DFU/GPM CONVERSION

Plumbing codes are very clear on the relationship between water supply fixture units (fu) and flow in gallons per minute (gpm). On the other hand, they can be confusing to some in regards to the relationship between drainage fixture units (dfu) and gravity flow gpm.

Water supply fu's can be converted to gpm using Hunter's Curve, which takes simultaneous usage of multiple fixtures into consideration.

Drainage fixture units can also be converted to gpm by using Hunter's Curve. However, the flush tank curve should be used rather than the flush valve curve (even if the installation utilizes flush valve toilets). The flush tank curve is closer to the reality of the actual drainage flow that will be experienced.

This method can be verified by comparing the drainage pipe sizing tables within the plumbing code with the Cast Iron Soil Pipe Institute's flow data tables.

1. CISPI Flow Tables   (Adobe PDF)
   
   
2. DFU to GPM Conversion Spreadsheet   (MS Excel)
   
   

PLUMB QUIZ

How many pounds of thrust are produced at the base of a 4" waste stack by ten feet of hydrostatic head?