Posted by **charlie** on Thursday, June 7, 2012 at 9:32am.

How can I calculate how long an amount of money will last if I spend x dollars per month, at the same time as the pot of money is attracting interest at y percent? Is thare a formula?

Thanks

- math -
**MathMate**, Thursday, June 7, 2012 at 10:24am
This is the same as the formula for borrowing money and repaying a monthly amount.

Let

P=principal

n=number of months it will last

y=interest rate, compounded monthly at y/12, (use 0.10 for 10% per annum)

x=monthly spending

then

p(1+y/12)^n=x((1+y/12)^n-1)/(y/12)

Unfortunately, to solve for n, you need to solve it by trial and error or Newton's method.

There exist compound interest calculators, such as:

http://www.thecalculatorsite.com/finance/calculators/compoundinterestcalculator.php#results

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