Ah, the key question. One rule of thumb is that you'll need 70% of your pre-retirement yearly salary to live comfortably. That might be enough if you've paid off your mortgage and are in excellent health when you kiss the office good-bye. But if you plan to build your dream house, trot around the globe, or get that Ph.D. in philosophy you've always wanted, you may need 100% of your annual income - or more.
It's important to make realistic estimates about what kind of expenses you will have in retirement. Be honest about how you want to live in retirement and how much it will cost. These estimates are important when it comes time to figure out how much you need to save in order to comfortably afford your retirement.
One way to begin estimating your retirement costs is to take a close look at your current expenses in various categories, and then estimate how they will change. For example, your mortgage might be paid off by then - and you won't have commuting costs. Then again, your health care costs are likely to rise. For more help making a precise estimate, use this calculator.