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Water Tank Calculator

Calculate water storage tank capacity, current volume, remaining space, water weight, and filling/draining times. Supports vertical/horizontal cylinders, rectangular, cube, and capsule tanks.

CalculatorHow to CalculateFormulas & ExamplesFAQs

Select a tank shape, enter dimensions, and click Calculate.

Tank Shape

Tank Dimensions

ft
ft

Current Water Level (Optional)

Enter either the current water height or the fill percentage. If both are left blank, the tank is assumed to be 100% full.

ft
%

Flow Rates & Time Estimates (Optional)

Enter flow rates to estimate how long it will take to fill the remaining capacity or drain the current volume.

Dimension Guide
Diameter (D)Height (H)

Results Panel

Awaiting Calculations

Specify the tank shape, dimensions, optional water level and flow rates, then click Calculate to view estimates.

Formula Details

Tank Capacity Formula
Vertical Cylinder: V = π × R² × H Horizontal Cylinder: V = π × R² × L Rectangular: V = L × W × H Cube: V = S³ Capsule: V = π × R² × L_cyl + (4/3) × π × R³

Calculates the total geometric volume of the tank. For capsules, the volume is the sum of the cylindrical middle section and the two hemispherical ends (forming a sphere). The result is converted to Liters or Gallons.

Variables Explained
  • R
    Tank Radius:Half of the tank diameter (D / 2).
  • L_cyl
    Cylinder Length:The straight middle section of a capsule tank.
  • H
    Tank Height:The vertical height of the tank.
Water Weight Formula
Weight = Current Water Volume × Water Density

Estimates the weight of the water currently inside the tank. Fresh water density is assumed by default.

Variables Explained
  • Metric Density
    Fresh Water:1.0 kg per Liter of water.
  • Imperial Density
    Fresh Water:8.3454 lbs per US Gallon (or 62.428 lbs per Cubic Foot).
Fill & Empty Percentages
Fill % = (Current Water Volume / Total Capacity) × 100 Empty % = 100 - Fill %

Determines the filled ratio and the remaining empty space in the tank as a percentage.

Filling & Draining Times
Filling Time = Remaining Capacity / Inlet Flow Rate Draining Time = Current Water Volume / Outlet Flow Rate

Estimates the duration to fill the empty portion of the tank or drain the current volume based on flow rate inputs. Flow rates are normalized to matching volume units per minute.

Calculation Example

Residential Overhead Water Tank (Imperial)
Scenario

Estimate total capacity, current volume, remaining space, and water weight of a vertical cylindrical overhead tank measuring 6 ft in diameter and 6 ft high. The current water level is measured at 3.9 ft.

Calculation Steps
  1. 1.Calculate radius.
    R = 6 ft / 2 = 3 ft
  2. 2.Calculate total capacity.
    Volume = π × 3² × 6 = 169.646 ft³ Convert to Gallons: 169.646 × 7.48052 = 1,269.0 US Gallons
  3. 3.Calculate current water volume (height = 3.9 ft).
    Water Volume = π × 3² × 3.9 = 110.270 ft³ Convert to Gallons: 110.270 × 7.48052 = 824.9 US Gallons
  4. 4.Calculate fill level and remaining capacity.
    Fill % = (3.9 ft / 6 ft) × 100 = 65.0% Remaining Capacity = 1,269.0 - 824.9 = 444.1 US Gallons
  5. 5.Calculate water weight.
    Weight = 824.9 Gallons × 8.3454 lbs/gal = 6,884.0 lbs
Final Quantity NeededCapacity: 1,269 gal, Water Volume: 825 gal (65%), Remaining: 444 gal, Weight: 6,884 lbs
Underground Rectangular Cistern (Metric)
Scenario

An underground concrete rectangular cistern measures 3.5 m long, 2.5 m wide, and 2.0 m deep. It is currently 40% full. Estimate the volume in Liters, remaining capacity, water depth, and draining time using a pump with a discharge rate of 120 L/min.

Calculation Steps
  1. 1.Calculate total capacity.
    Volume = 3.5 m × 2.5 m × 2.0 m = 17.5 m³ Convert to Liters: 17.5 × 1,000 = 17,500 Liters
  2. 2.Calculate current volume at 40% full.
    Water Volume = 0.40 × 17,500 L = 7,000 Liters
  3. 3.Calculate remaining space and depth.
    Remaining = 17,500 - 7,000 = 10,500 Liters Water Depth = 0.40 × 2.0 m = 0.8 meters
  4. 4.Calculate draining time.
    Draining Time = 7,000 Liters / 120 L/min = 58.33 minutes 58.33 min = 58 min 20 sec
Final Quantity NeededCapacity: 17,500 L, Water Volume: 7,000 L (40%), Depth: 0.8 m, Draining Time: 58 min 20 sec
Agricultural Horizontal Capsule Tank (Metric)
Scenario

A vineyard uses a horizontal capsule tank (cylinder with hemispherical ends) with a diameter of 2.0 m and a middle cylinder length of 3.0 m. Estimate full capacity and the time required to fill it from empty at a solar pump rate of 50 L/min.

Calculation Steps
  1. 1.Calculate radius.
    R = 2.0 m / 2 = 1.0 m
  2. 2.Calculate volume of the cylindrical middle section.
    V_cyl = π × 1.0² × 3.0 = 9.4248 m³
  3. 3.Calculate volume of hemispherical ends (combined as one sphere).
    V_sphere = (4/3) × π × 1.0³ = 4.1888 m³
  4. 4.Calculate total capacity.
    Total Volume = 9.4248 + 4.1888 = 13.6136 m³ Convert to Liters: 13.6136 × 1,000 = 13,613.6 Liters
  5. 5.Calculate filling time from empty.
    Filling Time = 13,613.6 L / 50 L/min = 272.27 minutes 272.27 min = 4 hours 32 minutes
Final Quantity NeededCapacity: 13,614 L, Filling Time: 4 hours 32 minutes

How to Use

Follow these simple steps to calculate your water tank capacity, volume, weight, and flow times:

  1. Choose your unit system: Select either **Imperial** or **Metric** at the top of the calculator.
  2. Select the tank shape: Pick from Vertical Cylinder, Horizontal Cylinder, Rectangular, Cube, Vertical Capsule, Horizontal Capsule, or Custom Volume.
  3. Enter the required dimensions: Input values like diameter, height, length, or width in your preferred length units.
  4. Provide current level (optional): Enter either the current water depth or the fill percentage. If omitted, the tank is assumed to be 100% full.
  5. Configure flow rates (optional): Input inlet flow rate (for filling time) and outlet flow rate (for draining time) along with flow units.
  6. Click Calculate: View the results panel with total capacity, current volume, remaining capacity, weight, fill percentage, and time estimates together.

Frequently Asked Questions

How do I calculate water tank capacity?
Water tank capacity is calculated by finding the geometric volume of the tank's shape. For instance, a vertical cylinder's volume is calculated using π × Radius² × Height. The volume is then multiplied by standard conversion ratios to express it in Liters or Gallons.
How much does water weigh?
Under normal conditions, fresh water weighs exactly 1.0 kg per Liter (or 1,000 kg per cubic meter). In imperial units, fresh water weighs approximately 8.34 lbs per US Gallon or 62.43 lbs per cubic foot. Knowing the water weight is essential to verify structural supports or concrete pad strengths.
Can I estimate filling or draining time?
Yes. If you know the flow rate of the inlet pipe (filling rate) or outlet pipe (discharge rate), you can estimate times. Filling time is the remaining empty capacity divided by the inlet flow rate. Draining time is the current water volume divided by the outlet flow rate.
What happens if I enter the current water level instead of a percentage?
If you input the physical water depth (level), the calculator will use the tank's shape geometry to compute the corresponding water volume and fill percentage. If you input a fill percentage instead, the calculator runs a numerical bisection solver to calculate the corresponding water depth.
Which tank shape should I select?
Vertical cylinders are the most common shape for outdoor overhead or cistern systems. Horizontal cylinders are common in mobile tankers or shallow underground storage. Rectangular and cube tanks are ideal for maximizing space in basement corners or utility rooms. Capsule tanks are typically utilized as high-strength underground vessels.
How accurate are these calculations?
The calculations are mathematically precise. However, real-world tanks may have slightly lower capacities due to wall thickness, internal reinforcement ribs, dome caps, and fittings. Water weight calculations assume fresh water; salty or chemical-treated water will be slightly heavier.

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Disclaimer

Calculations are estimates based on the values you enter and standard engineering formulas. Results are intended for planning and estimation only. Always verify critical measurements, specifications, and local building requirements before purchasing materials or beginning construction.