How Mortgage Amortization Works
Fifteen years into a 30-year mortgage — half the payments made — you still owe about 73% of what you borrowed. That is not a bank trick; it is arithmetic. Here is the machinery behind it, and why small extra payments bend it so powerfully.
Your payment is fixed. What it pays for is not.
Borrow $400,000 over 30 years at 6.5% and your payment is $2,528.27 a month, every month, 360 times. It feels like rent — a flat price for living in your house. But inside that unchanging payment, a slow handover is taking place. In the early years almost all of it is interest. In the later years almost all of it is principal. The schedule of that handover is called an amortization schedule, and understanding it explains nearly everything confusing about mortgages.
Principal = $2,528.27 − $2,166.67 = $361.61
Your very first payment — the one that feels like it should make a dent — reduces your loan by $361.61. The bank collects $2,166.67. That is not a trick or a front-loading fee. It is simply what 6.5% of $400,000 costs for one month. Interest is always charged on whatever you currently owe, and at the start, you owe all of it.
Why the balance barely moves for years
Each month the pattern repeats: interest is skimmed off the current balance, and whatever is left of the fixed payment chips away at principal. Because the balance falls, the interest shrinks a little each month, so the principal slice grows a little — a snowball rolling uphill, very slowly at first:
| Moment | Of this payment, interest is | Balance remaining |
|---|---|---|
| Payment #1 | $2,166.67 (86%) | $399,638 |
| After year 1 | $25,868 of the $30,339 paid that year | $395,530 |
| After year 15 (halfway) | — | $290,237 (73% still owed) |
| Payment #233 (year 20) | Principal finally exceeds interest | — |
| Payment #360 | $13.64 | $0 |
Sit with the middle row for a second. Fifteen years — half the loan's life, $455,000 of payments — and you have repaid barely a quarter of the principal. This is the single most surprising fact in home finance, and it follows directly from the first-month arithmetic: while the balance is large, the payment is mostly rent paid to the bank for the money, and the balance stays large because the payments are mostly rent. The curve bends only gradually, which is why the principal portion doesn't pass the interest portion until month 233 — well into year 20.
By the end, the totals are stark: 360 payments of $2,528.27 sum to $910,178, for a house you borrowed $400,000 to buy. The interest alone — $510,178 — is more than the loan itself.
See it with your own numbers
Everything above, live. Change the numbers to match your own situation:
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Extra payments: attacking the balance while it's big
Now the first-month arithmetic becomes good news. Since interest is charged on the balance, every dollar of principal you remove early stops accruing interest — not once, but every month for the remaining life of the loan. Extra payments are therefore worth far more early on, which is exactly when the schedule feels most discouraging.
| Strategy on the same loan | Paid off in | Total interest | Interest saved |
|---|---|---|---|
| Minimum payments only | 360 months | $510,178 | — |
| +$100 a month | 322 months | $446,261 | $63,917 |
| +$200 a month | 293 months | $398,286 | $111,892 |
| $10,000 lump sum at the start | 335 months | $454,851 | $55,327 |
That single $10,000 lump sum is the cleanest illustration of the whole guide: it "earns" $55,327 — a better than 5-to-1 return — because it removes $10,000 of interest-bearing balance for nearly three decades. No investment you can buy offers a guaranteed return like that, which is precisely how to think about prepayment: each extra dollar earns a risk-free, after-tax return equal to your mortgage rate. At 6.5%, that is a genuinely good return. At a 3% mortgage rate, it is a mediocre one, and investing the surplus instead usually wins on the maths (if not on the peace of mind). The rate on your loan is the hurdle any alternative has to beat.
And the 15-year mortgage?
Same $400,000, same 6.5%, but a 15-year term, costs $3,484.43 a month — $956 more — and a total of $227,197 in interest: $282,981 less than the 30-year. Nothing magical is happening; a shorter loan forces the principal slice to be big from month one (about $1,318 of the first payment), so the balance — and therefore the interest charged on it — collapses quickly. In practice 15-year loans also carry lower rates than 30-year loans, which widens the saving further. The catch is purely human: the higher payment is compulsory, in good months and bad, while the 30-year-plus-prepayments route lets you choose your overpayment month by month. Many borrowers sensibly take the flexible option and then imitate the 15-year schedule voluntarily.
A popular variant deserves a footnote: biweekly payments. Pay half your monthly payment every two weeks and you make 26 half-payments a year — the equivalent of 13 monthly payments, not 12. That one extra payment a year, worth $2,528 annually on our loan, behaves almost exactly like the +$200 strategy with the training wheels on: the mortgage clears in about 290 months and total interest falls to roughly $393,833, saving some $116,345. If your lender offers a genuine biweekly plan without fees, it is the rare piece of financial packaging that does what the brochure says.
The other lever: one percentage point of rate
Everything so far held the rate at 6.5%. Watch what a single point does to the identical $400,000, 30-year loan:
| Rate | Monthly payment | Total interest |
|---|---|---|
| 5.5% | $2,271.16 | $417,616 |
| 6.5% | $2,528.27 | $510,178 |
| 7.5% | $2,796.86 | $606,869 |
One point of rate is worth $92,562 across the loan — which is why refinancing a large balance when rates fall by a point or more is usually worth its paperwork, and why the rate you negotiate at purchase follows you for decades. It also reframes the prepayment decision from the previous section: extra payments earn your loan's rate. On a 5.5% loan that is a 5.5% guaranteed return; on a 7.5% loan it is 7.5%, which very few safe investments can promise.
The three takeaways
- Amortization is just interest-on-the-balance, repeated. There is no front-loaded fee; there is a front-loaded balance.
- The balance is a glacier for 15 years, then an avalanche. Expect to owe about three-quarters of the loan at the halfway mark of a 30-year term.
- Early extra dollars hit hardest. Small overpayments in the early years are worth several times their face value in saved interest — run your own rate and balance through the calculator above to see your schedule bend.
All figures: author's calculations for a $400,000 loan at 6.5% fixed, monthly payments, no taxes, insurance or fees. Payment #360's interest shown is the final month's interest charge. Your lender's schedule will match to the cent; that is the point of amortization.
Frequently asked questions
Why is so much of my early payment interest?
Because interest is charged on the balance, and the balance starts at its largest. On a $400,000 loan at 6.5%, the first month's interest is $2,166.67 of a $2,528.27 payment. The interest share falls only as the balance does — slowly at first, then faster.
How much of my loan is left halfway through a 30-year mortgage?
More than you would guess: about $290,237 of the original $400,000 — 73% — is still owed after 15 years of payments. The halfway point of the balance does not arrive until around year 20, when the principal portion of each payment finally passes the interest portion.
Is it better to make extra payments or invest the money?
An extra payment earns a guaranteed return equal to your mortgage rate — 6.5% in our example, after tax, with zero risk. Investing might beat that, or might not. The mortgage side of the trade is a sure thing; the investing side is an expectation. Many people split the difference, and the right answer also depends on liquidity: extra payments are locked in the house.
Does one extra payment a year really help?
Yes. Paying an extra $200 a month on our example loan clears it 5.6 years early and saves $111,892 of interest. Even a single $10,000 lump sum paid at the start saves $55,327 — because every dollar of principal removed early is a dollar that never accrues interest again for decades.
Why does a 15-year mortgage cost so much less overall?
Two reasons: you are borrowing for half the time, and the balance falls fast from day one. At the same 6.5% rate, a 15-year loan on $400,000 costs $227,197 of interest against $510,178 for the 30-year — though the monthly payment jumps from $2,528 to $3,484, and 15-year loans usually carry lower rates in practice, widening the gap further.
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