I find retirement calculators usually have 2 major flaws:

1.) They make you calculate how long it will take you to retire based on INCOME instead of SPENDING
2.) They make no mention of how to invest that money or how a nest egg grows.

The simple answer to both of these questions is The Trinity Study. The Trinity Study introduced the 4% Rule of Thumb - in short a portfolio of 80% stocks and 20% bonds allows you to withdrawal 4% a year to live on and has an 85% chance of success (still having money in it) after 30 years.

Thus, to estimate how much you would need to retire, you need to know how much you spend - if you spend $40k/year, you need $1 million.

The beauty of this formula is that inflation IS INCLUDED! A split of 80/20 stocks and bonds will yield average returns of between 6-8%, allowing for 2-3% inflation and 4% living expenses taken out.

The Trinity Study and 4% Rule of Thumb are easy to understand and much more straightforward than most retirement calculators/formulas.

The most important thing is is to give people somewhere to start - having to fill out a list of math problems is the nail in the coffin for retirement planning - an already painful process for many people.

The 4% Rule isn't foolproof- there's room for failure and it takes monitoring and adjustments to completely make it work. But those adjustments come DOWN THE ROAD, not before you've even gotten started. And for most people, figuring out where to start is the hardest part.