Eaves Gutter & Downpipe Sizing Calculator

AS/NZS 3500.3:2018 & 2025 (Figures 3.5.4 / 3.5.5 nomographs, Table 3.5.2, Table 3.4.3.2)  |  NSB 151/152/153 & CSIRO Technical Paper No.1 (Experimental Building Station, 1978)

0  Project — appears on the printed summary

1  Roof & rainfall

Plan (horizontally projected) area of roof + gutters
Used for the catchment multiplier F
NSB 151 cl. 2.01: add ½ the exposed wall area (extreme exposure)
Australian place names and coordinates from GeoNames (CC BY 4.0). The BOM IFD grid is 0.025° (≈2.8 km), so the grid cell may differ slightly from the site.
Site coordinates — used to open the BOM IFD tool at this point.
Look the point up on the BOM page with the button below, type the 5-minute 5 % AEP intensity above, and note here where it came from.

2  Gutter system

NSB 152 Table 2 (position of outlet / distance of outlet from bend)

3  Overflow measures — optional, for the project record

Gutter length draining to one downpipe
Sets the minimum inflow per metre
Required for the 2025 overflow volume Q*

Results

Design catchment area
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Max roof catchment per downpipe
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Downpipes required
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Downpipe size
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Gutter required (if sizing the gutter to the catchment)
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Overflow measures
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Calculation report (full working) 

        

      


      

        
Subcatchment check

        

          

            
            
          

          

            
            
          

        

        

      

    

  


  
  

    
Effective cross-sectional areas — product library

    
Table 2 of the Lysaght NSW Rainwater Solutions guide, values calculated to AS/NZS 2179.1. Click a row to load it as the selected gutter.

    

  


  

    
Verification against the Standards' own worked examples

    

  


  

    
Method, formulas and sources