It also assumes that you need an annual retirement income equivalent to 55 or 80% of your pre-retirement income to live comfortably. Depending on your spending habits and medical expenses, spending may need to be increased or decreased. However, between 55 and 80% is a good estimate for many people. A common guideline is that you should try to replace 70% of your annual pre-retirement income.
This is what the calculator uses by default. You can replace your pre-retirement income with a combination of savings, investments, Social Security, and any other source of income (part-time work, pension, rental income, etc.) The Social Security Administration website has several calculators that help you estimate your benefits.