CMT vs SOFR ARM 2027 Comparison Calculator
An adjustable-rate mortgage resets to an index plus a fixed margin. This calculator compares the payment on the same loan under two indexes: the 1-year Constant Maturity Treasury (CMT) and SOFR (Secured Overnight Financing Rate). Enter the current value of each index and your margin. The index values pre-filled are examples only, not live rates.
Index Plus Margin
After the initial fixed period, an ARM's rate is the index value plus the margin written in your note. The margin does not change; the index does. Two loans with the same margin but different indexes will have different payments whenever the indexes differ.
The Two Indexes
SOFR is a benchmark rate based on overnight borrowing secured by US Treasury securities, published by the Federal Reserve Bank of New York. ARMs usually reference an average of SOFR over a period rather than a single day; your note says which. The 1-year CMT is a US Treasury yield published by the Treasury and the Federal Reserve. Enter the specific value your loan documents refer to.
Worked Example
$450,000 loan, margin 2.50%. With CMT at 4.55% the rate is 7.05% and the 30-year payment $3,008.99. With SOFR at 4.32% the rate is 6.82% and the payment $2,939.66, a difference of $69.33 a month. These index values are illustrations.
Disclaimer
Real resets are also limited by periodic and lifetime caps and are applied to the balance outstanding at the time. Not financial advice.
Method: standard amortisation formula on user inputs. Index values: New York Fed — SOFR. Last updated: October 2026.