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Student loan payments and opportunity costs
I agree with the advice given, but I'll add another angle from which to look at it. It sounds like you are already viewing the money used to either pay off the loan early or invest in the market as an investment, which is great. You are wise to think about opportunity cost, but like others pointed out, you are overlooking the risk factor. The way I would look at this is: I could take a guaranteed 6.4% return by paying off the loan or a possible 7% return by investing the money.  If the risk pays off modestly, all you've done is earned 0.6%, with a huge debt still hanging over you. Personally, I would take the guaranteed 6.4% return by paying off the debt, then invest in the stock market. Now this is looking at the investment as a single, atomic pool of money.  But you can split it up a bit.  Let's say the amount of extra disposable income you want to invest with is $1,000/mo.  Then you could pay an extra $500/mo to your student loan and invest the other $500 in  the stock market, or do a 400/600 split, or whatever suits your risk tolerance. You mentioned multiple loans and 6.4% is the highest loan.  What I would do, based on what I value personally, is put every extra penny into paying off the 6.4% loan because that is high.  Once that is done, if the next loan is 4% of less, then split my income between paying extra to it and investing in the market. Remember, with each loan you pay off, the monthly income that previously went to it is now available, and can be used for the next loan or the other goals.