Did you know the #1 method people use to estimate how much money they need for retirement is guessing? (According to a study done by the Transamerica Center for Retirement Studies.)
Simple Method to Calculate Recommended Retirement Savings by Age
If you are the typical household making around the median wage and plan to retire at the normal retirement age, then this simplified rule of thumb is a good place to start (certainly better than guessing!)
Fidelity's calculations take into account assumptions such as how much of your income will be replaced by social security, the age you retire, and estimated investment returns.
Of course there are a lot of assumptions involved, but as a rule of thumb, it is a great place to start. Most people do not drastically alter their lifestyle after retirement, so basing it on your income is a reasonable assumption. And this method gives a quick and easy method to calculate if you are on track or not.