In his recent book, Stop Acting Rich, Thomas Stanley (author of The Millionaire Next Door), describes his "wealth index," a measure that he says gives people a good indication of whether they are on the right track to financial independence.
The formula is: Invested assets should be greater than or equal to 10% times your age times your income.
So, according to the index, if you are 50 years old, you should have 5 times your income in invested assets.
Is this a valid formula?
In my experience, this formula is a reasonably good "first look" for people under the age of 50.
For example, a 40 year old making $100,000 per year with $400,000 invested is probably doing pretty well, assuming he accumulated that amount via a frugal lifestyle, and continues to save and invest.
Like most rules of thumb, though, it is too simplistic and doesn't capture the myriad variables associated with financial planning.
Is he married? How many kids does he have and is his wealth going to be used to send them to college? Does he have any income sources during retirement? How long does he plan to work?
Additionally, the formula tends to fall apart for most people over age 50 (the formula is linear but the mathematics of retirement investing are not).
Example: according to the formula, a 65 year old making $100k should have $650k invested. If he retires at that age, he can reasonably expect to safely draw 4%, or $26k from his portfolio in the first year.... way short of the $100k he's used to earning. Social Security, pension, and other income sources might make up the difference but as in the case of the younger person, it depends greatly on a number of variables that are specific to each person.
In Dr. Stanley's defense, the point of the WX indicator is to segregate large samples of people into different categories to further study the behavioral traits that characterize certain groups. It is not intended to replace an individualized financial plan.
Bottom line: The WX indicator is a fast, easy way to see if you are falling short of a prudent savings path. It should only be used as a negative indicator-- if you fall short, you are likely NOT on the right track, but if you exceed the threshold, it doesn't necessarily mean that you are on the road to financial independence.