When you sell stock short, IBKR must locate and borrow shares to make delivery to the purchaser at settlement. Specific cost to borrow those shares depends upon the supply of shares available to borrow and the demand from other short sellers to borrow shares.
IBKR provides a number of tools to estimate the cost to borrow before entering your order. Note that borrow costs tend to fluctuate daily along with supply and demand and the actual cost can only be determined at the end of the day when the necessary borrows have been sourced. Outlined below are tools to determine the indicative borrow rate:
- The Shortable column within TWS displays the number of shares available and the projected borrow rate for each of the stocks you are monitoring. See: How do I see whether a stock is shortable on TWS?
- The SLB Rates tool within TWS. To open the tool, select the SLB Rates option from the Analytical Tools menu located on the TWS top menu bar. Then, enter the symbol for the stock of interest. This tool also provides a bar graph of historical rates for the prior 10 days.
The shortable shares data displayed in the Shortable column is in real-time and no subscription is required.
Find more information and examples on calculating the cost of borrowing stocks on the IBKR website.