Everyone loves a good rule of thumb. Knowing that the number of cricket chirps in 15 seconds plus 37 is the approximate temperature _[1] is great . I usually do the old 100 plus my age comparison when I get my blood pressure checked as well. These ballpark estimates are easy to remember and give a close enough approximation for many things in life. Unfortunately, there are often way too many variables for a rule of thumb to be of any real value for personal finances.

There are some financial rules that are valuable, but they tend to be the ones that are math based. An example would be “Double your hourly rate and add three zeros to get approximate yearly salary.” Of course, even such a rule makes the assumption that you work 40 hours per week. These “rules of thumb” are what I generally file under Quick Calculations. They are more mathematical approximations

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