Assume the person starts working at 25 and earns the average salary for whatever year it is at the time (changing every year). Assume that they save 20% of every pay cheque and invest it in an account that earns 7% interest tax-free which gets reinvested. How many years does it take them to save 25 times their current annual salary? How much money have they saved?
Years ago I wrote a brief discussion of a book titled The Number. In it, I link to a spreadsheet that will do exactly as you request. It opens looking like this
I used a starting salary of 17338 as that's what was needed to get an age 62 salary of 60,000. The assumed raise was 3%. The 15% saved was (in my approach) 10% deposit and 5% company match. You can adjust that up to 20%, and adjust the return down to 7%. You can adjust the salary increase, maybe more at the beginning, less later on.
Keep in mind a few issues -
- You won't be saving in retirement for retirement, therefore, 20% of your budget just got eliminated.
- Ditto for 7.65% Social Security withholding.
- Ditto for mortgage (perhaps) and college savings.
- Social Security replaces some of your budget income. 40-50% for most people.
Personal Finance is just that, personal. Look at your own timing, budget, etc, along with forecast Social Security benefit, and see what "Number" you need to make up the difference.