Water Tank Calculator

Fluid Mechanics & AWWA Water Storage Tank Metrology

Water storage calculations convert geometric volume into liquid capacity and structural mass:

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• Geometric Volume Formulas**:

• Vertical / Horizontal Cylinder: V = π ((D) / (2))^2 H cu ft

• Rectangular / Square Cistern: V = L × W × H cu ft

• Spherical Tank: V = (4) / (3) π ((D) / (2))^3 cu ft

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• Volumetric Conversion Constants**:

• 1 cu ft = 7.48052 US Gallons = 28.3168 Liters

• 1 US Gallon of Water = 8.3404 lbs = 3.7854 kg

• 1 Metric Ton of Water = 1,000 Liters = 1.0 m^3 = 264.17 US Gallons

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• Hydrostatic Base Water Pressure Formula**:

P_base = Water Depth (ft) × 0.4335 PSI/ft

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• Standard Tank Sizing Benchmarks**:

• 4 ft Dia × 6 ft L (Horizontal): 564 Gallons | 2,135 L | 2.35 Tons | 1.7 PSI

• 8 ft Dia × 10 ft H (Poly Cistern): 3,760 Gallons | 14,233 L | 15.68 Tons | 4.3 PSI

• 10 × 10 × 8 ft (Concrete Cistern): 5,984 Gallons | 22,653 L | 24.96 Tons | 3.5 PSI

• 12 ft Dia × 12 ft H (Homestead): 10,152 Gallons | 38,430 L | 42.34 Tons | 5.2 PSI

How to Use This Calculator

Enter your Water Tank Geometry & Orientation — choose from Vertical Cylinder (Standard Poly / Rainwater Cistern), Horizontal Cylinder (Underground / Skid Tank), Rectangular / Square Cistern (Concrete / Modular), Diameter / Width (e.g. 8 in ft), Height / Cylinder Length (e.g. 10 in ft), Fill Level / Operating Water Depth — choose from 100% (Maximum Full Capacity), 90% (High Water Overflow Line), 75% (Three-Quarters Full), and Storage Tank Benchmark — choose from Custom Dimensions, 8 ft Dia × 10 ft H Vertical Poly Tank → 3,760 US Gallons / 15.7 Tons / 4.3 PSI, 12 ft Dia × 12 ft H Large Homestead Cistern → 10,152 US Gallons / 42.3 Tons. Measure twice before entering dimensions — small measurement errors scale up significantly in material estimates.

The Formula

Hydrostatic Bottom Pressure: P = rho g h = 0.4335 PSI per foot of head

Worked Example

A homeowner installing an 8 ft diameter by 10 ft high poly rainwater cistern calculates that the tank holds 3,760 gallons (15.68 tons of water weight), engineering a 6-inch reinforced concrete slab on compacted gravel to prevent slab sinking and tank rupture.

Understanding Your Result

Construction estimates are starting points — site conditions, material specifications, and local code requirements will affect final quantities and costs. Always obtain at least three contractor bids for major projects. Local building departments can confirm permit requirements and inspection milestones. Add waste factors appropriate to the material type and complexity: 10% for simple rectangular shapes, 15–20% for complex geometries or custom cuts.