The Mortgage Payment Formula
Your monthly principal and interest payment is calculated using this formula:
M = P × [r(1+r)^n] / [(1+r)^n - 1]
Where:
- M = monthly payment
- P = principal (loan amount)
- r = monthly interest rate (annual rate ÷ 12)
- n = total number of payments (years × 12)
A Worked Example
Let's say you're borrowing $400,000 at 6.5% annual interest over 30 years.
- P = $400,000
- r = 6.5% ÷ 12 = 0.5417% = 0.005417
- n = 30 × 12 = 360
M = $2,528/month (principal + interest only)
What's Not Included in That Number
The P&I payment is only part of your actual monthly cost. Most lenders roll in:
- Property tax — typically 1–2% of home value annually, divided by 12
- Homeowner's insurance — roughly $100–200/month depending on location and coverage
- PMI (Private Mortgage Insurance) — required if your down payment is under 20%, typically 0.5–1% annually
- HOA fees — if applicable to your property
How Interest vs. Principal Changes Over Time
In the early years of a mortgage, almost all of your payment goes to interest. By year 30, almost all goes to principal. This is called amortization.
On that $400,000 loan at 6.5%:
- Month 1: $2,167 to interest, $361 to principal
- Month 180 (year 15): $1,702 to interest, $826 to principal
- Month 359 (year 30): $23 to interest, $2,505 to principal
Why a Shorter Term Saves Dramatically
A 15-year mortgage on the same $400,000 at 6.5%:
- Monthly payment: $3,488 (vs $2,528 for 30 years)
- Total interest paid: $227,872 (vs $509,882)
- Savings: $282,010
Skip the Math — Use the Calculator
ToolForge's Mortgage Calculator handles all of this instantly. Enter your loan amount, rate, term, and optional property tax/insurance/HOA, and you'll get your full monthly breakdown plus a complete amortization schedule showing every payment over the life of the loan.