Planning & Retirement
Mortgage Amortization — How the Schedule Front-Loads Interest
An amortizing mortgage payment is constant, but the interest/principal split shifts dramatically over time. Here is the formula, what early payments really do, and when refinancing or pre-paying actually helps.
A $400,000 mortgage at 6.5% over 30 years has a fixed monthly principal-and-interest payment of about $2,528. In month one, roughly $2,167 of that payment is interest and only about $361 reduces the principal. By month 360, the split is reversed almost completely. The payment never changes, but where the money goes shifts dramatically — and that shift is the entire story of why the first decade of a mortgage feels like running uphill, and why a small extra payment in year one is worth multiples of the same payment in year twenty.
The amortization formula
The standard expression for a fixed-rate amortizing payment is:
M = P * (r * (1+r)^n) / ((1+r)^n − 1)Where:
- M is the monthly payment (principal and interest only — taxes and insurance are separate).
- P is the principal — the amount borrowed.
- r is the monthly interest rate (annual rate divided by 12; 6.5% annual is 0.0054167 monthly).
- n is the total number of monthly payments (30 years × 12 = 360).
For the $400,000 example: M = 400,000 × (0.0054167 × 1.0054167^360) / (1.0054167^360 − 1) ≈ $2,528.
How the interest/principal split shifts over time
Each month the bank computes interest on the current outstanding balance: interest = balance × r. Whatever is left of your fixed payment after that interest is paid goes to principal. Because the balance starts large and shrinks, interest starts large and shrinks. Because the payment is constant, the principal portion has to grow by exactly the amount the interest portion shrinks.
Plotted over 360 months, the interest portion is a slowly decaying curve and the principal portion its mirror image. The two lines cross around year 19 to 20 on a 30-year loan at typical rates. By year 25, almost every dollar is principal. This is not a coincidence or a bank trick — it is the unavoidable consequence of charging interest on a balance and holding the payment constant.
Why early payments matter so much
Consider an extra $1,000 paid against principal in month one of a 30-year loan at 6.5%. That $1,000 would have otherwise sat on the books accruing interest for the remaining 359 months. Eliminating it does not just save $1,000 — it saves all the compounded interest that $1,000 would have generated, which on a 6.5% loan is roughly $5,500 to $6,500 over the life of the loan.
The same $1,000 paid in year 25 of the same loan saves only the interest for the remaining 60 months — perhaps $200 in lifetime savings. Early payments are not just better than late payments; they are an order of magnitude better. The leverage point is at the start of the schedule.
Refinancing math
The breakeven rule is brutally simple: closing costs divided by monthly savings equals the number of months to recoup the refinance. If a refi cuts your payment by $250 a month and costs $7,500 in closing fees, breakeven is 30 months. Below breakeven you lose money; above it you win — provided you stay in the loan past that point.
The trap is term reset. Refinancing a 25-years-remaining loan into a fresh 30-year resets the amortization clock back to the front-loaded interest phase. Your monthly payment drops, but you may pay more total interest despite the lower rate because the new balance is being amortized over a longer horizon. Always compare lifetime interest, not just monthly payment, and consider matching the new term to your remaining original term where possible.
Common mistakes
Three errors recur. First, sending extra money to escrow instead of principal — escrow holds taxes and insurance and does nothing to reduce the loan balance; always specify “apply to principal.” Second, paying for bi-weekly conversion services when the entire benefit can be replicated for free by adding 1/12th of your payment to principal each month. Third, pre-paying a 3% mortgage while ignoring the opportunity cost — that capital invested at expected market returns of 7% real over decades would compound to substantially more than the interest saved on a low-rate loan.
Worked example
A $400,000 30-year loan at 6.5% with no extra payments costs about $510,000 in total interest, with the loan paid off in month 360. The same loan with $200 extra to principal every month from day one pays off in roughly month 290 — about six years early — and total interest drops to roughly $410,000 to $420,000. That is approximately $90,000 saved in lifetime interest from $200 a month, with the savings front-loaded into the early years of the schedule. Exact figures depend on lender rounding and whether extras are applied at month-start or month-end.
Where to go next
Run your own numbers in the /mortgage-calculator/ and stress-test extra payments. For the underlying math behind the formula, see /compound-interest/. To compare mortgage pre-payment against other debt, see /debt-payoff/. For a more general view across loan types, see /loan-calculator/.
This article is educational and is not financial, legal, or tax advice. Mortgage products, tax treatment, and prepayment rules vary by lender and jurisdiction — verify any decision against your loan documents and a qualified advisor.
Frequently asked questions
Should I pay off my mortgage early or invest the difference?
The honest answer is a function of your mortgage rate, your expected after-tax investment return, and how you weigh psychological certainty. A guaranteed 6.5% saving from pre-paying a mortgage is roughly equivalent to a 6.5% risk-free return — historically competitive with bond yields and below long-run equity returns of about 7% real. If your rate is 3% the math leans heavily toward investing; at 7% or higher it leans toward pre-paying. Most balanced plans split the difference: max retirement match first, then evaluate.
What is a recast and how is it different from refinancing?
A recast applies a lump-sum principal payment to your existing loan and re-amortizes the remaining balance over the original term, lowering the monthly payment without changing the rate or the payoff date much. Refinancing replaces the loan entirely — new rate, new term, new closing costs (typically 2% to 5% of the balance). Recasts cost a few hundred dollars and keep your original rate; refinances make sense only when rates have dropped enough to recoup closing costs within a few years.
Why does the first half of my mortgage feel like I'm not paying down anything?
Because you mostly aren't. On a 30-year loan at 6.5%, after 15 years you have paid roughly 60% of total interest but only about 25% of total principal. The amortization formula keeps your monthly payment flat by front-loading interest when the balance is largest. The principal portion overtakes the interest portion only somewhere in years 18 to 21 on typical 30-year loans. This is not a flaw — it is mathematical inevitability of a constant payment against a shrinking balance.
Do bi-weekly payments really save money through compounding?
Mostly no — the savings are almost entirely from making one extra monthly payment per year. Bi-weekly means 26 half-payments, which equals 13 full monthly payments instead of 12. That extra payment goes straight to principal and shaves four to six years off a 30-year loan. The compounding-frequency effect within a single month is negligible. You can replicate the entire benefit by adding 1/12th of your payment to principal each month, with no third-party service required.
Is mortgage interest still worth deducting on taxes?
Less often than people assume. Standard deductions in most jurisdictions are now high enough that itemizing only pays off above a certain interest threshold, usually corresponding to a large balance or a high rate. Even when you do itemize, a deduction is not a credit — at a 24% marginal rate, $10,000 in interest reduces taxes by $2,400, not $10,000. Treat the deduction as a small offset to the cost of borrowing, not as a reason to keep the loan around. Tax rules vary by country and year; verify with a local advisor.