How to calculate overflow without junction diameter in SWMM

[Buddy]

There is not any parameter that can reflect the diameter of Junction. How SWMM calculate volume of overflow without junction size?
I would greatly appreciate any reply.
Thank you!

[KillianTobin]

SWMM assumes every junction has the same default diamete. If you wish to specify the specific volumes available at different depths then you can use a storage unit in place of the junction.