Part of understanding how much you owe in student loans is understanding how repayment of your loan works. As such, it may be helpful to use a loan repayment calculator to simulate what your payments might look like once you begin to repay your loans.
The government offers a Loan Simulator. You can log on to the website using your Federal Student Aid ID (FSA ID), which is the same ID that you use to complete your FAFSA each year. Once logged in, the Loan Simulator will pull your federal loan balances from Federal Student Aid website. The Simulator will allow you to see what your monthly payments would look like under a variety of different payment plans, from the standard plan to income-based repayment plans.
As a disclaimer, this repayment calculator does not take into account any private, alternative student loans that you may have borrowed to help finance your education. It is important to factor those into your budget as well.
Other, more simplistic, loan calculators exist. For example, you can find a sample repayment calculator on the FinAid website. Please note that the information you receive from this calculator is based on the estimated information you enter, and should only be used for approximations.