Cost of Savings Index is an index used for certain adjustable-rate mortgages (ARMs). The index is a weighted average of the rates of interest on the deposit accounts (sometimes called cost of savings) of the federally insured depository institution.

The COSI index is a stable index. Historically, it is not as unstable as some other mortgage indexes, such as the one-month LIBOR index. It also tends to delay other mortgage indexes when it comes to adjusting to changes in interest rates.