The math behind this decision is rarely presented clearly. Knowing the break-even post-intro rate — the rate above which a fixed mortgage becomes cheaper — lets you make a fully informed choice rather than guessing.
Two Loan Structures, One Decision
Everyone taking out a mortgage faces this choice. The key insight: ARM savings early in the loan are worth more than identical savings later — because you owe the most money at the start.
A mathematical perspective on this decision — factors that many people overlook.
Your outstanding principal is highest at the start. A 1% rate difference on a $1.2M loan saves $12,000/yr in year 1 vs. only ~$5,000/yr in year 20 when the balance is lower. Same rate difference, very different dollar impact.
ARM savings are front-loadedMoney saved in years 1–7 can be invested and compound over the rest of your holding period. Each dollar of early savings is worth more than a dollar saved in year 15. This compounds the ARM's advantage significantly.
Time value of money favors ARMOne practical move: use savings from the ARM's lower payment to prepay extra principal. That immediately reduces your balance, lowers future interest costs, and reduces your exposure if the rate resets higher later.
Lower balance = lower reset riskHistorically, a majority of mortgage holders sell or refinance before 10 years. If you sell or refi before the ARM intro period ends, you never experience the rate reset — and you captured all the savings.
Median hold period ~8 yearsThe break-even post-intro rate tells you: "If I believe post-intro rates will stay below X%, ARM wins." With 5.5% ARM / 6.5% fixed / 7-yr intro / 15-yr hold, the break-even is 8.21%.
Calculate yours below ↓None of this is to say ARM is always better. A 30-year fixed provides genuine peace of mind and eliminates rate risk entirely. If you value certainty or don't want to track rates, fixed is a perfectly rational choice.
Certainty has real valueAdjust the four inputs to instantly see the break-even post-intro rate — the ARM reset rate above which the 30-year fixed becomes the cheaper option.
This topic comes up frequently — everyone taking out a mortgage faces this decision. Here is a mathematical perspective and some observations that people often overlook.
Your outstanding principal is highest at the start, which means the interest rate you pay early on applies to the largest balance. A small rate difference in the first few years can have a bigger impact than a similar difference much later, when the balance is smaller.
Savings at the beginning of the loan are worth more because money saved today can earn a return over time. An ARM's lower intro-period payment isn't just $X saved — it's $X that can be invested or used to pay down principal, compounding its value over the rest of your holding period.
One practical move is to use the ARM's lower payment during the intro period to prepay principal. That immediately reduces what you owe, which lowers future interest costs even further. It also reduces your exposure if the ARM rate rises later, since a higher rate would apply to a smaller remaining balance.
If interest rates come down at some point — often during a recession — you may be able to refinance into a better loan and reset the ARM intro period. In addition, a majority of people will hold the mortgage for less than 10 years. If you sell or refi before the ARM intro period ends, you never experience the rate reset and capture all the savings.
With an ARM at 5.5%, a 30-year fixed at 6.5%, a 7-year intro period, and a 15-year holding period, the ARM doesn't become worse than the fixed unless the post-intro ARM rate rises higher than 8.21%. That is the break-even rate for those inputs. Run the calculator above with your actual numbers to find yours.
None of this is to say an ARM is always better. A 30-year fixed provides real peace of mind and protects you from future rate increases. The point is simply: make an informed decision using the math that actually tells you which option will be cheaper for you.
Prefer working in a spreadsheet? Use the original Excel/Sheets model — enter your data into the yellow fields and see the results directly.
Enter your ARM intro rate, fixed rate, intro period, and holding period into the yellow fields. The sheet calculates break-even rates and total cost comparison.
Open in Google Sheets