?

As shown in the example, a 20 ft × 1 ft × 1 ft footer is 20 cubic feet. An 80-lb bag yields about 0.6 cubic feet, so you need 20 ÷ 0.6 ≈ 33.3, meaning 34 bags. Always round up and add 10% for waste, so about 37 bags.

Can I use a footer calculator for a deck footing?

Yes, but you need to know the dimensions of the footing – typically a round or square pad. For a round footing, use the formula π × (radius)² × depth. For a square, use length × width × depth. The calculator works for any shape if you enter the correct area.

What is the difference between a footer and a foundation?

A footer is the wide, shallow concrete pad that distributes the weight of the structure to the soil. A foundation is the entire system that supports the building, including the footer, foundation walls, and sometimes a slab. The footer is the bottom part of the foundation.

Should I add rebar to my footer?

Most building codes require rebar in footers to prevent cracking and provide tensile strength. The amount and placement depend on the size of the footer and the load. Consult your local code or a structural engineer. The calculator does not include rebar, but it’s an important part of the pour.

How much does a cubic yard of concrete cover?

One cubic yard covers 27 cubic feet. If your footer is 1 foot thick, a cubic yard covers 27 square feet. If it’s 1.5 feet thick, it covers 18 square feet. Use the calculator to find the exact coverage for your dimensions.

Related

• Board Foot Calculator Formula
• Board Foot Calculator: Lumber Volume
• Board Foot Calculator From Log: How to Estimate Lumber

in l | Length | ft | 20 | 0
in w | Width | ft | 1 | 0
in d | Depth | in | 12 | 0
out vol_ft | Volume (cubic ft) | ft³ | l * w * (d / 12) | 2
out vol_yd | Volume (cubic yd) | yd³ | (l * w * (d / 12)) / 27 | 2
out bags80 | 80-lb bags | bags | ceil(((l * w * (d / 12)) * 0.6) / 0.6) | 0
out bags60 | 60-lb bags | bags | ceil(((l * w * (d / 12)) * 0.45) / 0.45) | 0