← Blog · May 21, 2026 · finance, mortgage
Mortgage Amortization Explained: Why Early Payments Are 80% Interest
Pull up the first month of your mortgage statement and you'll see something infuriating: out of a $2,661 payment on a $400,000 30-year loan at 7%, about $2,333 goes to interest and only $328 to principal. You're paying 88% interest in month one. By the final year of the loan that ratio inverts. What's going on?
The math behind the curve
Mortgage interest is computed every month on the remaining balance. Month one, you owe the full $400,000, so the interest portion is $400,000 × 7% / 12 = $2,333. The lender forces a fixed total payment of $2,661 every month — whatever's left after interest goes to principal.
Next month, your balance is $328 lower (~$399,672). Interest is now $399,672 × 7% / 12 = $2,331. Principal portion bumps up by $2 to $330. Repeat 358 more times. The principal portion grows slowly at first, then accelerates as the balance shrinks.
By month 180 (halfway through a 30-year), you've only paid off about 31% of the loan. The remaining 69% gets crunched in the second half — that's when the curve finally bends.
Total cost over 30 years
On that $400k loan at 7% over 30 years:
- Total paid: $957,960
- Of which interest: $557,960
- Of which principal: $400,000
You pay more in interest than the original loan amount. This is normal. The bank is lending you a small fortune for three decades — that's a service worth paying for. But it does mean the rate matters enormously: dropping from 7% to 6% saves about $96,000 over the life of the loan.
How extra principal changes everything
Every extra dollar applied to principal does two things: reduces today's balance and reduces all future interest charges on that dollar. The savings compound.
Pay an extra $200/month on that same loan and you:
- Pay off the mortgage in 24 years instead of 30 (6 years early)
- Save about $130,000 in interest
Pay an extra $500/month and you're done in 20 years with about $230,000 less interest.
The biweekly trick
Some lenders offer a "biweekly" payment plan: instead of one monthly payment of $2,661, you pay half ($1,330.50) every two weeks. There are 26 biweekly periods in a year, so you make 13 monthly equivalents instead of 12. The extra month goes to principal — exactly the same effect as paying an extra 1/12 of your payment each month.
This trick alone shaves 4-7 years off a 30-year mortgage. See our working days and compound interest tools to understand why small recurring amounts compound to large outcomes over years.
When to NOT pay off early
Paying off the mortgage isn't always the right move. The argument against:
- Opportunity cost: If your mortgage is at 4% and you can earn 7% in a low-cost index fund, the math favors investing.
- Liquidity: Money in the mortgage is locked. Money in a brokerage is accessible for emergencies.
- Tax deduction: US mortgage interest is deductible (with limits). For high-income borrowers, the after-tax rate is lower than the headline.
- Inflation hedge: A fixed mortgage rate is a great inflation hedge. Inflation erodes the real cost of your future payments.
The argument for paying off:
- Guaranteed return: Paying off a 7% mortgage is a guaranteed 7% return. The stock market isn't.
- Peace of mind: No mortgage = no risk of foreclosure. Hard to value.
- Cash flow: A paid-off house means lower monthly expenses, which lets you save/invest more.
At today's 6-7% rates, paying off is usually the better play unless you have an unusually high risk tolerance.
Use our calculators
Run your own numbers:
- Mortgage Calculator — monthly payment + full amortization table
- Refinance Calculator — should you refi at today's rates?
- Compound Interest — the alternative if you invest instead
- Rent vs Buy — sometimes the answer is "don't buy at all"