ARM Calculator
This ARM calculator estimates the intro payment on an adjustable-rate mortgage. It also shows the potential payment shock when the rate first adjusts.
By Onias Derilus, Mortgage Capital · NMLS# 1859012 · Last Updated: June 2026
Payment Before vs After Adjustment
Models a 5/6 ARM: fixed for 5 years, then adjusting. The first-adjustment cap limits how far the rate can jump initially. Estimate only — index and margin drive real rates.
An adjustable-rate mortgage offers a lower fixed rate for an introductory period — often five years — then adjusts periodically based on a market index plus a margin. The lower start can save money if you sell or refinance before it adjusts.
The risk is payment shock: when the rate resets, your payment can jump. Rate caps limit how far it can move at the first adjustment and over the life of the loan.
These figures are estimates. For neutral, official guidance on mortgage costs and what lenders can charge, see the CFPB's Owning a Home guide.
How to Use This Calculator
- 1
Enter your loan amount.
- 2
Set the introductory fixed rate offered during the initial period.
- 3
Estimate the fully-indexed rate the loan could reach after adjusting.
- 4
Set the first-adjustment cap, then compare the intro payment to the post-adjustment payment.
The Formula & Assumptions
Intro P&I = payment at intro rate
After 5 years, balance amortizes,
then re-amortizes over 25 years
at min(fully-indexed, intro + cap)
Shock = adjusted − intro payment
We model a common 5/6 ARM: a fixed rate for five years, then adjustments. After the fixed period the remaining balance is re-amortized over the remaining term at the new rate.
The first-adjustment cap limits how far the rate can rise at the initial reset, even if the fully-indexed rate is higher. Lifetime caps limit the total increase over the loan.
Real ARM rates after adjustment equal an index (such as SOFR) plus a fixed margin, subject to the caps. Because the index moves, the post-adjustment payment shown here is an illustration, not a guarantee.
Frequently Asked Questions
What does 5/6 ARM mean?
The first number is the years the rate stays fixed — five — and the second is how often it adjusts afterward, every six months. A 7/6 ARM stays fixed for seven years before adjusting semiannually.
When does an ARM make sense?
An ARM can save money if you expect to sell or refinance before the fixed period ends, or if intro rates are meaningfully lower than fixed rates. The risk is being caught in the adjustable period when rates are high.
How high can my ARM payment go?
Rate caps limit it. There is usually a cap on the first adjustment, a cap on each later adjustment, and a lifetime cap. The fully-indexed rate cannot exceed the lifetime cap above your start rate.
How is the adjusted rate calculated?
After the fixed period, the rate equals a published index plus a fixed margin set in your note, subject to the caps. Because the index moves with the market, your future payment is not fixed.
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Rates are illustrative only. APR and payments vary by credit score, loan amount, and market conditions. Subject to credit approval. Not a commitment to lend. NMLS# 1859012. Equal Housing Lender.
It estimates payments on an adjustable-rate mortgage across the fixed introductory period and the adjustment periods that follow. It includes worst-case rate caps.
ARM Calculator in Florida
ARMs can appeal to Florida buyers who expect to sell or refinance before the fixed period ends — common in a state with seasonal residents and frequent relocations. Just stress-test the worst case: if you might still own the home when it adjusts, make sure the capped payment is one you could absorb.
How the ARM Calculator Works
The calculator amortizes at the low intro rate during the fixed period, then projects payments at the maximum the rate can reach under the caps, so you see both the best case and the worst case after the loan adjusts.
The ARM Formula, Explained
Adjusted rate = index + margin, bounded by periodic and lifetime caps
An ARM like a 5/1 is fixed for the first five years, then adjusts annually. The new rate equals an index plus a fixed margin. Periodic and lifetime caps limit how far it can jump at each adjustment and over the life of the loan.
The calculator's worst-case projection applies those caps to show the highest possible payment. Comparing it to the intro payment reveals your exposure if rates rise after the fixed period ends.
The Complete ARM Calculator Guide
An adjustable-rate mortgage trades long-term certainty for a lower initial rate. For the right borrower — one who plans to move or refinance before the fixed period ends — that lower intro rate is real savings. For everyone else, it is a bet on future rates.
The key terms are the caps. They limit how much your rate can rise at each adjustment and over the loan's life, which bounds your worst case. Never evaluate an ARM on the intro payment alone; evaluate it on the capped maximum.
Run the worst-case scenario in the calculator and ask whether you could afford that payment. If the answer is no and you might still own the home when it adjusts, a fixed-rate loan is the safer choice despite the higher starting rate.
ARM Calculator FAQ
What does 5/1 ARM mean?
The rate is fixed for the first five years, then adjusts once a year after that. The new rate is set by an index plus a margin, limited by the loan's rate caps.
Is an ARM riskier than a fixed loan?
It can be, because the payment can rise after the fixed period. The risk is bounded by the caps, and it is lower if you plan to sell or refinance before the loan adjusts.