A good 43.5 million Americans owe money in the form of student loans. And with the average student loan balance surpassing the $37,000 mark, it's easy to see why that debt is such a burden for so many people.

If you're looking at colleges or graduate programs, you may be wondering how much student loan debt you can reasonably afford to rack up. And the truth is that there's no preset formula to answer that question. But if you follow these two rules, you can potentially set yourself up to avoid financial struggles in the course of repaying your student loans.

A person at a computer.

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1. Don't borrow more than your starting salary out of college

There's a general rule floating around stating that your total student loan balance should not exceed your expected starting salary out of college. So if, based on your desired profession, you anticipate making $50,000 your first year after college, you wouldn't want your student loan balance to exceed $50,000.

This rule is a good one to follow in limited circumstances, but it hinges on you being sure of what you want to do once you graduate. If you have no idea what profession you want to pursue, your best bet may be to simply research the average starting salary across all college graduates and use that as your personal benchmark.

But if you know for a fact that you want to go into advertising, you'd be wise to look up starting salaries in that industry based on your preferred geographic location. From there, you can determine how much to borrow, keeping in mind that just because the average entry-level advertising role pays $62,000 doesn't mean you'll manage to earn that much.

2. Look at your lifetime earnings to determine how much debt to take on

Some professions don't pay a huge salary right away, but can pay a lot in time. So it's also important to look at the big picture when determining how much student debt to take on.

Let's say you're thinking of going to medical school. The average total cost of medical school is $218,792, according to the Education Data Initiative.

Clearly, that's a lot of money, and many medical school graduates do not make a lot their first few years on the job when they're doing their internships. However, in this sort of situation, you'll want to look more so at lifetime earnings than near-term earnings.

Let's say that based on the field you intend to pursue, the average post-residency salary is $250,000 a year. That makes it much easier to justify a student loan balance of almost $219,000.

Taking on too much student loan debt could wreak havoc on your finances. It could also impede a lot of the financial goals you set for yourself, whether it's buying a home or retiring comfortably. There's unfortunately no exact science to calculating how much of a loan balance is reasonable. But if you follow these two rules to the best of your ability, you may be more likely to end up borrowing a sum that isn't overly burdensome.