Most lenders use the weekly value on their ARMs.
These indexes reflect the state of the economy, and respond quickly to economic changes. Most one-year ARMs are tied to the Constant Maturity of the One Year Treasury Security. Your new interest rate will be determined by the T-Sec value 30 to 45 days before your next rate change. Your mortgage disclosure specifies the exact index, whether the weekly or monthly value applies, the lead time for your index, the margin, and any caps. See our ARM Check Kit for complete instructions and worksheets.