Fixed vs Adjustable Rate Mortgage
Understand the real difference between fixed-rate and ARM loans, when each makes financial sense, and how to calculate which saves you more money.
The Core Difference
Every mortgage has an interest rate. The question is whether that rate stays the same for the life of the loan or whether it can change over time. That single distinction — fixed or adjustable — shapes your payment stability, your risk exposure, and your total cost of borrowing.
Fixed-rate mortgage: The rate you lock in on closing day is the rate you pay on payment 1 and payment 360. Your principal and interest payment never changes. Taxes and insurance can vary, but the core mortgage payment is perfectly predictable.
Adjustable-rate mortgage (ARM): The rate is fixed for an initial period — typically 3, 5, 7, or 10 years — and then adjusts periodically, usually once per year, for the remainder of the loan term. The starting rate is generally lower than comparable fixed rates. But after the fixed period, the rate moves with the market.
How ARMs Are Structured
ARMs are described with a shorthand like "5/1" or "7/6". The numbers tell you:
- First number: How many years the initial rate is fixed (5 years, 7 years, etc.)
- Second number: How often the rate adjusts after the fixed period (1 = annually, 6 = every 6 months)
So a 5/1 ARM is fixed for 5 years, then adjusts once per year. A 7/6 ARM is fixed for 7 years, then adjusts every 6 months.
The Index and Margin
When an ARM adjusts, the new rate is calculated as:
Rate Caps — Your Protection Against Payment Shock
ARM rate caps limit how much the rate can change at any given adjustment. They are expressed in a three-number format, such as 2/1/5:
- Initial adjustment cap (2): The rate cannot rise more than 2% at the first adjustment after the fixed period ends.
- Periodic adjustment cap (1): At each subsequent adjustment, the rate cannot change by more than 1%.
- Lifetime cap (5): The rate can never be more than 5% above the initial rate over the entire loan term.
Worst-case scenario with 2/1/5 caps — initial rate 6.50%
| Initial rate (years 1–5) | 6.50% |
| Year 6 rate (max first adjustment) | 8.50% (+2.00%) |
| Year 7 rate (max periodic cap) | 9.50% (+1.00%) |
| Year 8+ rate (lifetime cap reached) | 11.50% (6.50% + 5.00% max) |
Fixed Rate — Pros and Cons
Advantages
- Payment certainty: Your P&I payment never changes, making long-term budgeting straightforward.
- Protection against rate increases: If market rates rise sharply, you are insulated — your rate was locked in years ago.
- Simplicity: No need to track index rates or recalculate your budget when rates adjust.
- Better for long-term holders: If you plan to stay in the home for 10+ years, predictability is worth the premium.
Disadvantages
- Higher starting rate: Fixed rates are typically 0.5–1.5% higher than comparable ARM initial rates, meaning a higher payment from day one.
- You pay for certainty you may not need: If you sell in 5 years, you paid for 30 years of rate protection you never used.
- Less benefit when rates fall: If market rates drop significantly, you must refinance to capture savings (which costs money and time).
Adjustable Rate — Pros and Cons
Advantages
- Lower initial rate: ARM starter rates are typically meaningfully below fixed rates, reducing your early payments.
- Ideal for short-term holders: If you sell before the rate adjusts, you captured savings with zero downside.
- You benefit if rates fall: When the market rate drops, your rate (and payment) drop at the next adjustment.
- May allow you to qualify for more: Lower initial rate means lower payment, which can improve your debt-to-income ratio.
Disadvantages
- Payment uncertainty: Your payment can increase significantly after the fixed period — even with caps, a 5% lifetime cap is a large potential increase.
- Complexity: Understanding index rates, margins, caps, and adjustment schedules requires more attention than a fixed loan.
- Risk if plans change: You might plan to sell in 5 years but circumstances could change, leaving you exposed to adjustments.
Side-by-Side Comparison
$400,000 loan — 30-year fixed at 7.25% vs. 7/1 ARM at 6.375%
| Fixed-rate monthly payment (P&I) | $2,729 |
| ARM monthly payment (years 1–7) | $2,495 |
| Monthly savings with ARM | $234 |
| Total ARM savings over 7 years | $19,656 |
| ARM balance at year 7 | $368,200 |
| Potential ARM payment at first adjustment (worst case) | $3,119/month |
When to Choose Fixed
- You plan to stay in the home for more than 7–10 years
- Your income is steady but not expected to grow significantly
- You cannot absorb payment increases in your budget
- You value predictability and sleep better knowing your payment is locked
- Current rate spreads between fixed and ARM are narrow (less than 0.5%)
When to Choose an ARM
- You have a specific plan to sell or refinance before the fixed period ends
- The ARM-to-fixed rate spread is significant (1%+ savings)
- Your income is likely to grow, giving you room to absorb future payment increases
- You are in a high-rate environment and expect rates to fall
- You are buying a starter home and plan to move up within 5–7 years