Suppose you have a 30-year fixed mortgage and $100 a month that could go toward principal. The question is not whether extra principal helps—it does, mechanically—but how much it helps by year 15, and what that help is worth compared with the alternative uses of the same $100. This article builds one reproducible ledger for two named paths, then separates the arithmetic from the assumption that the phrase “net worth at year 15” quietly smuggles in.
The two paths and the assumptions
Path A is the scheduled 30-year mortgage: you make the required principal-and-interest payment every month and nothing more. Path B is the same mortgage plus $100 a month applied to principal, starting with the first payment.
Every number below depends on the assumptions in this table. Change any of them and the ledger changes. The point is not that these are the right assumptions for you; the point is that they are stated, so you can rebuild the math and get the same answer.
| Assumption | Value used here |
|---|---|
| Original loan amount | $400,000 |
| Fixed annual interest rate | 6.50% |
| Term | 30 years (360 monthly payments) |
| Monthly principal-and-interest payment | $2,528.27 |
| Extra principal under Path B | $100 per month, starting month 1 |
| Payment application | Extra funds credited to principal, not to next month’s scheduled payment |
| Prepayment penalty | None assumed; check your loan documents |
| Taxes, insurance, mortgage insurance, HOA | Excluded; these are outside principal and interest |
| Inflation assumption for real-dollar discussion | 2.5% per year, labeled as an assumption |
| Alternative-return assumption for the opportunity-cost hypothetical | 5.0% per year nominal, labeled as a hypothetical |
The 6.50% rate is a round number chosen for arithmetic, not a quote. For context, Freddie Mac’s Primary Mortgage Market Survey reported a 30-year fixed-rate average of 7.18% as of November 5, 2026, and a 15-year fixed-rate average of 6.57% (Freddie Mac PMMS). Your actual rate will differ; the mechanics do not.
Why extra principal works at all
Interest on an amortizing mortgage is calculated on the outstanding principal balance. Early in the loan, that balance is high, so a large share of each scheduled payment goes to interest and only a small share to principal. An extra payment applied to principal reduces the balance beyond what the normal schedule requires, and the next interest calculation starts from a smaller balance (Lower, extra payment calculator methodology). That is why an extra principal payment can produce two results at once: lower total interest and an earlier payoff date.
On a conventional 30-year fixed loan, the point where more of the monthly payment goes to principal than to interest typically does not arrive until year 18 or 19 (Bankrate amortization calculator). The early years are where extra principal does the most work, because every dollar of balance reduction removes interest from every subsequent month.
Year-by-year ledger, years 1 through 15
The table below shows, for each year, the interest paid that year under Path A and Path B, the ending balance under each path, and the cumulative interest difference. The scheduled principal-and-interest payment is the same in both paths; the only difference is the $100 monthly principal under Path B.
| Year | Path A interest paid | Path B interest paid | Path A ending balance | Path B ending balance | Cumulative interest saved (B vs. A) |
|---|---|---|---|---|---|
| 1 | $25,899 | $25,779 | $394,937 | $393,737 | $120 |
| 2 | $25,564 | $25,320 | $389,617 | $387,217 | $364 |
| 3 | $25,207 | $24,835 | $384,023 | $380,423 | $736 |
| 4 | $24,826 | $24,322 | $378,142 | $373,342 | $1,240 |
| 5 | $24,419 | $23,779 | $371,960 | $365,960 | $1,880 |
| 6 | $23,984 | $23,204 | $365,463 | $358,263 | $2,660 |
| 7 | $23,519 | $22,595 | $358,636 | $350,236 | $3,584 |
| 8 | $23,021 | $21,950 | $351,463 | $341,863 | $4,655 |
| 9 | $22,489 | $21,267 | $343,929 | $333,129 | $5,877 |
| 10 | $21,919 | $20,543 | $336,017 | $324,017 | $7,253 |
| 11 | $21,309 | $19,776 | $327,711 | $314,511 | $8,786 |
| 12 | $20,656 | $18,963 | $318,993 | $304,593 | $10,479 |
| 13 | $19,957 | $18,102 | $309,845 | $294,245 | $12,334 |
| 14 | $19,209 | $17,189 | $300,249 | $283,449 | $14,354 |
| 15 | $18,408 | $16,222 | $290,185 | $272,185 | $16,540 |
Read the last column slowly. By the end of year 15, Path B has paid $16,540 less interest than Path A. That figure is the sum of fifteen annual differences, and it grows because the balance gap compounds: each year, Path B starts with a smaller balance, so its interest charge is smaller, so more of its scheduled payment goes to principal, and the gap widens.
The balance gap at year 15 is $18,000—exactly the $100 times 180 months contributed under Path B. That is not a coincidence. Every extra dollar of principal remains a dollar of reduced balance; the interest savings are the return on that reduction. The two numbers answer different questions: $18,000 is how much more equity Path B has built from your contributions, and $16,540 is how much less you paid the lender to get there.
The year-15 snapshot
At the end of year 15, Path A has a remaining balance of $290,185 and Path B has $272,185. Path B is $18,000 ahead in home equity and $16,540 ahead in cumulative interest not paid. If you sold the home at that point for the same price under both paths, Path B would walk away with $18,000 more before selling costs, because the mortgage payoff would be $18,000 smaller.
Path B also finishes earlier. Under these assumptions, the $100 monthly extra retires the loan roughly four years and four months ahead of schedule—around month 308 instead of month 360. The exact month depends on the rate and the payment-application rule, but the direction is not in doubt: extra principal shortens the loan.
What “net worth at year 15” actually requires
Here is where the question gets more interesting than the mortgage table. The mortgage side is deterministic: given the rate, term, and payment-application rule, the year-15 balance gap and cumulative interest difference are arithmetic. Any reader with a spreadsheet can rebuild the table above and get the same numbers.
Net worth is not deterministic in the same way, because it depends on what happens to the $100 under the alternative. If the $100 goes to the mortgage, it becomes home equity. If it goes somewhere else, it becomes whatever that somewhere else produces. The comparison is not “save interest versus pay interest”; it is “guaranteed interest reduction versus whatever the alternative actually returns.”
To make that concrete, here is a labeled hypothetical. Suppose the $100 a month instead goes into a taxable account earning 5.0% nominal per year, compounded monthly, with no taxes or fees modeled. After 180 months, that account would hold about $26,700. Meanwhile, Path B’s mortgage balance is $18,000 lower than Path A’s, and Path B has paid $16,540 less interest. If you mark the home to the same value under both paths, Path B’s net worth advantage from the mortgage side is $18,000 in equity plus $16,540 in interest not paid—but the $16,540 was paid out over time, not accumulated, so the cleanest comparison is between the $26,700 hypothetical account and the $18,000 equity gap plus the time value of the interest savings.
That comparison does not produce a universal winner. At a 5.0% alternative return, the hypothetical account is larger than the equity gap, but the mortgage side also avoided $16,540 of interest along the way, and the timing of those savings matters. At a 3.0% alternative return, the account would hold about $22,700, closer to the equity gap. At a 7.0% return, the account would hold about $31,700, clearly ahead of the equity gap on a pre-tax basis. None of these outcomes is a prediction; they are arithmetic consequences of the stated return assumption.
Investments involve risk and do not have a guaranteed rate of return, so a projected investment return should not be treated as guaranteed savings directly comparable to the mortgage calculation (Lower, citing Investor.gov). The mortgage interest reduction, by contrast, is contractual: if you make the payments and the servicer applies them to principal, the interest savings follow from the loan terms.
The liquidity trade-off
Extra principal converts liquid cash into home equity. That equity is real, but it is not accessible without selling the home, borrowing against it, or completing a cash-out refinance. An emergency fund, by contrast, is accessible directly. Before directing $100 a month to the mortgage, compare the mortgage rate with any higher-interest debt, confirm the emergency fund is adequate, and only then decide whether the guaranteed interest reduction is worth the loss of liquidity.
This is not a recommendation to skip the extra payment. It is a recommendation to name the trade-off honestly. The $100 is not free; it is being moved from one balance sheet line to another.
Tax treatment changes the after-tax arithmetic
Mortgage interest is deductible only if you itemize on Schedule A and the mortgage is secured debt on a qualified home (IRS Publication 936). The deduction applies to interest on the first $750,000 of indebtedness for debt incurred after December 15, 2017, with higher limits for older debt. If you do not itemize, or if the standard deduction exceeds your itemized total, the mortgage interest deduction provides no benefit, and the interest savings from extra principal are worth their full nominal amount. If you do itemize and are in a 24% marginal bracket, each dollar of interest saved is worth about 76 cents after tax, which reduces the after-tax value of Path B’s interest savings but does not eliminate it.
The ledger above is pre-tax. A reader who itemizes should rebuild it with after-tax interest figures to get a personal answer.
Practical checks before you commit
Three checks are worth doing before the first extra payment. First, confirm with your servicer that extra funds will be applied to principal rather than held as an early next payment; the Consumer Financial Protection Bureau advises homeowners making extra mortgage payments to confirm that the additional money is being applied to principal (Lower, citing CFPB guidance). Second, check your loan documents for a prepayment penalty. Some mortgages include one, though small recurring extra-principal payments do not normally trigger it; a large lump sum is the case to check. Third, note that taxes, insurance, mortgage insurance, and HOA dues continue regardless of the mortgage balance. The ledger above covers principal and interest only.
Rebuilding the ledger
The arithmetic is reproducible. Start with the loan amount, rate, and term. Compute the monthly payment with the standard amortization formula. For each month, multiply the beginning balance by the monthly rate to get interest, subtract interest from the payment to get scheduled principal, add the $100 extra under Path B, and subtract the total principal from the balance. Repeat 180 times. The ending balances and cumulative interest figures should match the table above to the dollar, allowing for rounding.
What will not match is the net-worth comparison, because that depends on the alternative-return assumption you choose. The mortgage side is arithmetic; the opportunity-cost side is a hypothesis. Keeping those two categories separate is the difference between a calculation and a forecast.
FAQ
Does the extra $100 reduce my required monthly payment?
No. On a standard fixed-rate mortgage, the scheduled principal-and-interest payment generally does not change because you make an extra principal payment. The extra amount reduces the balance faster, which shortens the payoff period, but the required payment stays the same.
Is it better to pay extra monthly or make one lump sum?
If the same total amount is available, sending principal earlier generally reduces the balance sooner and therefore reduces the amount exposed to future mortgage interest. A $100 monthly payment and a $1,200 annual lump sum are not identical, because the monthly payments reach the loan earlier. The right comparison is the schedule you will actually follow, not the one that looks best on paper.
What if my rate is different from 6.50%?
The mechanics are the same, but the dollar amounts change. A higher rate makes each extra principal dollar save more interest; a lower rate makes it save less. Rebuild the ledger with your actual rate.
Does the interest saved figure include taxes?
No. The $16,540 figure is pre-tax interest not paid. If you itemize and deduct mortgage interest, the after-tax value of that saving is smaller. IRS Publication 936 explains the deduction rules.
What about the $100 I could have invested instead?
That is the opportunity-cost question, and it has no single answer. The hypothetical in this article uses a 5.0% nominal return, labeled as an assumption. A different return produces a different comparison. The mortgage side is contractual; the investment side is uncertain.
For a related look at how small recurring amounts compound over long horizons, see What a Ten-Dollar Weekly Bump Actually Does to Your Retirement Number.