BYTETOOLS

ARM Calculator

Project an adjustable-rate mortgage through its caps: a fixed teaser period, then index plus margin, re-amortised at each adjustment in three scenarios.

$1,703.37
Starting payment
6.750%
Fully indexed rate
10.500%
Lifetime rate ceiling
$2,600.23
Highest payment (worst case)

Three scenarios

ScenarioFirst paymentHighest paymentHighest rateTotal interest
Best case (rate falls to the floor)$1,703.37$1,703.375.500%$188,424
Expected (index stays where it is)$1,703.37$1,916.466.750%$377,141
Worst case (fully capped rises)$1,703.37$2,600.2310.500%$573,430

Schedule — Expected (index stays where it is)

PeriodFrom monthRatePaymentEnd balance
Fixed start (5y)15.500%$1,703.37$277,382
Adjustment 1616.750%$1,916.46$272,973
Adjustment 2736.750%$1,916.46$268,257
Adjustment 3856.750%$1,916.46$263,212
Adjustment 4976.750%$1,916.46$257,817
Adjustment 51096.750%$1,916.46$252,045
Adjustment 61216.750%$1,916.46$245,872
Adjustment 71336.750%$1,916.46$239,269
Adjustment 81456.750%$1,916.46$232,207
Adjustment 91576.750%$1,916.46$224,652
Adjustment 101696.750%$1,916.46$216,572
Adjustment 111816.750%$1,916.46$207,929
Adjustment 121936.750%$1,916.46$198,684
Adjustment 132056.750%$1,916.46$188,795
Adjustment 142176.750%$1,916.46$178,218
Adjustment 152296.750%$1,916.46$166,904
Adjustment 162416.750%$1,916.46$154,803
Adjustment 172536.750%$1,916.46$141,859
Adjustment 182656.750%$1,916.46$128,014
Adjustment 192776.750%$1,916.46$113,204
Adjustment 202896.750%$1,916.46$97,364
Adjustment 213016.750%$1,916.46$80,421
Adjustment 223136.750%$1,916.46$62,298
Adjustment 233256.750%$1,916.46$42,913
Adjustment 243376.750%$1,916.46$22,178
Adjustment 253496.750%$1,916.46$0

At every adjustment the remaining balance is re-amortised over the months left in the term at the new rate, so the loan still finishes on schedule. The rate floor is assumed to be the margin, which is the common contract term. Nobody can predict where an index will go — the expected case simply holds today's index constant. Estimates only, not financial advice.

What is the ARM Calculator?

The ByteTools ARM Calculator projects an adjustable-rate mortgage the way the contract actually works.

  • Fixed teaser period followed by index-plus-margin adjustments
  • Initial, periodic and lifetime caps all applied, with a floor at the margin
  • Re-amortises the remaining balance over the remaining term at each reset
  • Best, expected and worst-case schedules side by side
  • Period-by-period rate, payment and balance table with CSV export
  • Private in-browser math; estimates only, not advice

How to use the ARM Calculator

  1. 1

    Enter the loan amount, total term, fixed start rate and the length of the fixed period.

  2. 2

    Choose how often the rate adjusts after the fixed period ends.

  3. 3

    Enter the index rate and your margin — added together these give the fully indexed rate.

  4. 4

    Set the initial, periodic and lifetime rate caps from your loan documents.

  5. 5

    Compare the three scenario rows, then switch scenarios to see the detailed schedule and download it.

About the ARM Calculator

The ByteTools ARM Calculator projects an adjustable-rate mortgage the way the contract actually works. Your rate is fixed for an initial period, then at each adjustment it resets to the index plus your margin — but clamped by three separate caps: how far it can move at the first adjustment, how far at each one after that, and how high it can ever go above the start rate.

At every adjustment the remaining balance is re-amortised over the months still left in the term, which is why an ARM payment can jump so much even though the loan still finishes on schedule. The tool builds three full schedules: a best case where the rate falls to its floor, an expected case that holds today's index constant, and a worst case where every cap is hit.

Switch between the three to see period-by-period rates, payments and balances, and export any of them as a CSV. Nobody can forecast an index, so treat the expected case as an assumption rather than a prediction. Everything is computed locally in your browser. Estimates only, not financial advice.

Frequently asked questions

What does 5/1 ARM mean?

The first number is how many years the rate stays fixed, and the second is how often it adjusts afterwards. A 5/1 ARM is fixed for five years then adjusts once a year for the rest of the term. Set the fixed period to 5 and the frequency to once a year to model it here.

How is an ARM rate calculated after the fixed period?

The new rate is the current index value plus your margin, which is fixed for the life of the loan. That fully indexed rate is then limited by the caps, so a big index move can only reach your payment gradually.

What do ARM rate caps mean?

Caps are usually quoted as three numbers such as 2/2/5: the most the rate can change at the first adjustment, the most at each later adjustment, and the most it can ever rise above the start rate. This calculator applies all three and shows the resulting worst-case payment.

How bad can an ARM payment get?

The worst-case scenario in this tool answers that precisely — it raises the rate by the full cap at every adjustment until it hits the lifetime ceiling. Looking at that number before you sign is the single most useful thing you can do when comparing an ARM to a fixed-rate loan.

Is an ARM a good idea?

It can be if you are confident you will sell or refinance before the fixed period ends, or if the teaser rate is meaningfully below fixed-rate quotes. The risk is that plans change and you are still holding the loan when rates reset, so judge it against the worst-case payment, not the starting one.

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