This calculator was developed in accordance with the equations and procedures found in the Handbook for PVC Pipe Design and Construction (5th Edition). The user is encouraged to reference the document if more information on external load calculations is needed. Conservative assumptions have been used to provide a conservative design with appropriate safety factors. Designers are responsible for the proper use of this program for all projects.

The formula used to calculate deflection is called the Modified Iowa Equation and is as follows:

Where:

- ∆Y/D = Percent Deflection, %
- K
_{X}= Bedding Constant - T
_{L}= Time Lag Factor - W
_{E}= Prism Earth Load, psi - W
_{L }= Live Load on the Pipe, psi - PS = Pipe Stiffness, psi
- E’ = Modulus of Soil Reaction, psi