Whether you’re Retirement Calculator close to retirement, figuring out whether you’re on track to retire when — and how — you want isn’t easy. But if you’re smart about it, you can avoid some common traps and make better decisions based on a little back-of-the-napkin math.

The most important tool in your arsenal for retirement planning is a good calculator. It can help you model your financial situation and test different scenarios to help you figure out how much you’ll need in retirement, if you have enough saved and how you might make your savings last throughout the rest of your life.

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Most retirement calculators use the same basic inputs: your desired retirement age, your expected life expectancy, your annual rate of savings and investment growth, and your expected expenses in retirement. But not all calculators are created equal. Some may be misleading because they rely on unrealistic assumptions, such as reading goat entrails to predict the future or an arbitrary assumption like 4% as a safe annual withdrawal rate from your retirement account.

It’s also important to consider how other sources of income will affect your final retirement amount, including your Social Security benefits and any pensions you may have. And don’t forget to factor in the cost of inflation, which can dramatically decrease your spending power over time. You should also consider any other supplemental sources of income, such as rent, dividends and the sale of substantial assets.