An adjustable-rate mortgage, or ARM, has an interest rate that can change after an initial period. The rate is commonly based on a published index plus a lender-set margin, subject to floors and caps in the note.
Decode the shorthand
An ARM described with two numbers typically uses the first for the initial fixed period and the second for how often adjustments occur afterward. Confirm the exact definition in the disclosure; labels are not a substitute for the note.
Three caps matter
- Initial adjustment cap: the maximum change at the first reset.
- Periodic cap: the maximum change at a later reset.
- Lifetime cap: the maximum rate over the loan’s life.
A payment cap is different from an interest-rate cap. If a payment limit prevents the payment from covering all interest due, the unpaid amount may affect the balance under the loan’s terms. Ask explicitly about negative amortization and minimum-payment options.
Stress-test the payment
Calculate the payment at the initial rate, after the first permitted increase, and at the lifetime maximum. Add taxes, insurance, mortgage insurance, association dues, and maintenance. The maximum is not a forecast, but it shows the contractual exposure.
Compare with fixed financing
Use the same loan amount, term, points, credits, and lock assumptions. Consider how long you expect to own the property, but also model the possibility that you stay longer. A lower starting payment is valuable only when the remaining risk is affordable.
Keep every adjustment notice and check the new index, margin, rate, payment, and effective date against the note. An ARM is understandable when its formula is visible and survivable when its downside fits the budget.