The Mortgage Payment Formula
Your fixed monthly payment is calculated using the same formula used worldwide for amortizing loans:
Where:
M = Monthly payment
P = Loan principal (amount borrowed)
r = Monthly interest rate (annual rate ÷ 12)
n = Total number of payments (years × 12)
Example — $400,000 Mortgage at 6.5% for 30 Years
- P = $400,000
- r = 6.5% ÷ 12 = 0.5417% per month (0.005417)
- n = 30 × 12 = 360 months
- Monthly Payment (M) = $2,528
Total paid over 30 years: $2,528 × 360 = $910,177
Total interest paid: $910,177 − $400,000 = $510,177
You pay more in interest than the original loan amount. This is the reality of a 30-year mortgage at current rates.
The Amortization Schedule — How Payments Split Over Time
Here is how the $2,528 monthly payment is divided between interest and principal at different points in the loan:
| Payment # | Payment | Interest | Principal | Balance |
|---|---|---|---|---|
| Month 1 | $2,528 | $2,167 | $361 | $399,639 |
| Month 12 | $2,528 | $2,143 | $385 | $395,610 |
| Month 60 (Year 5) | $2,528 | $2,037 | $491 | $375,491 |
| Month 120 (Year 10) | $2,528 | $1,862 | $666 | $343,739 |
| Month 198 (Year 16.5) | $2,528 | $1,264 | $1,264 | $232,788 |
| Month 240 (Year 20) | $2,528 | $1,090 | $1,438 | $200,628 |
| Month 300 (Year 25) | $2,528 | $656 | $1,872 | $119,508 |
| Month 360 (Year 30) | $2,528 | $14 | $2,514 | $0 |
Key observations:
- Month 1: 86% of your payment ($2,167) goes to interest. Only 14% ($361) reduces your balance.
- Month 198 (year 16.5): This is the crossover point — interest and principal are equal for the first time.
- Month 360: Almost the entire payment goes to principal.
⚠️ The First 5 Years Trap
After 5 years of payments ($151,680 total), your balance has only decreased by $24,509. That means 84% of your first 5 years of payments went to interest, not building equity. This is why selling a home within the first few years after purchase often results in little or no equity gain.
The Power of Extra Payments
Because extra payments go directly to principal (not interest), they have an outsized effect:
| Strategy | Payoff Time | Total Interest | Interest Saved |
|---|---|---|---|
| Standard payments | 30 years | $510,177 | — |
| +$200/month extra | 24.2 years | $390,247 | $119,930 |
| +$500/month extra | 19.8 years | $291,484 | $218,693 |
| 1 extra payment/year | 25.5 years | $428,020 | $82,157 |
| Bi-weekly payments | 25 years | $418,890 | $91,287 |
💡 The Bi-Weekly Payment Trick
Instead of 12 monthly payments, make 26 bi-weekly half-payments. This results in 13 full payments per year instead of 12 — one extra payment annually. It is the easiest way to save $80,000+ and pay off your mortgage 5 years early without changing your budget significantly.
15-Year vs 30-Year Mortgage
The interest difference between a 15-year and 30-year mortgage is dramatic:
| Term | Rate | Monthly Payment | Total Interest |
|---|---|---|---|
| 30-year | 6.50% | $2,528 | $510,177 |
| 15-year | 6.00% | $3,375 | $208,713 |
| Savings with 15-year | $301,464 | ||
The 15-year mortgage costs $847 more per month but saves over $300,000 in total interest. If you can afford the higher payment, a 15-year mortgage builds wealth dramatically faster.
When to Consider Refinancing
Refinancing makes sense when:
- Rate drop of 1%+: A $400K mortgage going from 7.5% to 6.5% saves $280/month. Break-even on closing costs typically occurs within 2-3 years.
- Shortening term: Refinancing from a 30-year to 15-year at a lower rate dramatically reduces total interest.
- Removing PMI: If your home has appreciated and you now have 20%+ equity, refinancing eliminates private mortgage insurance.
⚠️ Refinancing Resets Amortization
If you are 10 years into a 30-year mortgage and refinance to a new 30-year term, you restart the amortization clock. You go back to paying mostly interest. To truly benefit, refinance to a shorter term (20 or 15 years) or continue making your old higher payment even if the new minimum is lower.
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Use Mortgage Calculator →How We Research and Update This Guide
We review current federal guidance, plan limits, lender documentation, and publicly available source material before revising calculations or examples.
- Federal thresholds, contribution limits, and lender or plan references are reviewed before revising the guide.
- Examples are recalculated and matched against the related tool or amortization logic used on the site.
- State-specific edge cases are separated from federal guidance where possible to reduce ambiguity.
Frequently Asked Questions — Mortgage Amortization
Amortization is the process of paying off a mortgage through regular monthly payments over a fixed period (typically 15 or 30 years). Each payment is split between interest and principal. In the early years, most of your payment goes to interest. As the principal balance decreases, the interest portion shrinks and the principal portion grows. By the end of the loan, almost all of each payment goes to principal.
Interest is calculated on the outstanding balance. At the start of a $400,000 mortgage at 6.5%, your balance is at its highest. Monthly interest is approximately $2,167 (6.5% ÷ 12 × $400,000). Since your total payment might be $2,528, only $361 goes to principal. As years pass and the balance shrinks, the interest portion decreases and more goes to principal reduction.
Extra payments go directly to principal reduction, which has a compound effect. On a $400,000, 30-year mortgage at 6.5%, one extra payment per year saves approximately $82,000 in total interest and pays off the loan about 4.5 years early. Even rounding up your payment by $100-200/month can save tens of thousands over the loan term.
A 15-year mortgage has higher monthly payments but significantly lower total interest. On a $400,000 loan at 6.5%, a 30-year mortgage costs $510,177 in total interest. The same loan at 15 years (typically 0.5% lower rate at 6.0%) costs only $208,713 in interest — saving over $300,000. Choose 15 years if you can comfortably afford the higher payment without straining your budget.
Refinancing replaces your current mortgage with a new one — usually at a lower interest rate. However, it resets the amortization schedule. If you are 10 years into a 30-year mortgage (where you are finally paying more principal than interest) and refinance to a new 30-year loan, you restart at a high-interest, low-principal split. Consider refinancing to a shorter term (15 or 20 years) to maximise savings.