If you borrow $400,000 for a house at 6.5% for 30 years, you will pay back roughly $910,000 in total. That extra $510,000 is mortgage interest — and understanding how it accrues is the difference between a loan you control and a loan that controls you.

The monthly formula

Every month, your lender charges interest on your remaining balance, not the original loan. With a 6.5% annual rate, the monthly rate is 6.5% ÷ 12 = 0.5417%. On a $400,000 balance, month one's interest is $400,000 × 0.005417 = $2,166.67.

Your fixed monthly payment on this loan is $2,528.27. So in month one:

  • Interest: $2,166.67
  • Principal: $2,528.27 − $2,166.67 = $361.60
  • New balance: $399,638.40

Next month, interest is charged on $399,638.40 — slightly less. That tiny shift repeats 360 times, and that slow drift is called amortization. Early payments are interest-heavy; late payments are principal-heavy. By year 15 you have paid about $380,000 but still owe roughly $300,000. The loan feels endless because, mathematically, the first decade mostly services interest.

Why the rate matters more than the price

A half-point rate difference sounds trivial. On a $400,000, 30-year loan it is not:

  • At 6.0%: $2,398/month, $463,000 total interest
  • At 6.5%: $2,528/month, $510,000 total interest
  • At 7.0%: $2,661/month, $558,000 total interest

Each half point costs about $47,000 in lifetime interest. That is why rate shopping — getting quotes from at least three lenders — is the highest-paid hour in home buying.

Three levers that cut interest

1. Bigger down payment. Putting 20% down instead of 10% on a $400,000 home borrows $80,000 less and eliminates PMI, saving well over $150,000 across 30 years.

2. Shorter term. A 15-year loan at 6.0% on $320,000 costs $2,701/month but only $166,000 in total interest — versus $434,000 on the 30-year version. The payment is higher, but the savings are enormous.

3. Extra principal payments. Paying just $200 extra per month toward principal on the loan above cuts roughly 7 years off the term and saves about $130,000 in interest. Extra payments attack the balance directly, which shrinks every future interest charge.

The takeaway

Mortgage interest is not mysterious — it is a monthly rent charged on whatever you still owe. Keep the balance shrinking fast (bigger down payment, shorter term, extra payments) and keep the rate low (shop lenders, protect your credit score), and you keep hundreds of thousands of dollars that would otherwise go to the bank.

Want to run your own numbers? Try our mortgage calculator to see the payment breakdown and payoff curve for any home price and rate.

Estimates for planning only — not financial advice. Actual loan terms depend on your lender.