Mortgage Total Interest 30 Year Calculator

📖 Overview

Use this calculator to understand the long term interest cost of a fixed mortgage.

Use these default and higher-pressure example inputs to explore how sensitive this calculator is before using your real numbers.

Input	Base Case	Higher Pressure Case
Loan Amount ($)	450,000	517,500
Annual Interest Rate (%)	6.25	7.5

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.

Use this tool when you need a fast decision during active planning or execution.
Use this before committing money, time, or tradeoffs that are hard to reverse.
Use this to compare options using the same assumptions across scenarios.

Results can be misleading if key inputs are missing, stale, or unrealistic.
Very small or very large values may amplify rounding effects and interpretation risk.
If assumptions change mid-decision, recalculate before acting.

💡 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.

❓ 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.