Adjustable-rate mortgages reset to a fully indexed rate subject to caps. Caps are not “customer service limits”—they are contract math that bounds how fast your note rate can jump at each anniversary and over the life of the loan.

Fully indexed rate = index + margin

At each reset, the note rate is typically rounded from index value + your fixed margin, subject to caps and floors. Common indices include SOFR-based ARM indices (replacing older LIBOR-based disclosures on legacy paper). Your margin is locked; the index moves.

What “2/2/5” caps constrain

Caps are usually quoted as initial / periodic / lifetime (percentage points). A common 5/1 structure might read 2/2/5:

  • Initial cap (2%): max change at the first adjustment after the fixed introductory period.
  • Periodic cap (2%): max change at each subsequent adjustment.
  • Lifetime cap (5%): max above the start rate for the life of the loan (verify your note—some disclosures phrase relative to start rate vs contract floor).

Build a worst-case rate staircase

Example only (not your loan): start rate 5.75%, first adjustment cap +2.00% → first reset ceiling 7.75%. Second periodic cap +2.00% would imply 9.75%, third step 11.75%—but a +5.00% lifetime cap from the start rate would top out at 10.75% and bind before the third full periodic step. Always read which base the lifetime cap references.

Action: plug the stepped rates into the mortgage calculator or loan calculator with your remaining balance/term to see P&I at each stair—not just the teaser payment.

Why DTI under stress matters more than teaser APR

Lenders qualify you on documented income and debts (DTI guide). If a +2% reset pushes housing payment into a band where savings stops and revolving debt begins, the ARM’s “average case” return does not matter. Model first reset and second reset payments explicitly.

Where to read this in your loan paperwork

Use your ARM disclosure, Note, and Adjustable Interest Rate (AIR) table. For consumer rules and disclosures, refer to the CFPB and your closing package—not blog summaries—as the source of truth.

Choosing between fixed and floating starts with product fit: read fixed vs variable rate trade-offs before optimizing cap arithmetic alone.