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Water Tank Volume Calculator

March 10, 2023 ·

Water Tank Volume Calculator

Find the full and usable capacity of a rectangular, cylindrical or spherical tank, check the current stored volume at any water level, and estimate storage days and pump fill time.

Tank Shape & Dimensions

Tank Shape:

Water Level & Freeboard

Freeboard is the empty space left below the top (or below the shell, for cylindrical/spherical tanks) to prevent overflow; it reduces the usable capacity below the full brim capacity.

Daily Demand & Fill Rate (Optional)

Leave LPCD empty to use 135 litres/person/day (CPHEEO norm for a house with piped water supply and sewerage).

FULL CAPACITY (AT BRIM)
--litres
USABLE CAPACITY
--litres
CURRENT VOLUME
--litres
PERCENTAGE FULL
--%
Of usable capacity
DAILY WATER REQUIREMENT
--L/day
DAYS OF STORAGE AVAILABLE
--days
VOLUME TO FILL
--litres
Current level to usable capacity
ESTIMATED FILL TIME
--
Note:
  • Freeboard: A typical allowance is 100–150 mm for open/domestic tanks and more for tanks with mechanical float valves or turbulent inflow.
  • Per Capita Demand (CPHEEO / NBC guidance): commonly used design figures are 135 LPCD for housing with full plumbing and sewerage, 70–100 LPCD without a sewerage connection, and higher allowances for institutional or commercial occupancies; always confirm the figure required by your local authority.
  • Horizontal cylindrical and spherical tanks do not fill or drain at a constant rate for a given inflow, since the water surface area changes with level; the volumes shown here account for that, but fill time is still an average based on the total volume needed.

Reference Guide: Tank Volume Formulas

1. Rectangular Tank

Volume = Length × Width × Water Level

2. Cylindrical Tank – Vertical (Standing)

Volume = π × (Diameter / 2)² × Water Level

3. Cylindrical Tank – Horizontal (Lying on its Side)

A horizontal cylindrical tank does not fill linearly with level, since the circular cross-section is narrower near the top and bottom. The wetted cross-sectional area at a water level h (measured from the bottom, radius r) is a circular segment:

Segment Area = r² × arccos((r − h) / r) − (r − h) × √(2rh − h²)
Volume = Segment Area × Tank Length

4. Spherical Tank

The volume below a water level h (measured from the bottom, radius r) is a spherical cap:

Full Volume = (4/3) × π × r³
Volume at Level h = (π × h² / 3) × (3r − h)

5. Storage Days and Fill Time

Daily Requirement (litres) = Occupants × Per Capita Demand (LPCD)
Days of Storage = Current Volume (litres) / Daily Requirement
Fill Time = Volume to Fill (litres) / Inflow Rate
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K K Kranti
Shares practical MEP calculators, checklists and design guides on MEP Analysis.

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