Should I use the inside diameter or outside diameter for the "diameter" field for my pipes?

Product(s): WaterGEMS, HAMMER, WaterCAD, SewerGEMS, CivilStorm, StormCAD, PondPack, SewerCAD
Version(s): All
Area:  Layout and Data Input


Should I use the inside diameter or outside diameter for the "diameter" field for my pipes and conduits?


All of the OpenFlows products use the inside diameter (ID) for calculations. So, you should enter the inside diameter for each pipe's or conduit's "diameter" field.

Nominal Diameter for most common material types is close enough to Internal Diameter to be used directly for modelling. However, you should be careful for some materials, as the manufacturer's expressed "nominal diameter" may refer to the outside diameter, which could be substantially different from the inside diameter. Discrepancies in model calibration are usually related to errors in roughness coefficients, not diameters.

If needed, you can filter the pipe flex table based on material and globally change the nominal diameter (say 4 in.) to the internal diameter (say 3.97 in) for each pipe material.
In most cases the difference between nominal and internal diameter is negligible. Some pipes like HDPE may have a notable difference.

To help avoid confusion when other users are reviewing the model, consider the following:

  1. If annotating pipes, set them up in such a way to differentiate between inside and outside diameter. For example add Diameter to the annotation and use the suffix option to append something like "(ID)" or "(inside diameter)", so as to show like "600 mm (ID)"

  2. Add a User Data Extension field for Outside Diameter and use the ordering option to place it just below the regular "Diameter" field in the physical section of the properties. To populate it with the outside diameters, consider:
    1. Fixed wall thickness: use the formula-derived UDX option to add the fixed wall thickness value to the diameter value.
    2. Varying wall thickness:
      1. Use filtering (by diameter) along with global edits to set these in the pipe flextable. (filter on diameter, then global edit the outside diameter field, repeat.)
      2. Set up a connection to a spreadsheet in modelBuilder, sync out the inside diameter, set up a function in Excel to calculate the outside diameter based on the desired criteria, then Sync in the outside diameter column to the outside diameter UDX.