Tank Drawdown Calculator
Estimate how long a tank or cistern can supply a constant outflow, pump rate, or demand rate.
Drawdown time is based on usable storage volume and outflow rate.
Tank Drawdown Calculator
Estimate how long a tank or cistern can supply a constant demand or pump rate.
Actual drawdown depends on controls, pump cycling, minimum water level, and available storage.
What this calculator does
This calculator estimates drawdown time using usable tank volume and a constant outflow or demand rate.
Inputs explained
- Usable tank volume: Volume available between current water level and the minimum operating level.
- Outflow or demand rate: Constant withdrawal, pump rate, or use rate.
Limitations
Actual drawdown depends on controls, pump cycling, outlet conditions, minimum operating depth, inflow, and usable storage limits.
FAQ
Is usable volume the same as total tank volume?
No. Usable volume is the portion of storage that can actually be drawn down for the intended use.
Can this be used for cistern sizing?
Yes, it is useful for early planning, but final cistern sizing should include inflow, demand patterns, overflow, water quality, and controls.
Does this account for variable pump rate?
No. It assumes a constant outflow rate.
Related tools
Need help reviewing this result?
Use this calculator for planning-level checks, then request help if you need a second look at assumptions, units, stormwater criteria, pump sizing, storage volume, or report-ready calculations.