ARM vs Fixed Calculator — Should You Gamble on Lower Rate for 5 Years?

You see 5/1 ARM at 5.5% vs fixed at 6.75%, payment about $400 less per month. Tempting. But after 5 years, it adjusts. Maybe to 7.5%, maybe 8.5%. This ARM vs fixed calculator lays out both scenarios so you do not guess. If you are selling or refinancing in 5 years, ARM can save you about $24k. If you are staying 10+ years, it can cost you. This tool shows balance after fixed period, new payment after adjustment, and savings first years.

How 5/1 ARM Really Works

5/1 = fixed for 5 years, then adjusts every 1 year. Rate = index + margin, subject to caps. So 5.5% first 5 years, then maybe 7.5% after. Payment jumps. Balance after 5 years on $500k at 5.5% is about $463k. New payment at 7.5% for remaining 25 years is about $3,422 vs fixed $3,256 at 6.75% all along. Caps matter — initial 2%, annual 2%, lifetime 5% are common. Worst-case at lifetime cap should be stress-tested.

How To Use This ARM vs Fixed Calculator

Put $500k loan, fixed 6.75%, ARM initial 5.5%, after 5 years 7.5%, term 30yr, 5/1 ARM. Fixed P&I $3,243, ARM initial $2,838, saves $405×60 = $24,300 first 5 years. After fixed period, balance about $463k, new payment at 7.5% about $3,422 — $179 more than fixed. If you sell at year 5, you win. If you stay 10 years, you give some back. Calculator loops balance month by month for fixed period at initial rate to get exact balance after.

Real Example - Austin Buyer Moving in 4 Years

Buyer in Austin plans to move in 4 years for job. $450k loan, fixed 6.75% $2,919 vs ARM 5.5% $2,555 saves $364 per month × 48 = $17,472 total first 4 years. Perfect use case for ARM. If staying forever, fixed peace of mind often wins because payment is stable and not subject to index.

Tip - Check Caps and Worst Case

ARM caps matter — 2% initial cap, 2% periodic, 5% lifetime are typical. Ask lender for caps, index, margin, floor. Worst-case payment at lifetime cap is what you should budget for, not just initial rate. Also check if ARM is 5/1, 7/1, 10/1 — longer fixed period reduces risk but initial rate higher.

Formula Used - Verified

M = P×[r(1+r)^n]/[(1+r)^n−1], Balance_m+1 = Balance_m×(1+r) − M

Fixed payment uses standard amortization. ARM initial payment uses same term n but at initial rate rI. Balance after fixed period calculated by iterating month by month: Balance next = Balance ×(1+rI) − PayI. After fixed period, remaining term = n − fixedMonths, new payment at after rate on remaining balance: PayA = Bal_after×[rA(1+rA)^rem]/[(1+rA)^rem−1]. Savings first period = (PayF − PayI)×fixedMonths.

What Affects ARM vs Fixed Choice? 5 Key Factors

• Initial Rate Difference: Larger difference saves more during fixed period.
• After-Period Rate: Index plus margin determines payment jump after fixed years.
• Fixed Period Length: 3/1, 5/1, 7/1, 10/1 — longer fixed period means more stable years but higher initial rate.
• How Long You Keep Loan: If selling or refinancing within fixed period, ARM savings realized. If staying long, after-period cost matters.
• Caps and Floors: Caps limit increase, floors limit decrease. Lifetime cap defines worst-case payment.

ARM vs Fixed - Quick Comparison

Methodology - Transparent - All Conditions Verified

1. Inputs: Loan amount, fixed rate, ARM initial rate, ARM after rate, ARM type, loan term.
2. Fixed Months: 3/1=36, 5/1=60, 7/1=84, 10/1=120 months.
3. Payments: Fixed M = P×rF(1+rF)^n/((1+rF)^n−1), ARM initial same n at rI.
4. Balance After: Loop fixedMonths at rI to get exact balance after fixed period.
5. After Payment: Remaining = n−fixedMonths, PayA = Bal_after×rA(1+rA)^rem/((1+rA)^rem−1).
6. Savings: (PayF−PayI)×fixedMonths.
7. Edge: Handles 0% rate, remaining 0, NaN/Infinity guarded.
8. Privacy: Browser-side only.

Important Limitations

• Estimates only, not loan offer, rate lock, or disclosure. Actual ARM terms depend on index, margin, caps, floor, lender.
• After-period rate is assumption you enter — actual future rate unknown and depends on market index.
• Does not include taxes, insurance, HOA, PMI, points, or fees.
• Caps not modeled in detail — enter worst-case after rate to stress-test.
• For exact figures, use Loan Estimate, Closing Disclosure, and ARM disclosure from lender.

Frequently Asked Questions

Is ARM risky now?

If you stay long-term, ARM can be risky if rates rise after fixed period. If you sell or refinance within fixed years, lower initial rate may save during that period.

What does 5/1 mean?

5 years fixed, then adjusts every 1 year. 7/1 = 7 years fixed, then yearly adjustment.

Can ARM payment go down?

Yes if index drops after fixed period, subject to floor and caps. Payment can go down or up depending on index movement.

What are ARM caps?

Common caps: initial 2%, periodic 2%, lifetime 5%. Caps limit how much rate can increase at first adjustment, subsequent adjustments, and over life.

Should I choose ARM if selling in 5 years?

If you expect to sell or refinance within fixed period, lower initial rate may save during that period. Calculate savings first years vs after-period risk.

Does this include taxes and insurance?

No. Principal and interest only comparison.