Duct Bank Concrete Calculator: Volume & Cost

This duct bank concrete calculator works out the volume of concrete needed to fill a rectangular duct bank trench, and estimates the cost based on the price you enter per cubic yard or cubic meter. You input the trench length, width, and depth, plus the number and diameter of conduits (ducts) inside, and it subtracts the conduit volume to give the true concrete fill volume and its cost.

Formula

The calculator uses the standard geometric volume formula for a rectangular prism, then subtracts the space taken up by the conduits (which are treated as cylinders running the full length of the trench).

Concrete volume (yd³) = [ (L × W × D) − (N × π × (d/24)² × L) ] ÷ 27

• L = trench length (ft)
• W = trench width (ft)
• D = trench depth (ft)
• N = number of conduits
• d = outer diameter of one conduit (in, converted to ft by dividing by 24 because radius = d/2, then inches to feet = /12, so radius in ft = d/24)
• Ï€ ≈ 3.14159
• 27 = cubic feet per cubic yard (since 1 yd = 3 ft, so 1 yd³ = 27 ft³)

The result is the net concrete volume in cubic yards. To get cubic meters, multiply the cubic yard result by 0.7646.

Worked Example

Using the default values: length = 100 ft, width = 3 ft, depth = 2 ft, conduits = 6, outer diameter = 4 in, price = $150/yd³.

1. Gross trench volume = 100 × 3 × 2 = 600 ft³
2. Conduit radius in ft = (4/2)/12 = 2/12 = 0.1667 ft
3. One conduit cross-section area = π × (0.1667)² = 0.08727 ft²
4. All 6 conduits volume = 6 × 0.08727 × 100 = 52.36 ft³
5. Net concrete volume = 600 − 52.36 = 547.64 ft³
6. Convert to cubic yards = 547.64 ÷ 27 = 20.28 yd³ (rounded to 20.28)
7. Cost = 20.28 × $150 = $3,042

The calculator shows 20.28 yd³ and $3,042 for the defaults.

Why Conduit Volume Matters

Many people order concrete based on the trench dimensions alone, which overestimates the fill. For a 100 ft run with six 4-inch conduits, the conduits take up about 52 cubic feet – that’s roughly 2 cubic yards of concrete you’d pay for but not need. In a large duct bank, this can easily mean ordering one less ready-mix truck. The calculator accounts for this so you order closer to the actual amount.

Note that this calculator assumes the conduits are straight, run the full trench length, and are completely surrounded by concrete. It does not account for spacers, sand bedding, or rebar – those occupy space too, but they are usually small compared to conduits. If you use a sand layer at the bottom (common for PVC duct banks), subtract that depth from the trench depth before entering it.

How to Use the Calculator Correctly

To get a realistic estimate, measure the trench dimensions from the actual excavation plans. The width should be the trench width at the bottom, not the top, unless the sides are perfectly vertical. The depth should be from the bottom of the trench to the top of the concrete, not to the ground surface if there is a soil cover above the concrete.

• Use the outer diameter of the conduit, not the inner diameter. Conduit wall thickness adds up.
• For multiple layers of conduits, just enter the total count. The calculator doesn’t care about layout.
• If conduits are different sizes, use an average outer diameter – or better, calculate each size separately and add the volumes.
• Add 5–10% for waste, spillage, and uneven trench walls. The calculator gives the theoretical volume; real pours always use a bit more.

Common Mistakes and Misconceptions

The most frequent error is mixing units. The trench length, width, and depth are entered in feet, but the conduit diameter is in inches. The calculator handles that conversion, but if you’re doing it by hand, remember to divide the diameter by 12 to get feet before using it in the area formula.

Another misconception is that the concrete volume equals the trench volume. That’s only true when there are no conduits. Even a single 4-inch conduit over 100 feet takes up 0.87 cubic feet – not much, but with 20 conduits, it’s over 17 cubic feet, which is over half a cubic yard.

Some people also confuse the outer diameter with the nominal conduit size. A 4-inch PVC conduit has an outer diameter of about 4.5 inches. Always check the manufacturer’s spec sheet for the actual outer diameter.

Finally, the calculator gives the concrete volume for the duct bank itself. It does not include concrete for manholes, pull boxes, or end bells. Those need separate calculations.

Cost Estimation: What’s Included and What’s Not

The cost output is simply the concrete volume multiplied by the price per cubic yard you enter. That price should be the delivered, poured cost of the concrete, which typically includes the material, delivery, and sometimes the pump. It does not include:

• Excavation and backfill
• Conduit material and installation
• Spacers, rebar, or wire mesh
• Concrete testing or inspection
• Finishing or curing
• Overhead and profit for the contractor

If you’re budgeting a project, add 10–20% to the concrete cost for these extras. The calculator is a starting point, not a full bid.

When to Use This Calculator (and When Not To)

Use this calculator for any rectangular trench that will be filled with concrete around conduits – typical for electrical duct banks, telecommunications, or street lighting runs. It works for any number of conduits, as long as they are roughly cylindrical and run the full length.

Do not use it for:

• Trenches that are not rectangular in cross-section (e.g., V-shaped or stepped)
• Conduits that are not straight or do not run the full length
• Concrete-encased duct banks with a different shape (e.g., round or oval)
• Calculating concrete for footings, slabs, or columns – those use different formulas

For a simple rectangular trench with no conduits, set the number of conduits to 0 and the calculator will give the plain trench volume.

Frequently Asked Questions

How much concrete do I need for a duct bank?
Measure the trench length, width, and depth in feet, multiply them to get the gross volume, then subtract the volume of all conduits (π × (diameter/24)² × length × number of conduits). Divide by 27 to get cubic yards. Add 5–10% for waste.

What is the difference between duct bank and conduit?
A duct bank is the concrete structure that encases multiple conduits (pipes). The conduits are the individual pipes that carry cables. The concrete protects the cables from damage and heat.

How many conduits fit in a duct bank?
There’s no fixed limit – it depends on the trench width and depth, conduit diameter, and required spacing. Common layouts have 2 to 24 conduits. The calculator works for any number, but you must ensure the trench is wide enough for the conduits plus the required concrete cover (usually 3 inches minimum).

Should I use the inner or outer diameter of the conduit?
Always use the outer diameter. The concrete fills the space around the outside of the conduit, so the outer dimension is what takes up space in the trench.

How much does a yard of concrete cost?
Prices vary widely by region and mix design. As of recent years, a typical ready-mix concrete costs $100–$200 per cubic yard, but you should get a local quote. The calculator lets you enter your own price.

Does the calculator include rebar or spacers?
No. Rebar and spacers take up a small volume, usually less than 1–2% of the trench volume. The calculator ignores them, which is fine for estimation. If you want to be precise, you could reduce the trench volume by 1–2% manually.

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in len | Trench length | ft | 100 | 0
in wid | Trench width | ft | 3 | 0
in dep | Trench depth | ft | 2 | 0
in num | Number of conduits | - | 6 | 0
in dia | Conduit outer diameter | in | 4 | 0
in price | Concrete price | $/yd³ | 150 | 0
out vol | Concrete volume | yd³ | (len*wid*dep - num*pi*(dia/24)^2*len) / 27 | 2
out cost | Concrete cost | $ | vol * price | 0