How the calculator works
- Filtration velocity: 10 m/h for sand and carbon filters, 12 m/h for multimedia filters, which are typical design values.
- Area per filter = flow per filter ÷ velocity. The calculator picks the smallest standard FRP vessel with at least that area.
- Carbon filters also need contact time: at least 5 minutes of empty-bed contact for chlorine removal. If the area-based vessel is too small for that carbon volume, the calculator moves up a size.
- Media volume is about 60% of the vessel volume (media plus support gravel), leaving freeboard for backwash.
- Backwash flow = vessel area × 30 m/h for sand and multimedia, 20 m/h for carbon. Your feed pump or a separate backwash pump must deliver it.
Related: FRP vessel size chart, MMF, ACF and PSF systems, multiport valve positions.
FAQs
What is the filtration rate of a pressure sand filter?
Typically 8 to 12 m³ per m² per hour (m/h). 10 m/h is a common design value.
How do I size an activated carbon filter?
Check both area (about 10 m/h) and contact time (at least 5 minutes empty-bed contact for chlorine, longer for organics). Use whichever needs the larger vessel.
What backwash flow does a sand filter need?
About 25 to 30 m/h across the filter area. For a 24-inch vessel that is roughly 8 to 9 m³/hr.