You may have multiple mutual funds with various ER in your portfolio.
For example, you may have VTSAX at 0.05 ER that holds 68% and VGTSX at 0.18 ER that holds 32% of the portfolio.
Calculating total ER for your portfolio is made easy with this online tool.
Here is the link to the online tool.
Best book to read before you start investing.
I like audiobook versions of this book