10/1 ARM Calculator

Background.

A 10/1 adjustable-rate mortgage buys a decade of certainty and then hands the rate over to the market. For the first 120 monthly payments the interest rate is locked; from year eleven it resets once a year to an index plus the lender's margin, held within whatever caps the loan note sets. The 10 is the fixed stretch in years and the 1 is how often the rate moves afterwards. Many newer loans tied to the SOFR index reset every six months instead and are sold as 10/6 ARMs, but their first ten years work the same way.

During that fixed decade the payment is worked out exactly as it would be for a fixed-rate loan at the introductory rate, spread over the full amortization term, usually 30 years. A $400,000 loan at 6% costs $2,398.20 a month either way. The attraction of the ARM is a starting rate that is often priced below the fixed-rate offer on the same day; the price is not knowing what payment 121 will be.

That unknown matters more than most borrowers expect, because so little principal has gone by then. On the $400,000 example, ten years of payments total about $287,784, yet only about $65,257 of that reduces the debt, leaving roughly $334,743 owed when the first adjustment lands. If the rate moved to 8% at that point, the payment over the remaining 20 years would be about $2,799.92.

The figure here is the starting principal and interest and nothing else. Property tax, homeowners insurance, mortgage insurance and association dues sit on top of it in a real monthly bill, and the payment after the reset depends on an index value nobody can know in advance.

What is 10/1 arm calculator?

The initial principal-and-interest payment of a 10/1 ARM is the level monthly amount that would repay the loan over its whole amortization term if the introductory rate never changed. Because that rate is fixed for 120 months, it is also the actual payment for the first ten years. It is not a forecast for year eleven onward, when the rate becomes index plus margin within the adjustment caps, and it excludes escrowed taxes and insurance.

How to use this calculator.

1. 01Enter the amount you are borrowing in Loan principal: the purchase price minus your down payment, plus any costs rolled into the loan, not the price of the home.
2. 02Type the introductory rate from the lender's Loan Estimate into Initial annual interest rate. Use the note rate rather than the APR, which folds fees in and would overstate the payment.
3. 03Leave Amortization term at 30 years unless your loan is written to repay over a different period. The ten-year fixed window does not shorten it.
4. 04Read the monthly principal and interest, then add your expected property tax, insurance and any mortgage insurance to estimate the full housing cost for the first decade.

The formula.

The payment comes from the standard amortization formula M = P × r ÷ (1 − (1 + r)^−n). P is the loan principal; r is the monthly rate, the introductory annual percentage divided by 1,200, so 6% becomes 0.005; n is the number of monthly payments, the amortization term in years times 12. The term (1 + r)^−n is what $1 due at the very end of the loan is worth today, so the denominator spreads the principal across n equal payments while interest is charged each month on whatever balance remains. At a 0% rate that expression would divide by zero, so the page simply divides the principal by the number of months. The rate moves the answer most: on a $400,000, 30-year loan each half-point shifts the payment by about $127 to $130 a month. The second output is always 120 months, because that is what the 10 in 10/1 means.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Consumer Financial Protection Bureau. Adjustable-rate mortgage handbook current 2026. ARM adjustment structure and consumer loan estimate. Retrieved 2026-08-06.
2. [2]U.S. Internal Revenue Service. Tax guidance current 2026. Official forms, instructions and tax topics. Retrieved 2026-08-06.
3. [3]OpenStax, Rice University. Principles of Finance, 2022. Chapters 6–10. Retrieved 2026-08-06.