One Extra $100/Month on Your Mortgage: 30-Year Math, Real Numbers
$100 extra a month turns a 30-year loan into a 26.5-year loan and kills $67,177.17 of interest. Every line of that math, plus what the same $100 is not.
A $360,000 mortgage at 6.75% costs $2,334.95 a month for thirty years, and $480,583.13 of what you pay over that time is interest. Pay $2,434.95 instead — the same payment plus one extra $100 — and the loan dies in 318 months instead of 360, taking $67,177.17 of interest with it.
That is the whole argument for the extra mortgage payment, and unlike most financial advice, it needs no assumptions about market returns. Here is the math, line by line, for a loan with numbers big enough to matter.
The baseline: where the first $2,334.95 goes
A 30-year amortization is brutal in the early years because interest is charged on the full balance every month:
| Line | Amount |
|---|---|
| Loan amount | $360,000.00 |
| Rate / term | 6.75% / 30 years |
| Monthly payment | $2,334.95 |
| First month's interest | $2,025.00 |
| First month's principal | $309.95 |
| Total interest over 30 years | $480,583.13 |
Look at the split: 86.7% of your first payment never touches your debt. It rents the money. The principal portion is $309.95, which means after a full year of paying on time you have retired less than $4,000 of a $360,000 loan.
What one extra $100 does
Send an additional $100 every month, directed at principal, and the simulation changes shape. The loan does not just finish a little early — the extra payment retires principal that would otherwise have accrued 6.75% interest, every year, for decades:
| Line | Minimum only | With +$100/mo |
|---|---|---|
| Monthly payment | $2,334.95 | $2,434.95 |
| Payoff time | 360 months | 318 months |
| Total interest | $480,583.13 | $413,405.96 |
| Interest saved | — | $67,177.17 |
| Time saved | — | 42 months (3 yr 6 mo) |
| Extra money deposited | — | $31,800.00 |
Read the last two rows together, because that is the trade: you deposit $31,800.00 extra over the years and the loan responds by erasing $67,177.17 of interest. The 30-year loan becomes a 26.5-year loan without a refinance, without closing costs, and without asking the lender for anything.
The mechanism is the first-month split from the table above. With the extra payment, your first month's principal jumps from $309.95 to $409.95 — roughly a third more principal retired from day one — and every dollar of principal you retire early stops generating 6.75% interest for the remaining life of the loan.
What the same $100 is not: a 15-year mortgage
The honest comparison is against the loan you did not take. At the same $360,000, a 15-year loan priced at 5.9% (lenders charge less for shorter terms) runs:
| Line | 15-year @ 5.9% | 30-year @ 6.75% |
|---|---|---|
| Monthly payment | $3,018.47 | $2,334.95 |
| Monthly difference | — | $683.52 |
| Payment as % higher | 29.3% | — |
| Total interest | $183,324.52 | $480,583.13 |
| Interest saved | — | $297,258.61 |
The 15-year loan costs $683.52 more per month — not $100 — and in exchange erases $297,258.61 of interest. That is 4.4 times what the extra $100 achieves, because $683.52 of extra principal compounds far harder than $100 does.
This is exactly why the extra-payment strategy is a middle path, not a substitute. If your income genuinely supports a $3,018.47 payment, the 15-year loan is the stronger move and always will be. If it does not, the extra $100 is the version of the same lever you can actually pull — and you can raise it. Every additional dollar against principal works identically; the payment difference between your current loan and the 15-year payment, applied as extra principal, reproduces the 15-year outcome on the 30-year contract.
The same trick on smaller loans, faster feedback
The mechanics are identical on any amortizing debt, and smaller balances show the effect sooner. Take a typical auto loan: $14,500 remaining at 7.9% APR with a $420 monthly payment. Adding $50 a month — half our mortgage example — saves $242 in interest and pays the loan off 5 months early.
Two things to check before sending extra money anywhere: that your lender applies overpayments to principal rather than as an early next-month payment, and whether your contract has a prepayment penalty. Most auto loans have none; mortgage prepayment penalties are rare on post-2014 loans but worth one phone call.
When the extra $100 is the wrong move
Three situations where this math should lose to something else:
Any debt with a higher rate comes first. The extra payment "earns" a guaranteed 6.75% — your mortgage rate. A credit card at 24% earns 24% by the same logic. Pay the expensive debt first; the ordering is arithmetic, not preference.
No emergency fund, no extra payments. Extra principal is illiquid. The bank will not let you withdraw it back when the water heater dies.
If you might need the money out of the house instead. Pulling cash back out later means a cash-out refinance — replacing the whole loan at today's rates to access yesterday's principal. That is the opposite direction of travel from prepaying, and the closing costs apply either way.
Run your own loan through the 15 vs 30 year mortgage calculator with this example pre-filled, then test the extra-payment mechanics on a smaller balance with the car loan payoff calculator — it shows payoff months and interest saved for any extra amount. If you are weighing pulling money out instead of putting it in, the cash out refinance calculator prices that side of the decision.
Run your own numbers: Open the 15 vs 30 year mortgage calculator with this example pre-filled