📖 Overview
Use this calculator to understand the long term interest cost of a fixed mortgage.
⚙️ How It Works
This model estimates total interest paid by calculating the fixed monthly payment and summing interest across 360 months.
The Formula
M = P × [r(1+r)ⁿ] / [(1+r)ⁿ − 1] | Total Interest = (M × 360) − P
| M | Fixed monthly payment |
| P | Principal loan amount |
| r | Monthly interest rate = Annual rate ÷ 12 ÷ 100 |
| n | 360 for a 30-year fixed loan |
💡On a $400,000 loan, the difference between 6% and 7% is roughly $95,000 in total interest. Even a 0.25% rate improvement is worth negotiating for.
Quick Reference
| Loan | 5% | 6% | 7% | 8% |
|---|---|---|---|---|
| $200k | $186k | $231k | $279k | $328k |
| $300k | $279k | $347k | $419k | $493k |
| $400k | $373k | $464k | $559k | $657k |
| $500k | $466k | $580k | $699k | $821k |
Practical Tips
💡 Compare total interest across rates, not only monthly payment.
💡 Extra principal payments can materially reduce long-term interest.
💡 Use this as a cost-of-borrowing decision metric.
Frequently Asked Questions
❓ Can total interest exceed the original loan?
Yes, especially at higher rates over long terms.
❓ Does this assume a fixed rate?
Yes, this estimate assumes a fixed interest rate for the full term.