ARM Formula Explained
The ARM formula explained in plain English: the equation behind the ARM Calculator, its variables, and the assumptions it makes.
By Onias Derilus, Mortgage Capital · NMLS# 1859012 · Last Updated: June 2026
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.
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 formula is only the starting point. Open the arm calculator to plug in your own numbers and see the result instantly. For a rate tied to your actual file, talk to a licensed broker before you decide.
Turn Your ARM Estimate Into a Real Pre-Approval
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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.