When you retire, the goal is to match your spending to the income from savings and other resources. So first figure out how much cash you can expect each month.
Social Security If you haven't already begun receiving benefits, you can estimate the size of the monthly Social Security check you'll get at ssa.gov (see link below).
Pension If you've worked for an employer that still offers a traditional check-a-month pension plan, your HR department can tell you how large a payment you're eligible for and when you can start drawing it.
Other resources You'll likely also have to turn to the savings you've socked away in 401(k)s, IRAs and other accounts. That can require a delicate balancing act. You want to draw enough from your investments to live well. But you don't want to pull out so much that you deplete your savings and jeopardize your security later on.
Most people, especially men, overestimate what a safe withdrawal rate is, giving answers of 10% or more in surveys. That's way too high. If you want to be reasonably sure your money will last at least 30 years, you should withdraw no more than 4% to 5% of the value of your investments the first year of retirement. You then increase this amount annually for inflation to keep your purchasing power in line with rising prices (see example in table above).
That withdrawal rate may seem stingy, but remember: A 65-year-old man has about a 50% chance of living to 85 and an 11% shot at making it to 95. The odds are even higher for women.
That said, you needn't be a slave to the 4% rule. You could take that extra vacation or treat yourself to other splurges in years when the market is on a roll and then pare back your spending in bad times. You've got to follow through on this in the down years, though. If you don't, the combination of investment losses and withdrawals could put such a big dent in your portfolio that it won't recover, and you could run out of money before you run out of time.