Tank & Trench Concrete: Volume Formulas with Worked Examples

Q: How much waste should I add?

Add 5–10% for spillage, over-excavation, and an uneven base. Trenches in loose or caving soil can lose more concrete, so use the higher end or up to 15%.

How Do You Calculate Concrete for a Tank or Trench?

A trench is a single rectangular prism: length × width × depth ÷ 27 cubic yards. A tank is two parts added together — the base slab plus the four walls. Both reduce to the same idea: break the structure into rectangular blocks, total their volumes, and add a 5–10% waste allowance.

Quick start: The Tank & Trench Concrete Calculator handles the base, walls, and waste in one step.

For converting the result between cubic feet, yards, and meters, see the concrete volume conversions guide.

Trench Concrete

A trench fill (or trench footing) is the simplest case:

Cubic yards = (Length × Width × Depth) ÷ 27

Worked example: a 50 ft trench, 18 in wide, 12 in deep.

Width: 18 ÷ 12 = 1.5 ft
Depth: 12 ÷ 12 = 1.0 ft
Volume: 50 × 1.5 × 1.0 = 75 ft³
Cubic yards: 75 ÷ 27 = 2.78 yd³ → order ~3.05 yd³ with 10% waste

A deep trench fill under a wall is really a strip footing — size it with the concrete footing calculations guide.

Tank Concrete (Base + Walls)

Estimate a rectangular tank in two pieces.

Worked example: exterior footprint 12 ft × 10 ft, walls 6 ft tall × 8 in thick, base slab 8 in thick.

Base slab:

12 × 10 × 0.667 = 80.0 ft³

Walls (count two full-length walls, then two shortened walls so corners aren't double-counted):

Long walls: 2 × (12 × 6 × 0.667) = 96.0 ft³
Short walls: 2 × (8.67 × 6 × 0.667) = 69.4 ft³ (8.67 = 10 − 2 × 0.667)
Wall total = 165.4 ft³

Total: 80.0 + 165.4 = 245.4 ft³ = 9.09 yd³ → order ~10.0 yd³ with waste.

Circular Tanks

For a round tank, the base is a disc and the wall is a ring:

Base   = π × r² × thickness
Wall   = π × (R² − r²) × height

where R is the outer radius and r the inner radius (R minus wall thickness). Add base and wall, then apply waste. The same calculator handles circular inputs.

Adjustments for Benches & Slopes

Real tanks and trenches aren't always clean rectangles:

Sloped trench floor: use the average depth — (start depth + end depth) ÷ 2 — in the formula.
Benched or stepped trenches: split into segments, compute each at its own depth, and sum.
Tapered tank walls: average the top and bottom thickness, or split the wall into a rectangular part plus a triangular wedge.
Top slab / lid: if the tank is covered, add the lid as a separate slab.

Frequently Asked Questions

How do you calculate concrete for a trench?

Multiply length × width × depth (all in feet) and divide by 27 for cubic yards. A 50 ft trench that is 1.5 ft wide and 1 ft deep holds 75 ft³, or 2.78 cubic yards before waste.

How do you estimate concrete for a tank?

Add the base slab volume to the four walls. Compute the base as length × width × thickness, then the walls around the perimeter — using two full-length and two shortened walls so the corners aren't counted twice — and finish with a 5–10% waste allowance.

Trench: L × W × D ÷ 27 — a 50 × 1.5 × 1 ft trench is 2.78 yd³.
Tank: base slab + four walls (avoid double-counting corners) — the 12 × 10 × 6 ft example is 9.09 yd³.
Use average depth for slopes and split benched or tapered shapes into parts.
Add 5–10% waste to the total.

Validate any tank or trench on the Tank & Trench Concrete Calculator.

Visit Concrete Calculator Max for the full suite of concrete estimation tools.