Rule of 70

http://en.wikipedia.org/wiki/Rule_of_70

The "Rule of 70" and the "Rule of 72" are approximation tools for determing exponential quantitative change, whether increase or decay. Dividing the rate of change into the constant 70 or 72, as appropriate, will yield a more-or-less reasonably close-enough approximation of the number of years required for the rate of change to either double or halve the initial quantitative value.

The Rule of 70 is most accurate with rate-of-change values of 3% or less, and most typically is used in applications involving biology, while the Rule of 72 is more accurate with rate-of-change values in the 6% to 8% range, and most typically is used in financial calculations.

A distantly related item is the "Rule of 78", also known as "Sum of the Digits", often used to calculate the unaccrued interest, or loan-payoff rebate amount, for loans which are paid-in-full prior to the contract expiration date. It rarely if ever works to the early-paying borrower's advantage, most often quite to the contrary, and is much loved by financial institutions.

The arithmetic functions made easily available by today's computer software and financial and scientific calculators render all of the above obsolescent, if not actually obsolete.