I am doing a retirement savings on excel, at age 21 I'm going to start making deposits from a percent of my salary and those deposits build interest at a rate of 6%. i plan to quit at age 65 and start withdrawing money while still earning interest. What i want to know is a formula to calculate the exact age and years my money is gone so if i have 200000 and want to withdraw 300000 how long will that 200000 last. The formula had to account for interest changing after i compute it with the rate being constant.