David Chen is a Certified Financial Analyst with over 10 years of experience in variable-rate mortgage analytics and amortisation modelling.
Use this free 15 year variable mortgage calculator to estimate starting payments, worst-case scenarios and total interest over the 15-year term when your rate tracks an index.
15 Year Variable Mortgage Calculator
Variable-Rate Mortgage Formula
Initial Payment: M = P × [ r(1+r)^n ] / [ (1+r)^n – 1 ]
Max Payment: Same formula with capped rate
Formula Sources: Investopedia | CFPB
Variables
- P: Original loan amount.
- r: Current monthly rate (annual ÷ 12).
- n: Total payments (15 × 12 = 180).
- Margin: Added to index to get your full rate.
- Cap: Highest rate allowed over life of loan.
Related Calculators
- 5-Year ARM Calculator
- Fixed vs Variable Calculator
- Interest-Only ARM Calculator
- Refinance Break-Even Calculator
What Is a 15-Year Variable Mortgage?
A 15-year variable mortgage amortises over 180 months but the interest rate moves with an index (e.g., SOFR, LIBOR) plus a fixed margin. Caps limit how high the rate can go, making this calculator useful for stress-testing future payments.
How to Calculate 15-Year Variable Mortgage (Example)
- Enter the loan amount (e.g., $350,000).
- Input the starting rate (e.g., 4.5%).
- Add the lender margin (e.g., 2.25%).
- Set the lifetime cap (e.g., 9.5%) and click “Calculate” to see initial and maximum monthly payments.
Frequently Asked Questions (FAQ)
How often does the rate adjust? Most 15-year variables adjust every 6 or 12 months—verify with your lender.
What index does the margin track? Commonly 1-year SOFR; older loans may use LIBOR or COFI.
Is a 15-year variable risky? Payments can rise, but lifetime caps and shorter term reduce total interest versus 30-year options.
Can I refinance later? Yes—many borrowers refinance before first adjustment or when caps approach.