Most people know their monthly mortgage payment but have no idea how it's calculated — or why the first years of payments are almost entirely interest. Understanding the math gives you real power to save tens of thousands of dollars over the life of a loan.
The Four Components of a Mortgage Payment
Your total monthly housing payment is often called PITI:
- Principal — the portion reducing your loan balance
- Interest — the cost of borrowing, front-loaded by your lender
- Taxes — property taxes, typically 1–2% of home value per year, divided monthly
- Insurance — homeowner's insurance, plus PMI if your down payment is under 20%
Mortgage calculators (including ours) calculate P+I. Add your local tax and insurance estimates for the full picture.
The Amortization Formula
Your monthly P+I payment is fixed for the life of the loan, calculated using: M = P[r(1+r)^n] / [(1+r)^n – 1], where P is the loan amount, r is the monthly rate, and n is total payments. On a $280,000 loan at 6.5% for 30 years, that works out to about $1,770/month.
Why You Pay So Much Interest Early
In month one of that same loan, roughly $1,517 goes to interest and only $253 reduces your balance. By year 15, the split becomes more balanced. By the final years, most of each payment is principal. This is amortization — and it's why refinancing early in a loan can reset you back to interest-heavy payments even if the rate is lower.
💡 Making one extra payment per year on a 30-year mortgage can cut the loan term by 4–6 years. On a typical $250,000–$400,000 loan, that translates to $20,000–$60,000 in interest saved — the exact amount depends on your loan balance and rate.
How to Lower Your Monthly Payment
- Larger down payment — reduces the principal and eliminates PMI if you hit 20%
- Longer term — 30 years vs 15 years lowers the monthly payment but roughly doubles total interest paid
- Lower rate — even 0.5% lower on a $300,000 loan saves ~$90/month and ~$32,000 total
- Buying points — paying upfront to "buy down" your rate makes sense if you'll stay 5+ years
LTV and PMI
Loan-to-value ratio (LTV) is your loan amount divided by home value. At 80% LTV or below, you avoid Private Mortgage Insurance (PMI), which typically adds 0.5–1.5% of the loan annually to your payment. On a $300,000 loan, that's $125–$375/month you can eliminate with a 20% down payment.