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ARM Mortgage Calculator
Use this free ARM mortgage calculator to see what an adjustable-rate mortgage really costs. Enter the loan, the initial rate and fixed period, the index and margin, and the caps — and it shows your initial payment, your first adjusted payment, and the worst-case payment, with a live timeline chart.
| Figure | Value |
|---|---|
| Initial monthly payment | $2,147.29 |
| First adjusted rate | 6.75% |
| First adjusted payment | $2,537.82 |
| Worst-case rate | 10.00% |
| Worst-case payment | $3,299.13 |
| Balance after fixed period | $367,314.93 |
How it works
An adjustable-rate mortgage, or ARM, is a home loan whose interest rate is fixed for an initial period and then changes at set intervals for the rest of the loan. A "5/1 ARM" is fixed for the first five years and then adjusts once a year. The lower starting rate is the attraction: it is usually below the rate on a fixed-rate loan, so the monthly payment starts lower. The trade-off is uncertainty — after the fixed period, the payment can rise, and this calculator shows by how much.
The model here is a fully amortizing loan, which means every monthly payment covers that month's interest plus a slice of the loan balance, so the balance reaches zero at the end of the term if every payment is made. After the fixed period ends, the new rate is the index plus the margin, subject to the caps. The index is a published benchmark rate that moves with the market; the margin is a fixed amount the lender adds on top, set when the loan is made and never changed. So if the index is 4.5% and the margin is 2.25%, the fully indexed rate is 6.75%.
The caps are what keep an ARM from jumping without limit. The initial cap limits how far the rate can move at the first adjustment — with a 2% initial cap, a 5% starting rate can rise to at most 7% at the first adjustment, even if the index plus margin says 8%. The periodic cap limits each later adjustment, usually to 1% or 2% a year. The lifetime cap limits the total rise over the life of the loan — with a 5% lifetime cap, a 5% starting rate can never exceed 10%, no matter what the index does. Each adjustment also re-amortizes the loan: the payment is recalculated on the remaining balance over the remaining term at the new rate.
This calculator shows three payments because one number is not enough to judge an ARM. The initial payment is what you pay during the fixed period. The first adjusted payment is what you would pay in the first year after the fixed period, at the index-plus-margin rate subject to the initial cap. The worst-case payment is the highest payment the loan could reach if the rate climbed by the periodic cap at every adjustment until it hit the lifetime cap. If you can afford the worst-case payment, the ARM is affordable in every scenario the caps allow; if you can only afford the initial payment, you are betting that rates stay low or that you will refinance or sell before the adjustments bite.
ARMs suit borrowers who expect to move, refinance, or pay the loan down before the fixed period ends, and borrowers whose income is likely to rise with their payments. They are a poor fit for anyone who needs a payment that never changes, or who would be stretched by the worst-case figure. Before signing, read the loan estimate carefully: it shows the index, the margin, and all three caps for your specific loan, and those are the numbers to type into this calculator rather than the examples shown here.
Worked example
A borrower takes a $400,000 loan over 30 years at an initial rate of 5%, fixed for 5 years. The index is 4.5%, the margin is 2.25%, the initial cap is 2%, the periodic cap is 1%, and the lifetime cap is 5%.
Step 1: The initial monthly payment at 5% over 360 months is $400,000 × (0.004167 × 1.004167360) ÷ (1.004167360 − 1) = $2,147.29.
Step 2: After 60 payments the remaining balance is $367,314.93.
Step 3: The fully indexed rate is 4.5% + 2.25% = 6.75%, which is inside the initial cap of 5% + 2% = 7%, so the first adjusted rate is 6.75%.
Step 4: Re-amortizing $367,314.93 over the remaining 300 months at 6.75% gives a first adjusted payment of $2,537.82.
Step 5: If the rate then rose by the 1% periodic cap each year until it hit the 10% lifetime cap, the highest payment reached is $3,299.13.
The result: an initial payment of $2,147.29, a first adjusted payment of $2,537.82, and a worst-case payment of $3,299.13. A second check: a $300,000 loan over 30 years at 4% fixed for 5 years gives an initial payment of $1,432.25 — the same formula, a smaller loan and a lower rate.
FAQ
What does 5/1 ARM mean?
The first number is how many years the rate stays fixed — five years — and the second is how often it adjusts after that, once a year. A 7/1 ARM is fixed for seven years, and a 3/1 ARM for three. The longer the fixed period, the closer the starting rate is to a fixed-rate loan.
What is the index and the margin on an ARM?
The index is a published benchmark interest rate that moves with the market, and the margin is a fixed amount the lender adds to it. Your adjusted rate is index plus margin. The margin never changes during the loan, but the index does, so the rate moves whenever the index moves.
How much can my ARM payment go up?
Only as far as the caps allow. The initial cap limits the first adjustment, the periodic cap limits each later adjustment, and the lifetime cap limits the total rise. With a 5% start and a 5% lifetime cap, the rate can never pass 10%. This calculator's worst-case payment shows the highest payment those caps permit.
Is an ARM better than a fixed-rate mortgage?
It depends on your plans. An ARM usually starts cheaper, which helps if you will move, refinance, or pay the loan off before the fixed period ends. A fixed-rate loan costs more at the start but the payment never changes, which suits anyone staying put for the long term or who could not afford the worst-case ARM payment.
What happens if I cannot afford the adjusted payment?
The loan still has to be paid, and missing payments damages your credit and can lead to foreclosure. Before taking an ARM, check that you could afford the worst-case payment shown here, and have a plan — refinancing, selling, or extra payments during the fixed period — for the day the rate adjusts.