Estimate concrete for foundation trenches and water tanks (rectangular or circular).
Assumptions: Tank volumes use the shell method (outer volume minus inner volume), which accounts for base slab, walls, and optional cover slab. Trench volume is a prismatic approximation (rectangular or trapezoidal section).
Always verify dimensions on drawings. For ordering, add waste according to site conditions and supplier guidance.
How to Calculate Tank or Trench Concrete
This guide explains exactly how much concrete you need for tanks (rectangular & circular) and foundation trenches. You’ll learn the core formulas, get worked examples, and see the most common pitfalls so you can order confidently with the right waste allowance.
Step 1: Use Consistent Units
Convert all inputs to a single system before calculating. Metric: meters (m). Imperial: feet (ft). If your drawings show cm or inches, convert them into meters or feet respectively to avoid errors.
Rectangular (Regular) Tank Concrete
For a water-retaining tank, the concrete volume is computed as the outer volume minus inner volume, which naturally includes the base slab, the walls, and (if provided) the cover slab.
Formulas
- Outer dims: Lo = Li + 2tw, Wo = Wi + 2tw, Ho = Hi + tb + tt
- Outer Volume = Lo × Wo × Ho
- Inner Volume = Li × Wi × Hi
- Concrete = Outer − Inner
Where Li, Wi, Hi are the clear internal dimensions; tw is wall thickness; tb base thickness; and tt cover slab thickness (0 if not provided).
Example (Metric)
Li=3.0 m, Wi=2.0 m, Hi=2.0 m, tw=0.20 m, tb=0.25 m, tt=0.15 m.
Lo=3.4, Wo=2.4, Ho=2.40 → Outer=3.4×2.4×2.4=19.58 m³
Inner=3.0×2.0×2.0=12.00 m³ → Concrete = 7.58 m³
Circular Tank Concrete
Similar idea: compute the outer cylinder and subtract the inner cylinder. Include the base (and top, if applicable).
Formulas
- Do = Di + 2tw, Ho = Hi + tb + tt
- Outer = π × (Do/2)² × Ho
- Inner = π × (Di/2)² × Hi
- Concrete = Outer − Inner
Example (Metric)
Di=2.5 m, Hi=2.0 m, tw=0.20 m, tb=0.25 m, tt=0.15 m → Do=2.9 m, Ho=2.40 m.
Outer = π×(2.9/2)²×2.4 = 15.83 m³; Inner = π×(2.5/2)²×2.0 = 9.82 m³ → Concrete = 6.01 m³
Trench Concrete
Trench volume is a prismatic approximation. Use a rectangular or trapezoidal cross-section depending on the excavation shape.
Formulas
- Rectangular: V = L × W × D
- Trapezoidal: V = L × D × (TopW + BottomW) / 2
Example (Trapezoidal)
L=24 m, D=0.9 m, TopW=0.8 m, BottomW=0.5 m
V = 24 × 0.9 × (0.8+0.5)/2 = 14.04 m³
Waste Allowance & Conversions
Add 5–10% for spillage, pump line losses, and minor over-excavation. Order in the supplier’s preferred units and round up to the nearest truck load if needed.
- 1 m³ = 35.3147 ft³
- 1 m³ = 1.30795 yd³
Design Notes & Best Practices
- Use clear (inner) dimensions for tanks when deriving outer dimensions via thicknesses.
- Confirm wall/base/top thickness, reinforcement, and joints per structural drawings—especially for water-retaining structures.
- For trenches, ensure the section shape (rectangular vs. trapezoidal) matches the excavation profile approved on site.
- Consider site access, pour sequence, and lift heights to minimize cold joints.
Common Mistakes to Avoid
- Mixing metric and imperial units in one calculation.
- Ignoring cover slab thickness when it’s specified.
- Underestimating trench width due to side batter or collapse.
- Forgetting to add waste allowance (5–10%).
In summary, calculate tanks by subtracting the inner volume from the outer (including base and optional cover), and compute trenches using the correct cross-section. Add waste, convert to supplier units, and round sensibly for delivery. Your on-page calculator will handle these steps in seconds and reduce errors.