Simple 401k Calculator
Your objective is to calculate the compound interest growth and future value of your monthly contributions. This can be achieved simply be entering your beginning balance, an aggregated estimate for all contributions (yours, employer match, catch-up), an estimate for return on investment, and the number of years until retirement, then click the "Compute" button.
Don't be deceived by more complex 401k calculators that include more variables. It might appear more accurate but that accuracy deceives.
For example, many calculators include salary growth but fail to subtract for inflation. Similarly, not all employer 401k contribution limits conform with federal guidelines (which change regularly) but many calculators provide no mechanism to adjust for these alternatives.
In short, it is wiser to keep this calculation very simple by assuming an average annual contribution in today's dollars that balances for future growth in earnings, your unique employer matching situation, catch-up contributions, highly compensated employee limitations, and other variables. Just aggregate these various factors into a simple annual contribution and think exclusively in terms of today's dollars. Otherwise, you complicate the process with variables you can never estimate accurately (future inflation, salary growth, changes in federal contribution limits, changes in employer policy).
While this simplified solution is not perfect, it is no worse than more complex alternatives as explained in this article here.