Governing Formulas
Tank capacity follows from the tank geometry; storage sizing derives the required volume from demand and the intended residence time; the refill duty is set by the fill (pumping) rate.
V = (π/4) × D² × H | V = L × W × H Where:
-
D= Cylindrical tank diameter [m] -
L, W= Rectangular tank length and width [m] -
H= Water depth (tank height or working depth) [m] -
Q_d= Daily water demand [m³/day] -
t_res= Storage residence duration [days]
Derived Equations:
V = (π/4) × D² × H V = L × W × H V_req = Q_d × t_res t_fill = Q_d / Q_fill How the Calculation Works
The capacity engine computes the usable volume for either a cylindrical or rectangular tank from the entered working dimensions. Capacity is reported in m³, liters, and US gallons (1 m³ = 1000 L = 264.17 US gal).
The storage module then compares capacity against the required storage (daily demand × residence time) and reports the daily pump running hours needed to refill the tank at the specified fill rate.
Worked Engineering Example
Design Scenario: Community Water Tank
A community needs 40 m³/day with 2 days of storage, refilled by a pump rated at 20 m³/h.
- Required storage:
V_req = 40 × 2 = 80 m³ - Daily pump running time:
t = 40 / 20 = 2.0 h/day - Capacity check for a Ø1 × 2 m cylindrical tank:
V = (π/4) × 1² × 2 = 1.57 m³ → 1570.8 L
Clearly insufficient for this duty — a single Ø1 × 2 m tank cannot meet the 80 m³ requirement; use the sizing check to iterate tank dimensions upward.
Engineering Notes & Design Benchmarks
| Application | Typical Storage Time | Notes |
|---|---|---|
| Domestic pressure / rooftop | 0.5 – 1 day | Peak-hour buffering, gravity feed |
| Municipal ground storage | 1 – 3 days | Peak demand + fire reserve |
| Treated water for industry | 0.5 – 2 days | Process continuity, filter backwash |
| Fire reserve only | Mass flow basis | Per code duration + required flow |
Practical sizing note: for potable water, over-large residence times risk stagnation and disinfectant decay — keep normal working residence modest and circulate the tank.
Assumptions & Limitations
- Capacity uses the working depth entered; freeboard and structural fill above the water line are not counted.
- No allowance for dead volume below the outlet, sludge, or overflow levels.
- Residence-time sizing is a heuristic; fire-flow, peak-hour, and treatment constraints may dominate the required volume.
Frequently Asked Questions
How many liters and gallons are in 1 m³?
1 m³ = 1000 liters = approximately 264.17 US gallons (US liquid). A cylindrical tank of 1 m diameter and 2 m water depth therefore holds 1.57 m³ ≈ 1570 L ≈ 415 US gal.
What residence time should I design for?
Municipal ground storage is commonly sized for 1–3 days of average daily demand (plus fire reserve), while domestic tanks use under a day. Longer residence must be balanced against stagnation in potable systems.
Does the sizing check tell me if my tank is big enough?
Yes — the calculator marks the result Adequate or Insufficient by comparing tank capacity with the required storage (demand × residence time). It assumes the full tank volume is usable, which is optimistic when depress sludges or outlet dead zones exist.
Engineering Disclaimer
Engineering Note: This calculator provides storage estimates for preliminary planning and education. Final tank sizing must meet code fire-flow, peak-hour, disinfection-residual, and structural requirements, and be verified by qualified engineering review.
Technical References
- AWWA D100: Welded Carbon Steel Tanks for Water Storage, American Water Works Association.
- AWWA D115: Tendon-Prestressed Concrete Water Tanks.
- EPA Drinking Water Infrastructure (DWSD) design guidance on storage residence time and disinfectant residual maintenance.