Cistern calculation information
The capacity values obtained from the calculator are advisory only.
The calculator advises a cisterns Actual Useable Capacity. (The toal useable volume of water available in the cistern) and dependant on its height, the cistern's estimated nominal capacity based on application.
A cistern's nominal capacity is its total internal volume calculated by multiplying its length x width x height.
For shallow cisterns, 1m or so deep, a significant proportion of nominal capacity is lost resultant on accommodating the height requirement of float valve, overflow fittings and position of outlet connection. A minimum loss of 25% in nominal capacity is to be expected. THis could increase, possibly to 60%, if large float valves and corresponding outlets are fitted as for break tank or booster pump set duties.
The greater the height of a cistern, for any given inlet size, the greater is its proportion of actual useable capacity available. Cisterns 3 to 4m in height will suffer only a modest 10-15% loss of nominal capacity and possibly only up to 30% when break tank and booster pump set duties are required.
Having determined a cisterns nominal size, whether it be shallow (1-2m) or deep (3-4m), the most appropriate cistern length, width and height can be determined based on space availability at the actual site location.
The break or booster tank sizing advised is a minimum buffer capacity only. No strategic storage is allowed for and its function is only in providing an anti-siphon pressure break or as a feed tank to a booster pump. Pump feed connections must not be rated greater than 1.5 m/s to minimise noise within buildings and inlet pipes must be sized to ensure inlet flow rates are always less than pump delivery capacities under all conditions.