Concrete Volume Calculator in CFT

This concrete volume calculator works out the amount of concrete you need in cubic feet (cft) when you enter the length, width, and depth of the area to be filled. You get the volume in cubic feet, and the calculator also gives you the equivalent in cubic yards and cubic meters for easy ordering. The result is the exact volume of concrete required, before any allowance for waste or over-excavation.

Formula

The basic formula for concrete volume in cubic feet is:

Volume (cft) = Length (ft) × Width (ft) × Depth (ft)

Where:

• Length = the longest horizontal dimension of the pour, in feet
• Width = the horizontal dimension perpendicular to the length, in feet
• Depth = the thickness of the concrete slab or fill, in feet (convert from inches by dividing by 12)

If you measure in inches, convert all dimensions to feet first: divide inches by 12. If you measure in meters, multiply the result by 35.3147 to get cubic feet, or use the metric output directly.

Worked Example

Using the default values from the calculator (length = 10 ft, width = 5 ft, depth = 4 in):

1. Convert depth to feet: 4 in ÷ 12 = 0.3333 ft
2. Multiply: 10 ft × 5 ft × 0.3333 ft = 16.67 cft
3. Convert to cubic yards: 16.67 ÷ 27 = 0.62 yd³
4. Convert to cubic meters: 16.67 × 0.0283168 = 0.47 m³

The calculator shows 16.67 cft, 0.62 yd³, and 0.47 m³, matching the manual calculation.

Why Cubic Feet Matter for Concrete

Ready-mix concrete is usually ordered by the cubic yard in the US and by the cubic meter in most other countries. But many DIY projects—small slabs, footings, fence posts, or stepping stones—are easier to plan in cubic feet because the dimensions are often in feet and inches. Knowing the cft volume lets you compare with bagged concrete (which lists coverage in cubic feet per bag) or convert to cubic yards for a ready-mix order. A 60-lb bag of concrete mix typically yields about 0.45 cft, and an 80-lb bag about 0.60 cft, so you can quickly estimate how many bags you need.

How to Use the Calculator Correctly

• Measure the length and width of the form or excavation at the top, not at the bottom if the sides slope.
• Use the actual depth of the concrete, not the height of the form if the form extends above the final grade.
• For round columns or footings, calculate the area of the circle (Ï€r²) and multiply by the height, then convert to cubic feet.
• If you are pouring multiple identical sections, calculate one section and multiply by the number of sections.

Common Mistakes and How to Avoid Them

The most frequent error is mixing units. Depth is often given in inches, but length and width in feet—forgetting to convert the depth leads to a result that is 12 times too large. Another mistake is measuring the outer dimensions of a form when the concrete will be inside it; subtract the form thickness if it is significant. Also, the calculator gives the theoretical volume of the finished shape. It does not account for:

• Waste and spillage (add 5–10% for small jobs, more for complex pours)
• Over-excavation or uneven ground that requires extra fill
• Sub-base material like gravel that reduces the concrete depth
• Reinforcement bars or wire mesh that displace a small amount of concrete (usually negligible)

How to Choose Between Bagged and Ready-Mix

For volumes under about 1 cubic yard (27 cft), bagged concrete is often more practical because ready-mix companies have minimum delivery charges. For example, a 4-inch slab that is 10 ft × 10 ft is 33.3 cft (1.23 yd³)—still manageable with about 74 bags of 60-lb mix, but you may prefer a short-load delivery. For larger pours, ready-mix is more economical and ensures a consistent mix. Always check the yield on the bag, as it varies slightly by brand and mix design.

Limitations of the Formula

The length × width × depth formula assumes a rectangular prism with straight, parallel sides. It does not work for irregular shapes, sloped surfaces, or curved forms. For those, break the area into rectangles and triangles, calculate each volume separately, and add them together. The formula also assumes the concrete is compacted to the specified depth; if you are pouring over a rough subgrade, the actual volume may be higher. For structural projects, always consult a professional engineer or local building codes—this calculator is an estimation tool, not a substitute for engineering judgment.

Frequently Asked Questions

How many cubic feet are in a 60 lb bag of concrete?
Most 60-lb bags of concrete mix yield about 0.45 cubic feet when mixed with water. An 80-lb bag yields about 0.60 cubic feet. Divide your total cft by the bag yield to get the number of bags needed.

How do I convert cubic feet to cubic yards?
Divide the cubic feet by 27, because there are 27 cubic feet in one cubic yard (3 ft × 3 ft × 3 ft). For example, 54 cft ÷ 27 = 2 yd³.

Should I add extra concrete to my calculation?
Yes, it is common to add 5–10% for waste, spillage, and uneven subgrades. For small DIY projects, 10% is a safe margin. For large commercial pours, 5% is usually enough.

Can I use this calculator for a round column?
No, this calculator is for rectangular slabs. For a round column, use the formula: Volume = π × (radius²) × height. Make sure the radius and height are in the same units, then convert to cft if needed.

What is the difference between concrete and cement?
Cement is one ingredient in concrete—it acts as the binder. Concrete also contains sand, gravel, and water. When you buy “concrete mix� at a store, it already contains cement, aggregates, and additives; you just add water.

How accurate is this calculator for ordering ready-mix?
It is accurate for the geometric volume of a simple rectangular pour. However, ready-mix trucks deliver by volume, and the actual amount placed can vary due to compaction, subgrade conditions, and waste. Always confirm with your supplier and consider ordering a little extra if the job is critical.

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in l | Length | ft | 10 | 0
in w | Width | ft | 5 | 0
in d | Depth | in | 4 | 0
out cft | Concrete volume | cft | l * w * (d / 12) | 2
out cy | Cubic yards | yd³ | cft / 27 | 2
out m3 | Cubic meters | m³ | cft * 0.0283168 | 2