How Is Your Monthly Mortgage Payment Actually Calculated?
Most homebuyers sign a 30-year mortgage without understanding how their monthly payment is calculated. Here's the exact formula — and what it means for your finances.
The Mortgage Payment Formula
Your monthly mortgage payment is calculated using the standard amortization formula:
Where:
M = Monthly payment
P = Principal (loan amount)
r = Monthly interest rate (annual rate ÷ 12)
n = Total number of payments (years × 12)
This formula ensures that every payment covers both your interest charges and a portion of the principal, so that after your final payment, the balance reaches exactly zero.
A Real-World Example
Let's say you take out a $300,000 mortgage at 6.5% annual interest for 30 years:
- P = $300,000
- r = 6.5% ÷ 12 = 0.5417% per month
- n = 30 × 12 = 360 payments
Over 30 years, you'll pay $682,633 total — meaning $382,633 goes to interest alone. That's more than the original loan amount!
How Your Payment Splits Between Principal and Interest
Here's the surprising truth: in the early years of your mortgage, most of each payment goes to interest, not principal.
| Year | Monthly Payment | Goes to Interest | Goes to Principal | Balance Remaining |
|---|---|---|---|---|
| Year 1 | $1,896 | $1,625 (86%) | $271 (14%) | $296,753 |
| Year 5 | $1,896 | $1,553 (82%) | $343 (18%) | $279,163 |
| Year 10 | $1,896 | $1,432 (76%) | $464 (24%) | $253,433 |
| Year 20 | $1,896 | $1,079 (57%) | $817 (43%) | $178,456 |
| Year 30 | $1,896 | $10 (1%) | $1,886 (99%) | $0 |
This front-loading of interest is called negative amortization bias and is why paying extra in the early years saves the most money.
What Factors Affect Your Monthly Payment?
1. Loan Amount (Principal)
The larger the loan, the higher the payment. A $400,000 loan at the same rate costs about $635 more per month than a $300,000 loan.
2. Interest Rate
Even a 1% difference matters enormously. On a $300,000 loan at 6.5% vs 7.5%, you pay $191 more per month — that's $68,760 extra over 30 years.
3. Loan Term
A 15-year mortgage typically has a higher monthly payment than a 30-year, but you pay far less interest overall. The 15-year on a $300,000 loan at 6.5% costs $2,613/month but saves $218,000 in interest.
4. Down Payment
A 20% down payment ($60,000 on a $300,000 home) reduces your loan to $240,000, cutting your monthly payment to $1,517 instead of $1,896.
Calculate Your Exact Mortgage Payment
Use our free loan calculator to see your monthly payment, total interest, and full amortization schedule.
Try the Mortgage Calculator →Key Takeaways
- Your monthly payment is fixed, but the split between principal and interest changes every month
- In early years, most of your payment goes to interest
- A lower interest rate saves more money than a shorter term in most cases
- Extra payments made early have a dramatically larger impact than extra payments made later
- Always use a mortgage calculator before committing to a loan