Model how changing interest rates affect your total loan cost. Add multiple rate periods and compare fixed vs. variable scenarios.
Enter your loan amount, term, and starting interest rate. Then add rate change periods to model when your variable rate adjusts — specify the month the change takes effect and the new rate. Hit Calculate Interest Costs to see your total interest paid, monthly payments, and a year-by-year breakdown. The optional comparison field lets you see how a fixed-rate loan stacks up.
Variable rate loans — including ARMs (Adjustable-Rate Mortgages), HELOCs, student loans, and business lines of credit — can save you money when rates drop but cost you significantly more when rates rise. The difference isn't always obvious upfront.
Consider a $300,000 mortgage starting at 5.5% that adjusts to 7.5% after 5 years. Over 30 years, that shift costs over $80,000 more in interest compared to locking in 6.5% from day one. Or the reverse: if your ARM drops from 6% to 4.5% after year 3, you save thousands — but you need to see the full picture to decide whether the risk is worth it.
This tool is designed for homebuyers evaluating ARMs, HELOC borrowers, small business owners with variable rate credit lines, and anyone who wants to understand exactly what rate volatility costs in real dollars — not just percentages.
For each rate period, the calculator uses standard amortization math:
Monthly Payment = P × [r(1+r)ⁿ] / [(1+r)ⁿ − 1]
Where P = remaining principal, r = monthly rate (annual rate ÷ 12), n = remaining months in the loan term. When the rate changes, the new payment is recalculated based on the remaining balance and remaining months (if "Recalculate on Change" is selected) or held fixed (standard amortizing). Total interest = sum of all interest portions across every monthly payment.
An ARM (Adjustable-Rate Mortgage) is a specific type of variable rate loan for real estate. Variable rate loans include ARMs, HELOCs, personal loans, and business lines of credit with rates that change based on an index like SOFR or the Prime Rate. Both follow the same mathematical principles this calculator uses.
This calculator is highly accurate for modeling scenarios you define. Real ARM loans also have rate adjustment caps (e.g., 2% per adjustment, 6% lifetime cap), adjustment frequency rules, and index-based pricing. Use your loan's specific terms to set the rate values in each period for the most realistic results.
When a variable rate loan adjusts, most lenders recalculate your monthly payment based on the new rate and the remaining balance and term — this is "Recalculate on Change." The "Fixed Payments" option keeps your original payment amount and applies the new rate, changing how much goes to interest vs. principal each month.
Yes. Add as many rate change periods as you need. For example: 5% for years 1–3, 7% for years 4–6, then 6% for years 7–30. The calculator handles any combination of rate changes across the loan term, in any order.