Use this to compare an Adjustable-Rate Mortgage (ARM) against a Fixed-Rate mortgage. You’ll see: estimated starting payments, how an ARM payment can change after the intro period, total interest paid up to your “keep it” timeline, and a simple break-even view that includes remaining payoff balance.
Tip: the single most important input is your how-long-you’ll-keep-it horizon. Many people choose an ARM because it’s cheaper today — but the real question is whether it stays cheaper over the time you’ll actually keep the loan.
Mortgages look complicated, but the core math is very consistent: every month you pay interest on the remaining balance, then the rest of your payment reduces principal. Over time the interest portion shrinks and the principal portion grows.
For a fixed-rate mortgage, the monthly payment (principal + interest) is computed so that the loan reaches a balance of $0 at the end of the term. The standard amortization formula is:
Monthly Payment = P × r × (1+r)n / ((1+r)n − 1) where P = loan principal, r = monthly interest rate (APR/12), and n = number of months.
An ARM uses the same payment formula — but the rate can reset. Most ARMs have:
This calculator models the ARM rate at each adjustment as: New Rate = Index + Margin, but then applies a per-adjustment cap (so it can’t jump too much at once) and a lifetime cap (so it can’t exceed the start rate + lifetime cap).
After each adjustment, the payment is re-calculated so the remaining balance amortizes over the remaining months. That is why ARM payments can jump even when the rate increases “only a little” — because the math must still hit $0 by the end date.
When people compare mortgages, they often look only at monthly payment. That’s useful, but incomplete. If you sell or refinance, what matters is:
That’s why the results show “Total out-of-pocket + payoff.” It’s a practical way to compare the two choices over your horizon.
Here’s the best way to use an ARM vs fixed calculator: treat it like a decision stress-test. You’re not trying to predict the future perfectly. You’re checking whether your choice is robust under plausible rate paths.
Suppose you borrow $550,000 for 30 years. Fixed is 6.75% with $1,500 in fees. ARM starts at 5.75% with a 5-year intro, adjusts yearly. Index starts at 4.75%, margin 2.25%. Caps are 2% per adjustment and 5% lifetime. Horizon: 6 years.
In this case, the ARM can look attractive because most of your timeline occurs during the intro period. But the key check is what happens in year 6 if rates have drifted up. Even one adjustment can change the result. Use the “Index change per adjustment” slider to see the sensitivity.
With a long horizon, the fixed mortgage often becomes more appealing because you are exposed to many ARM adjustments. Even with caps, a long sequence of moderate increases can create a big difference in total interest. In other words: the longer you keep the loan, the more valuable payment stability becomes.
If you want to model a falling-rate environment, move the index drift negative (e.g., −0.25%). This can make ARMs look better. However, note that many ARMs have floors (minimum rates), so the real-life benefit might be smaller than a perfect decline scenario.
A good viral share is to post two screenshots: “Rates flat” vs “Rates rising” with the same horizon, and ask friends which one they’d choose. People love seeing how the answer changes with a single slider.
Most mortgage regret comes from a mismatch between the loan and real life. You don’t regret a 30-year fixed because rates moved. You regret it because you felt “stuck.” Likewise, you don’t regret an ARM because ARMs exist — you regret it because you didn’t plan for the payment path.
If you are highly confident you’ll move or refinance before the ARM intro ends, the ARM’s lower start rate can be compelling. If you’re not confident, the “discount” of an ARM is often compensation for taking uncertainty.
Use the index drift slider as a worst-case proxy. If you bump drift up and the ARM payment becomes uncomfortably high, fixed may be the better fit for your lifestyle. Financially, the “best” loan is the one you can safely keep.
Two loans can have similar monthly payments but different payoff balances at your horizon. If you refinance or sell, that payoff matters. This is why APR and fees can change the comparison even when the note rate looks similar.
Typically a lower starting interest rate than a fixed mortgage, which can save money if you sell/refinance before major adjustments.
Payment shock. If the index rises, your rate can increase (within caps), and your monthly payment can jump.
The index is a market rate used by your loan; the margin is a fixed add-on set by the lender. Together they form the adjusted rate (subject to caps).
No. Caps limit how fast the rate can change, but payments can still rise meaningfully because the remaining balance must amortize over the remaining term.
If you sell or refinance, you must pay off the remaining balance. Comparing “paid so far + payoff” is a practical total-cost view.
No. Those are real costs, but they usually don’t change the ARM vs fixed comparison much because they apply similarly to both options.
Use it as a scenario knob. Start at 0.00% for a stable-rate world, then try +0.50% or +1.00% as a stress test.
Not always. Some ARMs have floors, and your timeline still matters. Use a falling scenario (negative drift), then confirm you’re comfortable if the decline doesn’t happen.
No. It’s an educational estimate. Always confirm your specific loan terms (index, margin, caps, floors) in the lender’s disclosure.
Run “Rates flat” vs “Rates rising” with the same horizon and share both results. People love seeing how sensitive the decision is.
When you see an ARM advertisement, the headline number is the teaser / start rate. The real “gravity” of an ARM is the fully-indexed rate: index + margin. If your index is 4.50% and your margin is 2.25%, your fully-indexed rate is about 6.75% — even if your start rate is 5.50%. That doesn’t mean you’ll jump straight to 6.75%, because caps often slow the change. But it does tell you the direction the loan wants to move when the intro period ends.
Caps are usually described like “2/1/5” or “2/2/5” (first adjustment cap / periodic cap / lifetime cap). This calculator uses a simpler model that most people can reason about: per-adjustment cap (how much the rate can change at each reset) and lifetime cap (maximum increase above the start rate). If you have the detailed cap structure from your lender, you can approximate it by choosing a per-adjustment cap that matches your periodic cap, and a lifetime cap that matches your lifetime cap.
A rate increase does two things at once: it increases the interest you pay each month and it changes the payment needed to reach $0 by the end of the term. After 5–10 years, the remaining amortization window is shorter. So even a modest rate increase can cause a larger-than-expected payment jump because the remaining balance must be paid down faster.
Two loans can share the same note rate but have different fees (points, origination, lender credits). APR compresses some of that into a single number, which is why it’s useful for comparisons. But APR still won’t capture everything (like closing credits, prepayment penalties, or rate-lock extensions). If you want a “real cost” view, enter estimated fees for each option and compare totals over your horizon.
Many ARM decisions rely on refinancing later. That can work — but it isn’t guaranteed. Refinancing depends on (1) market rates, (2) your credit score, (3) your debt-to-income ratio, and (4) whether your home value supports the new loan. In a tight market, refinance options can be worse exactly when you most want them. This is why a good rule is: only choose an ARM if you could live with it even if you cannot refinance.
A market rate your ARM references. It can move up or down over time.
A fixed add-on set by the lender. Your adjusted rate is usually index + margin (before caps/floors).
The “raw” adjusted rate (index + margin), before caps are applied.
A limit on how much the rate can change (per adjustment) and how high it can go (lifetime).
The initial years where the ARM behaves like a fixed loan at the start rate.
The schedule that pays a loan down to $0 via monthly payments.
The remaining principal you must pay if you sell or refinance.
If you want to be extra careful, run 3 scenarios: drift 0.00%, +0.50%, and +1.00% and compare winners.
MaximCalculator provides simple, user-friendly tools. Always verify important numbers using lender disclosures and professional advice.