Calculator inputs
This page uses a single-column layout overall. The calculator fields shift to three, two, or one column based on screen width.
Example data table
| Field | Example value |
|---|---|
| Loan amount | $250,000.00 |
| Loan term | 30 years |
| Initial rate | 5.25% |
| Initial fixed period | 5 years |
| Adjustment frequency | 12 months |
| Projected index rate | 4.10% |
| Margin | 2.25% |
| First adjustment cap | 2.00% |
| Periodic cap | 1.00% |
| Lifetime cap | 5.00% |
| Floor rate | 2.50% |
| Monthly taxes + insurance | $420.00 |
Formula used
1) Fully indexed rate: Fully Indexed Rate = Projected Index + Margin
2) Lifetime ceiling: Lifetime Ceiling = Initial Rate + Lifetime Cap
3) Monthly interest rate: Monthly Rate = Annual Rate / 12 / 100
4) Standard payment formula: Payment = P × r / (1 − (1 + r)−n)
5) Interest each month: Interest = Outstanding Balance × Monthly Rate
6) Principal each month: Principal = Payment − Interest + Extra Payment
7) Balance update: Ending Balance = Beginning Balance − Principal Paid
Adjustment logic: Each reset targets the fully indexed rate, then limits the change using the first cap, periodic cap, floor rate, and lifetime ceiling. The payment is recalculated over the remaining term whenever a reset occurs.
How to use this calculator
- Enter the original loan amount and the full term.
- Choose the start date for the amortization schedule.
- Input the initial ARM rate and fixed period length.
- Set the future reset frequency in months.
- Enter your projected index and contract margin.
- Add first, periodic, and lifetime caps plus the floor.
- Include optional extra principal, taxes, insurance, and PMI.
- Submit the form to see payment summaries, rate changes, charts, and the full amortization table.
- Use the CSV or PDF buttons to export the generated schedule.
Frequently asked questions
1) What does this calculator estimate?
It projects monthly ARM payments, future rate resets, principal reduction, interest costs, escrow totals, and payoff timing using your cap structure and index assumption.
2) What is a fully indexed rate?
The fully indexed rate is the projected index value plus the lender margin. That number becomes the target rate before caps, floor limits, and lifetime ceiling rules are applied.
3) Why can the new rate differ from the target?
ARM contracts often limit how much the rate can change at each reset. First-adjustment caps, periodic caps, floor rates, and lifetime caps can all keep the modeled rate away from the target.
4) Does this include taxes and insurance?
Yes. Monthly taxes, insurance, and PMI are added as escrow items so you can compare the principal-and-interest payment with the total monthly cash outflow.
5) How do extra payments affect the result?
Extra principal reduces balance faster, lowers future interest, and may shorten the payoff date. It can also reduce the payment recalculation base at later adjustment points.
6) Are future rates guaranteed?
No. The schedule is a scenario model only. Actual future ARM rates depend on the real index path, lender terms, timing, and loan servicing rules.
7) Why is my first adjusted payment higher?
A higher payment usually happens when the rate rises after the fixed period. The loan then needs to amortize the remaining balance over fewer months at a higher rate.
8) Can I use this for comparison shopping?
Yes. Try different margins, caps, floors, and index assumptions to compare risk, peak payment exposure, interest totals, and payoff speed across loan offers.