Three years to the day after I first posted that article, a reader who goes by the name of "dunkelblau" has indeed come up with a new and improved formula! Below is summary of dunkelblau's comments taken from this post, and from my very popular Net Worth Update post. (Note: Comments have been edited.)
I agree that wealth should be measured in units of time rather than dollars -- as in how long you can holdout without any wage income. But the correct denominator is annual expenses, not annual income. You're ready to retire when your networth divided by your annual expenses reaches 100 minus your age in years.
I don't blame you if you didn't catch that. What the reader is saying is that if you take your net worth and divide that by annual expenses, that should equal 100 minus your age. The reader refers to net worth as the "numerator" and annual expenses as the "denominator". Reading on, dunkelblau continues by saying:
Investing well increases the numerator, but even investing badly still shrinks the denominator (assuming you don't borrow to invest). I think a conservative target number [for this quotient] is 100 minus your age in years. This way there's little chance of outliving your money.
So, as further clarification, another way to say this is that your net worth should equal your annual expenses times (100 minus your age):
Net Worth = Annual Expenses * (100 - age)
Lets take the example of a 40 year old who has saved $1 million for retirement. If this person can reduce expenditures to $40,000 per year, a common rule of thumb is that one can live off of the $1 million saved by assuming a conservative 4% safe withdrawal rate. See Resources for Early Retirement.
However, the "new and improved" wealth formula dictates that one needs (100 - age) times annual expenses to retire. In the case of a 40 year old, that would be 60X annual expenses, or a whopping $2.4 million for this individual to retire! Conversely, this person could further reduce their spending to less than $17,000 per year in order to live off of the $1 million. No wonder people assume that early retirement is impossible! The other curious thing about this new "formula" is that as one ages, the expected net worth actually goes down if annual expenses are kept constant. The one thing that I agree with dunkelblau about is that if you follow this formula, there is little chance of outliving your savings.
Anyway, my common sense tells me to take dunkelblau's formula with a grain of salt. In my opinion, this is definitely a new formula, but where is the improvement? What do the readers think?