How to Calculate Mortgage Payments
A mortgage is likely the largest debt most people will ever take on. Understanding exactly how your payment is calculated — and what factors affect it — helps you make smarter decisions when buying a home.
The Mortgage Payment Formula
The monthly mortgage payment for a fixed-rate mortgage is calculated using this formula:
M = P × [r(1+r)^n] / [(1+r)^n - 1]M = Monthly Payment · P = Loan Principal (amount borrowed) · r = Monthly Interest Rate (Annual Rate ÷ 12) · n = Total Number of Payments (Years × 12)
Step-by-Step Example
Let's calculate the monthly payment for a $300,000 mortgage at 6.5% for 30 years:
- Principal (P): $300,000
- Annual rate: 6.5%, so monthly rate (r): 6.5% / 12 = 0.5417% = 0.005417
- Total payments (n): 30 × 12 = 360 months
- M = $300,000 × [0.005417 × (1.005417)^360] / [(1.005417)^360 - 1]
- (1.005417)^360 = 7.0436
- M = $300,000 × [0.005417 × 7.0436] / [7.0436 - 1]
- M = $300,000 × 0.03815 / 6.0436 = $300,000 × 0.006321 = $1,896/month
Over 30 years, you'll pay 360 × $1,896 = $682,560 total — meaning $382,560 in interest on a $300,000 loan!
What Makes Up Your Total Payment (PITI)
Your actual monthly mortgage "payment" often includes more than just principal and interest:
| Component | Description | Typical Amount |
|---|---|---|
| Principal (P) | Pays down your loan balance | Varies by month |
| Interest (I) | Cost of borrowing | Varies by month |
| Taxes (T) | Property taxes (escrowed) | $200–$600+/mo |
| Insurance (I) | Homeowner's insurance (escrowed) | $80–$200/mo |
| PMI (if applicable) | Required if down payment < 20% | 0.5–1.5% annually |
15-Year vs. 30-Year Mortgage: The Real Cost
On a $300,000 loan at 6.5%:
| 30-Year | 15-Year | |
|---|---|---|
| Monthly Payment | $1,896 | $2,614 |
| Total Paid | $682,560 | $470,520 |
| Total Interest | $382,560 | $170,520 |
| Interest Savings | – | $212,040 |
The 15-year mortgage saves over $212,000 in interest, but costs $718 more per month. It's a trade-off between cash flow and long-term cost.
How to Lower Your Mortgage Payment
- Make a larger down payment — reduces the loan amount
- Improve your credit score — qualifies you for lower rates
- Choose a longer term — lower monthly payment (but more interest overall)
- Shop multiple lenders — rates can vary by 0.5-1%+
- Buy a less expensive home — simplest solution
- Consider points — pay upfront to reduce your rate