<prompt> Human: The other day I discussed this with my boyfriend, who couldnt believe that it could be this simple. So we both checked it, and the rule of 70 is a good appromixation. So the general rule is: if you want to know how long it will take for your money to double, with a given annual growth rate and compounding, then the number of years it will take is 70 divided by the interest rate so for a 7 interest rate, its 70 divided by 7: itll take ten years. Some examples: so if you put 1000 in a savings account at 1 interest rate, the rule of 70 says it will take 701: 70 years for the money to double. According to a compound interest calculator online, putting 1000 in and waiting for 70 years with 1 interest rate, gives you 2006 after those 70 years. So it doubled, plus a few bucks. For 2 interest rates, the money should double after 35 years, and this one is spot on. After 35 years the calculator tells me Ill have 2000 from 1000 original input.For 7 growth, after ten years youve got 1967. So its a little bit less than 2000. For 10 growth, after seven years its 1950. So again, just a little bit less than the expected 2000, but very close. So it works as a rule of thumb. How often will you need it? Maybe not so often, but maybe now and then...Assistant: TIL, rule of 70 vs rule of 72: Human: Thanks for the link.Err, they may be trying to explain what the difference is between the two rules, but I dont get it. Other than that the rule of 72 is of course a bit more pessimistic than the rule of 70...  <response>: 70 for compounding return and 72 for simple interest. Rule of 72 has the added benefit of being divisible by more integers: 2, 3, 4, 6, 8, 9, etc. Since its estimate anyway, its not a big deal. But rule of 72 is more popular from what Ive come across. 