Glossary · Definition
Mortgage interest
Mortgage interest is the cost of borrowing money to buy a home, calculated monthly on your remaining principal balance. Because each payment retires some principal, interest paid declines over time — and that’s why early prepayment saves dramatically more than late prepayment.
Definition
Mortgage interest is the cost of borrowing money to buy a home, calculated monthly on your remaining principal balance. Because each payment retires some principal, interest paid declines over time — and that’s why early prepayment saves dramatically more than late prepayment.
What it means
Each month, your lender computes interest as (current_balance × annual_rate / 12) and bills you for that amount plus enough principal to amortize the loan over the agreed term. On a 30-year $320,000 loan at 6.5%, your first month’s payment of about $2,022 splits into $1,733 interest and $289 principal. By year 15 it’s about even. By year 30, almost all of every payment retires principal. This is the ‘front-loaded’ pattern that makes early prepayments so impactful — every dollar of extra principal in year 1 skips 30 years of interest on that dollar.
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Formula
monthly_interest = (current_balance × annual_rate) / 12
Why it matters
Most homebuyers think of mortgage interest as a flat percentage but the actual dollar cost is non-linear. A $200/month extra payment in year 1 saves more interest over the loan’s life than the same $200/month in year 15. Mortgage-interest deductibility (capped at $750K of mortgage debt under the 2017 Tax Cuts and Jobs Act, dropping to $500K for new loans after 2025 sunset unless extended) further changes after-tax cost. The standard deduction increase in 2017 means most middle-income homeowners no longer itemize and thus get no tax benefit from mortgage interest at all.
Example
On a 30-year $320,000 loan at 6.5%, total interest paid over the loan is about $408,000 — more than the loan itself. Prepaying an extra $200/month from year 1 cuts the loan to ~22.5 years and saves about $135,000 in interest.
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Frequently asked questions
Is mortgage interest tax-deductible?
Federally yes if you itemize, but capped at $750K of acquisition mortgage debt (2017-2025 TCJA rules). Most middle-income homeowners take the standard deduction and get no benefit. State rules vary. Check your specific situation with a tax pro.
When is the best time to prepay?
Year 1 saves dramatically more than year 25. Front-loaded amortization means each dollar of extra principal in early years skips many years of future interest.
Does my interest rate change over time?
Fixed-rate mortgages: no, the rate is locked. ARM (adjustable-rate) mortgages: yes, after the initial fixed period. Most borrowers want fixed for predictability.
Related terms
- DefinitionAmortizationAmortization is the process of paying off a loan with equal periodic payments that are split between interest and principal. In the early months, most of your payment goes to interest; as the balance shrinks, more goes to principal.
- DefinitionAPRAPR (Annual Percentage Rate) is the total yearly cost of borrowing money, expressed as a percentage — including the interest rate plus most fees. It's the number you should compare between loans, not the 'interest rate'.
- DefinitionCompound interestCompound interest is interest earned on both your original money AND the interest it's already earned. Over long periods, this 'interest on interest' effect is what turns modest monthly contributions into retirement-level balances.