A Roth 401k is a type of retirement account that allows you to withdraw money tax-free upon retirement. Unlike a traditional 401k, the money that is invested in the account is taxed in the year that it is earned. Using a calculator can help you determine how to best plan for your retirement using one of these accounts.How Much Should You Invest Each Year?The amount of money that you will have to retire on depends on many different factors. One of these factors is how much money you put into it. Generally, you will be able to put in $5,000 a year. For those over 55, the amount is generally $6,000 a year. This is the amount that you should use when calculating how much you will have after interest when you retire.Is It Better To Use A Traditional Account Instead?You should never underestimate the impact of taxes on your retirement account. A good calculator will show you how much you will save by paying taxes on the money now as opposed to paying taxes on the money when you eventually decide to retire.When Should I Start Investing?Many people aren't sure when they should start investing into a 401k. To get yourself motivated, use a calculator to show you how much you would have if you start saving now as opposed to five years from now. The difference will certainly get you to open an account right away.Your retirement isn't something that you can just start planning for a couple of years before you actually retire. Instead, you must start thinking about your retirement as soon as you start working. Otherwise, you won't have anything to fall back when you stop working.