There's no single number, but a few well-known rules make it easy to estimate a realistic target.
The 25× rule
Multiply the annual income you want in retirement by 25. Want $50,000 a year? You'd aim for about $1.25 million. This comes from the 4% rule below.
The 4% rule
Research suggests you can withdraw about 4% of your savings in the first year of retirement, then adjust for inflation, and have a strong chance the money lasts 30 years. 4% of your nest egg should cover your yearly spending.
Don't forget other income
Social Security, pensions or part-time work reduce how much your own savings must provide. Subtract those from your target yearly income before applying the 25× rule.
Estimate your number
Enter a desired yearly income in the calculator and it will show the nest egg you'd need using the 25× guideline, next to your projected balance.