Flow Rate and Turnover


Flow rate refers to the speed at the pool water is circulating around the system and is usually expressed as m3/hr, meaning cubic metres per hour. Sometimes the flow rate is given as gallons per minute (GPM) or litres per minute (LPM).

Without knowing the flow rate, you will not be able to calculate the turnover time of the pool, which refers to the amount of time it takes for the pool volume to go through the circulation system. Flow meters should be maintained and calibrated on a regular basis to ensure that the information they provide is accurate.

Flow Rate: The speed at which water is travelling through the circulation system. Usually expressed as cubic metres per hour (m3/hr).

Turnover: The time it takes for a volume of water equivalent to that of the pool contents to make one pass through the circulation system. Usually expressed as hours and/or minutes.

If you were operating a swimming pool that is 25m long, 10m wide, with a 0.9m shallow end and a 1.9m deep end, the pool volume would be worked out like this:

25m x 12m x 1.4m =  350m3

If the flow rate was indicated as being 125m3/hr, then the pool turnover would be calculated like this:

350m3 / 125m3/h = 2.8 hours          

As can be seen, it would be impossible to calculate the turnover time, without knowing the flow rate. Therefore, all pools should ideally be fitted with a flow rate meter.

If the turnover time of a pool is too long, pollution levels will start to build up. If the turnover time is too short, the water will be travelling too fast through the system and this will have a negative impact on the effectiveness of the filtration system (slower filtration is usually better than faster filtration).

Recommended Turnover Times

  • Diving Pools 4 - 8 Hours
  • Domestic Pools 4 - 8 Hours
  • 50m Olympic Pools 3 - 4 Hours
  • 25m General Use Pools 2.5 - 3 Hours
  • Leisure Pools Over 1.5m Deep 2 - 2.5 Hours
  • Leisure Pools 1 - 1.5m Deep 1 - 2 Hours
  • Leisure Pools 0.5 - 1m Deep 0.5 - 1.25 Hours
  • Leisure Pools Less Than 0.5m Deep 10 - 45 mins
  • Hydrotherapy Pools 0.5 - 1.5 Hours
  • Teaching Pools 0.5 - 1.5 Hours
  • Waterside Splash Pools 0.5 - 1 Hour
  • Interactive Water Features 20 mins
  • Domestic Spas 15 mins
  • Commercial Spas 6 mins
  • Leisure Water Bubble Pools 5 - 20 mins

The turnover of water in shallow areas may be designed to be higher than in other areas. The turnover period of pools with moveable floors should be appropriate to the pool at its shallowest point (ie potentially biggest bathing load).

Pools with restricted access and thus bathing load (eg school, hotel, health club) may operate with a longer turnover period than indicated here, but the turnover rates given above should not be reduced where public use is introduced (eg. dual­-use school pools).

Pool water should circulate and be treated 24 hours a day. If the pool has a moveable floor or bulkhead (boom), the circulation system should ensure proper water distribution in all possible positions. Good circulation hydraulics are necessary to ensure that the whole pool is adequately served in terms of both disinfection and removal of pollutants.

The circulation rate and turnover rate established in accordance with the guidelines here should form the basis, along with water velocity and inlet and outlet issues, for the specification of circulation hydraulics, including pumps and pipework.