Your mortgage payment is flat. The split between interest and principal is anything but. In month one, almost all of your payment is interest — the bank charging you for the use of the money you just borrowed. By the final year, almost all of it is principal. Understanding this curve is the difference between "paying a mortgage" and owning your house.
The formula, in one paragraph
A fixed-rate mortgage uses a payment that stays identical every month for the full term. That number is calculated so that — across 360 payments on a 30-year loan — you pay back the original principal plus all the interest that accrued along the way. The formula:
P = L × [r(1+r)ⁿ] / [(1+r)ⁿ − 1]
Where P is the monthly payment, L is the loan amount, r is the monthly rate (annual ÷ 12), and n is the number of months. You will never memorize this. You don't have to — any mortgage calculator runs it instantly. The interesting question is what happens between the first payment and the last.
Why interest dominates the early years
Interest is charged on the outstanding balance. When your loan is brand new, the balance is at its peak — so the interest portion is at its peak. As each payment whittles the principal down, next month's interest charge shrinks a little, and a little more of your constant monthly payment goes to principal. The curve accelerates as the balance falls.
Let's make it concrete. A $400,000 loan at 7% for 30 years has a monthly P&I of $2,661. In month one:
- Interest — $400,000 × (7% ÷ 12) = $2,333
- Principal — $2,661 − $2,333 = $328
Only about 12% of your first payment went to principal. The other 88% went to the bank as pure cost. By month 120 (year 10), the split is ~22% principal. By month 240 (year 20), it's ~46%. By month 360 (year 30), it's essentially all principal.
Enter loan, rate, and term. Get the full month-by-month split — and model extra payments.
The crossover
The "crossover" is the month when principal first exceeds interest in the split. For a 30-year mortgage at 7%, it happens around year 21. Before that, you're paying more rent on the bank's money than you're paying down your house. Shorter terms and lower rates push the crossover earlier:
- 30-year at 4% — crossover at year 14.
- 30-year at 7% — crossover at year 21.
- 15-year at 7% — crossover at year 6.
This is a big part of why 15-year mortgages feel different despite the same rate. You're building real equity almost immediately, not just paying down interest.
The extra principal superpower
Every extra dollar you pay toward principal skips all the future interest that dollar would have accrued. It's a guaranteed return equal to your mortgage rate, and it stacks exponentially early in the loan.
On that same $400k / 7% / 30-year loan, an extra $200/month:
- Cuts the term from 30 years to ~25 years
- Saves roughly $110,000 in interest over the life of the loan
- Costs you only $60,000 in extra payments ($200 × 300 months)
That's a net benefit of ~$50k, plus payoff 5 years sooner. An extra $500/month nearly halves the term. No other "investment" offers a guaranteed return that high with no market risk.
The caveats
A few realities worth knowing. First, your monthly mortgage payment isn't just P&I — it usually includes property tax, homeowner's insurance, and sometimes PMI or HOA. Those don't amortize; they're ongoing costs whether you've paid off the loan or not.
Second, before throwing extra money at principal, make sure you've filled the higher-return basics: an employer 401(k) match (free money), high-interest credit-card debt (very expensive money), and a small emergency fund. Mortgage principal is a great next stop after those are handled.
Third, "pay off early" is a valid financial goal, but it's not obviously the best one. At 4% fixed-rate 30-year mortgages in low-rate environments, keeping the mortgage and investing the difference in a diversified portfolio historically outperforms. At 7%, the math tilts the other way. Run the comparison before committing.
Compare scenarios: 15 vs 30 year, different rates, extra principal. Shareable URL for every run.