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The City of Portland, Oregon

Environmental Services

working for clean rivers

Phone: 503-823-7740

Fax: 503-823-6995

1120 SW 5th Ave, Suite 613, Portland, OR 97204

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Presumptive Approach Calculator, SWMM 2020

The Presumptive Approach Calculator (PAC) has been rewritten to reflect the changes in the 2020 SWMM. It is available in a BETA form “test harness." It is an executable file with associated program files. In this form, reviewers may test facility configuration and sizing changes. The BETA version does not include the web interface or reporting functions currently available with the online PAC. These additional components will be completed before the SWMM goes into effect in fall 2020.

The technical background for the PAC is located in Appendix A.

Accessing and downloading the PAC program files

  1. Go to ftp site:
  2. Download the .zip file,, and extract all the files.
  3. Open the file in the top-most directory to open the PAC: BES.SWMM.PAC.TestHarness - Shortcut. If that does not work, open the sub folder and look at the PDF instruction for which file to open.
  4. Review the PAC Test Harness Instructions to learn how to use the BETA test harness. The file is also included in the .zip file.
  5. Provide comments in this Word document
  6. If you have difficulty downloading the PAC please contact Adrienne Aiona at or (503) 823-2051 to request an email attachment.

Learn more about the PAC

Informational presentations on May 26 and 27 gave an overview of the changes to the PAC.