Robinhood makes money on it's "gold" service, which is $10/month. For that $10 they float you the cash to immediately reinvest cash after stock sales (rather than waiting 3 days) and they give you $2000 in margin. Really it's pretty genius. They figured out that transaction costs for exchanges are essentially free. With their gold service they are monetizing at least 6% profit margin, i.e. you can think of the $120 yearly as a cost of borrowing $2000, which would boil down to 6% interest rate. Since that money is secured by assets in your account, this is actually a really attractive rate for the risk profile, and, likely scenario, few people are constantly maxed out at $2000. So membership / margin lent is their total profit margin. I bet its well over 10%. Not bad.