Concrete Foundation Calculator: Mix, Cost & Volume

This concrete foundation calculator works out how much concrete you need for a slab, footing, or wall by taking the length, width, and depth (or height) you enter, then returns the volume in cubic yards, cubic meters, and the number of 60-lb or 80-lb bags required. It also gives a rough cost estimate if you type in a price per cubic yard or per bag. You get an instant answer for volume and materials, which is the first number you need before ordering ready-mix or bagged concrete.

Formula

The basic formula for concrete volume is the same as for any rectangular prism:

Volume = Length × Width × Depth

Where:

• Length (l) – the longest horizontal dimension of the pour, in feet (or meters).
• Width (w) – the shorter horizontal dimension, in feet (or meters).
• Depth (d) – how thick the concrete is, in inches (or centimeters). Convert to feet by dividing by 12.

To convert cubic feet to cubic yards, divide by 27 (since 1 yard = 3 feet, and 3³ = 27). For metric, multiply length × width × depth in meters to get cubic meters directly.

The bag count uses standard coverage: a 60-lb bag makes about 0.45 cubic feet, and an 80-lb bag makes about 0.6 cubic feet. So the number of bags is the total cubic feet divided by 0.45 or 0.6, rounded up.

Worked Example

Using the calculator defaults: a slab that is 20 ft long, 10 ft wide, and 4 inches deep.

1. Convert depth to feet: 4 in ÷ 12 = 0.333 ft
2. Volume in cubic feet: 20 × 10 × 0.333 = 66.6 ft³
3. Convert to cubic yards: 66.6 ÷ 27 = 2.47 yd³
4. 60-lb bags: 66.6 ÷ 0.45 = 148 bags (round up to 148)
5. 80-lb bags: 66.6 ÷ 0.6 = 111 bags (round up to 111)
6. Cost at $150 per yard: 2.47 × $150 = $370.50

The calculator shows 2.47 cubic yards, 148 bags of 60-lb, 111 bags of 80-lb, and $370.50 for the concrete alone.

How to Use the Calculator Correctly

Measure the actual dimensions of your foundation, not the overall footprint of the house. For a slab, that’s the length and width of the slab surface. For a footing, it’s the width of the trench and the depth of the concrete. For a wall, it’s the length, height, and thickness.

Always add a safety margin. Concrete is not perfectly level, forms can bow, and you might spill some. A common practice is to order 10% extra. The calculator does not add that automatically, so multiply the result by 1.1 if you want a buffer.

Check your units: if you measure length in feet and depth in inches, convert depth to feet first. Mixing inches and feet without conversion is the most common error.

Common Mistakes and How to Avoid Them

• Forgetting to convert depth: Depth is usually given in inches, but length and width are in feet. Always divide inches by 12.
• Ordering too little: Ready-mix trucks charge for the full load even if you use less. It’s better to overestimate by 5–10% than to run short and have to pour a cold joint.
• Ignoring the subgrade: The calculator assumes a flat, even excavation. If your site is sloped or has soft spots, you’ll need more concrete to fill low areas.
• Rounding too early: Keep at least two decimal places until the final answer. Rounding each step can change the bag count by several bags.
• Not accounting for waste: Forms leak, the ground absorbs some water, and the chute always has leftover concrete. Add 10% unless you are very experienced.

Ready-Mix vs. Bagged Concrete

For large foundations (more than about 1 cubic yard), ready-mix from a truck is usually more practical and cheaper per volume. You order by the cubic yard, and the truck delivers it wet. For small projects like a shed pad or a small footing, bagged concrete is easier because you mix only what you need.

Bagged concrete comes in 40, 60, and 80-lb bags. An 80-lb bag yields about 0.6 cubic feet, a 60-lb bag about 0.45 cubic feet, and a 40-lb bag about 0.3 cubic feet. The calculator uses the two most common sizes, but you can adjust if you use a different size.

One important note: bagged concrete is more expensive per cubic foot than ready-mix, but you avoid the delivery fee and the minimum order charge. For a 2-cubic-yard slab, ready-mix is almost always the better deal.

Cost Factors and Estimates

The calculator’s cost estimate is just for the concrete material. The actual cost of a foundation includes several other items:

• Concrete – the material itself, per cubic yard or per bag.
• Delivery – ready-mix trucks charge a delivery fee, sometimes included in the per-yard price.
• Reinforcement – rebar or wire mesh, which adds strength but also cost.
• Formwork – wood or metal forms to hold the concrete in place.
• Labor – if you hire someone, labor often costs more than the material.

To get a realistic total, add 20–50% to the concrete cost for these extras. The calculator gives you the base material cost, which is the starting point for any budget.

Limits of the Formula

This calculator assumes a simple rectangular shape. Real foundations often have steps, corners, or irregular shapes. If your foundation is L-shaped or has a thickened edge, break it into rectangles, calculate each one, and add them together.

The formula also assumes the depth is uniform. If your footing is deeper in some spots (e.g., to reach frost line), you need to measure the average depth or calculate each section separately.

It does not account for the “fluff� factor – freshly placed concrete compacts slightly, so the actual volume in place is a bit less than the wet volume. In practice, this is negligible for small projects, but for large pours, contractors order slightly more than the calculated volume.

Frequently Asked Questions

How many 80-lb bags do I need for 1 cubic yard?

One cubic yard is 27 cubic feet. An 80-lb bag makes about 0.6 cubic feet, so 27 ÷ 0.6 = 45 bags. You can also use the calculator: enter length = 3 ft, width = 3 ft, depth = 36 in (which is 3 ft), and it will show 1 cubic yard and 45 bags.

Should I order extra concrete?

Yes, generally order 5–10% more than the calculated volume. This covers uneven ground, spillage, and forms that bulge slightly. For a 2-cubic-yard pour, that means ordering 2.1–2.2 cubic yards. It’s cheaper to have a little left over than to run short and have to make a second pour.

What is the difference between a slab, footing, and wall in the calculator?

They all use the same volume formula – length × width × depth. The only difference is what you measure. For a slab, measure the surface area and thickness. For a footing, measure the width and depth of the trench. For a wall, measure the length, height, and thickness. The calculator works for any rectangular pour.

How do I convert inches to feet for the depth?

Divide the number of inches by 12. For example, 6 inches is 6 ÷ 12 = 0.5 feet. The calculator does this automatically when you enter the depth in inches, but if you’re doing the math by hand, don’t forget this step.

Can I use this calculator for a round concrete column?

No, this calculator is for rectangular shapes only. For a round column, use the formula for a cylinder: π × radius² × height. The volume in cubic feet is then converted to cubic yards the same way (divide by 27).

Does the calculator include rebar or wire mesh?

No, it only calculates the concrete volume and the cost of the concrete itself. Reinforcement is a separate cost and is not part of the volume calculation. You’ll need to add that to your budget separately.

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in l | Length | ft | 20 | 0
in w | Width | ft | 10 | 0
in d | Depth | in | 4 | 0
in price | Price per cubic yard | $ | 150 | 0
out cy | Concrete needed | cubic yards | (l * w * (d / 12)) / 27 | 2
out bags60 | 60-lb bags | bags | ceil(((l * w * (d / 12)) / 27) * 54) | 0
out bags80 | 80-lb bags | bags | ceil(((l * w * (d / 12)) / 27) * 40) | 0
out cost | Estimated cost | $ | cy * price | 2