The cost of funds index (COFI) is one of the indexes that are used to determine interest rate changes for certain adjustable-rate mortgages (ARM). It represents the weighted-average cost of savings, borrowings, and advances of the financial institutions such as banks and savings & loans, in the 11th District of the Federal Home Loan Bank.

The Federal Cost of Funds Index (COFI) is calculated as the sum of the monthly average interest rates for marketable Treasury bills and for marketable Treasury notes, divided by two, and rounded to three decimal places.