What it means
An adjustable-rate mortgage reprices periodically against a benchmark index. Without limits, a rate spike at one adjustment could turn a manageable payment into an impossible one overnight.
The periodic cap is the shock absorber. A typical cap of two percentage points means that no matter how far the index has jumped, your rate can rise at most two points at that adjustment.
Caps usually come in a three-number structure. The first number caps the initial adjustment, the second caps each subsequent periodic adjustment, and the third caps the total change over the loan's life; a 5/2/5 structure means up to five points at first reset, two per reset after, five total.
The Consumer Financial Protection Bureau's adjustable-rate mortgage booklet explains these structures to borrowers, including how caps interact with the index and margin that set the underlying rate. Periodic caps protect monthly budgets, not lifetime cost.
A rate that wants to rise four points but is capped at two does not forget the difference; it catches up at the next adjustment if the index stays high. Payment caps are a dangerous cousin to know.
Some exotic loans cap the payment rather than the rate, and when the payment cannot cover the interest due, the shortfall gets added to the loan balance, negative amortisation, and the debt grows while you pay. The value of a cap depends on the rate environment.
In a falling-rate era caps feel decorative; in a rising-rate era they are the difference between strain and default. For a non-finance borrower, the periodic cap is one of three numbers to memorise before signing an ARM: how high the first jump can go, how high each later jump can go, and how high the rate can ever go.
In practice
Real-world examples.
Example
A borrower's ARM adjusts from 4 percent to only 6 percent despite the index implying 8, because a 2 percent periodic cap staged the increase.
Example
Shopping two ARMs, a buyer chooses the one with 2/2/5 caps over 5/2/6, valuing the tighter first-adjustment protection over a slightly lower start rate.
Example
A loan with a payment cap rather than a rate cap builds negative amortisation as unpaid interest is added to the principal each month.
Formula
Calculation
New rate at adjustment equals the previous rate plus or minus the index movement, bounded by the periodic cap, and never exceeding the lifetime cap. With a 2 percent periodic cap, a 5 percent rate can move only to between 3 and 7 percent at that reset regardless of the index.
Worked example. A fictional borrower has a $300,000 balance at 4.5%, a 2-point periodic cap, a 5-point lifetime cap, and an index plus margin that would imply 9%. The lifetime ceiling is 4.5% + 5% = 9.5%. At the first reset the rate can rise only to 4.5% + 2% = 6.5%, and at the next to 8.5%, before reaching 9% at the third.
To see the budget effect, take annual interest on the balance as an illustration. At 4.5% it is $300,000 x 4.5% = $13,500, and at 6.5% it is $300,000 x 6.5% = $19,500. The cap staged an increase of $6,000 a year, or $500 a month, instead of the $13,500 extra that a jump straight to 9% would have caused.Case study
Seen in the real world.
This case study is fictional and illustrative. A made-up couple in Phoenix takes a 5/1 ARM at 3.5 percent with 2/2/5 caps. Five years later the index has soared and their fully indexed rate computes to 8 percent, but the initial cap limits the first reset to 5.5 percent. Their payment rises by about a fifth, painful but budgeted.
The following year the index stays high and the second periodic cap lets the rate reach 7.5 percent, still short of the fully indexed level, with the lifetime cap of 8.5 percent looming as the absolute ceiling. Their neighbour, who took an exotic loan with a payment cap instead, discovers her balance has grown by $14,000 through negative amortisation even while making every payment on time. The couple refinances into a fixed rate when the index finally falls, grateful the caps staged the shock instead of delivering it at once.
Watch out
Common mistakes.
- Confusing periodic caps with lifetime caps; the periodic cap stages each jump, but the rate can still climb toward the lifetime ceiling over several resets.
- Assuming the capped rate equals the full cost; deferred increases return at later adjustments if the index stays elevated.
- Missing whether the cap limits the rate or the payment; payment caps can produce negative amortisation, where the balance grows despite on-time payments.
Questions
People also ask.
What is a periodic cap on an ARM?
A limit on how much the interest rate can change at each adjustment date, commonly one or two percentage points per reset.
What do the three cap numbers mean?
They cap the initial adjustment, each subsequent periodic adjustment, and the total lifetime change, in that order, as in a 5/2/5 structure.
Does a cap stop rates rising?
Only per adjustment; if the index stays high, the rate keeps climbing toward the fully indexed level at each reset until the lifetime cap binds. Borrowers should model the worst permitted path before signing.
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