I had the same question, and I found this answer:
Post# of 148187
“The borrow rate is a floating rate that can change day to day. In addition, the total daily fee will also fluctuate based on the value of the security. No fee is charged for a day trade. The fee formula is:
(Borrow rate) x (security's closing price) x (# of shares) / 365
That means that you pay for holding on the weekend and holidays as well.”
https://money.stackexchange.com/questions/124...f-the-loan