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.
http://www.squawkfox.com/tools/portfolio_mer_calculator/
Best book to read before you start investing.
The Simple Path to Wealth: Your roadmap to financial independence and a rich, free life
I like audiobook versions of this book