An adjustable mortgage can start with a smaller payment. The important question is what you would pay after that starting rate ends—and whether your budget could handle it.
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Fixed means the loan rate stays put
A fixed-rate loan keeps the same interest rate. With a normal loan that is gradually paid off, the required principal-and-interest payment stays the same until the final payment. Your full housing bill can still change when taxes or insurance change. CFPB: fixed and adjustable loan basics
Adjustable means the rate can change later
Think of an adjustable-rate mortgage, or ARM, as having two stages. First, the starting rate lasts for a set period. After that, the rate is checked at regular intervals and the loan payment may change. A lower starting rate gives you an early benefit, but does not tell you the whole cost.
The name gives clues. A new 5/1 ARM generally starts with five years at one rate and then changes yearly. A 5/6 ARM generally starts with five years at one rate and then changes every six months. Ask the lender to confirm the actual schedule in months; the second number does not always use the same units. CFPB: six-month ARM adjustment products
Where does the new rate come from?
Two numbers work together. The index is the part that moves with the market. The margin is the extra percentage points specified by the lender. For example, an assumed index of 3.5% plus a margin of 2.5 points gives 6%, before the loan’s limits are applied. Use the index named in your agreement—not a Fed headline or an average mortgage rate. CFPB: index and margin
Our steady-index scenario keeps the index unchanged after the first reset. It does not promise that the starting loan rate will stay unchanged: the index plus margin could already be different.
The limits matter as much as the starting rate
A rate cap works like a fence. One fence limits the first change, another limits each later change, and another sets the maximum increase over the starting rate. A floor sets the minimum allowed rate. The first two fences can delay a large increase without preventing it over several changes. Ask the lender for the highest payment the contract permits, even if you expect to move first. CFPB: ARM rate caps
A smaller bill today can become a bigger bill later
Here is a fictional example, not a lender quote. Both loans borrow $300,000 for 30 years. The fixed rate is 6.5%. The ARM starts at 5.5% for five years, then changes yearly. Its first and later change limits are two points, its maximum increase is five points, and its floor is 2.5%. Each loan has $3,000 in cash-paid fees.
- Fixed loan: about $1,896.20 a month for principal and interest.
- ARM at the start: about $1,703.37—roughly $192.84 less each month.
- Maximum-rate stress test: about $2,049.83 at the first change in month 61, eventually reaching about $2,600.23 in month 85.
Under that maximum path, ten-year interest and fees are about $212,820.36 for the ARM versus $184,872.87 for the fixed loan. The ARM starts cheaper each month, yet costs about $27,947.49 more over those ten years. Other rate paths give different results. Taxes, insurance, mortgage insurance and HOA are excluded.
Compare the same date, not just the first payment
Choose the number of months you expect to keep the loan. Our simulator shows the payments made, fees, interest and debt still owed at that same date for both choices. Remaining debt matters: making a smaller payment does not necessarily mean you have paid down as much of the loan.
It also shows four possible ARM paths: a steady index, a falling index, a rising index and the maximum rate path. These are tests, not predictions. The maximum path asks, “Could my budget handle this if it happened?” Full-term interest is shown separately for people who keep the loan to the end.
Do not make refinancing your only backup plan
A plan to refinance before the first change is not a guarantee. Future rates, available offers and your ability to qualify may change. Test what happens if you must keep the original loan longer than planned. Leave room for housing costs beyond principal and interest.
This simulator covers new, fully repaying loans with cash-paid fees and simple rate limits. Some agreements have different rules, including rounding, lookback dates or unused rate-change carryover. Interest-only loans, payment caps and loans whose balance can grow need a different model. Compare the agreement and lender disclosures with the calculator assumptions.
Try your own fixed and adjustable loan numbers → · Compare fixed-rate offers, points and financed fees →
Original MyLoanAlert educational guide. Checked October 5, 2026 against the linked CFPB resources. Hypothetical example calculated with our monthly-payment simulator. Not a rate offer, approval decision or personalized loan recommendation.