Loan Prepayment: How Extra Payments Kill Interest

Where extra money really goes

A normal payment covers the month's interest first, then reduces principal. An extra payment skips the line: the full amount reduces principal, because the month's interest is already covered.

That principal reduction is permanent. Next month's interest is calculated on the smaller balance, so it shrinks, which means more of your regular payment hits principal, which shrinks the following month's interest further. It is a virtuous spiral.

This is why small extras punch far above their weight early in a loan, when balances and interest portions are largest. An extra $200 in year two of a mortgage destroys more future interest than the same $200 in year twenty.

The headline example: $200 a month

Take a $250,000 loan at 7% for 30 years. The monthly payment is about $1,663, and over 360 payments you would pay roughly $348,800 in interest alone, more than the loan itself.

Add $200 a month toward principal. The loan dies in about 262 months instead of 360, roughly 8 years early, and total interest falls to about $239,000. You save roughly $110,000 in interest.

Read that again: $200 a month, the cost of a few dinners out, erases $110,000 of interest and frees you eight years sooner. Few financial moves offer that ratio.

Lump sums: timing is everything

A $10,000 lump sum applied at the very start of that same loan cuts the term by about 43 months and saves roughly $62,000 in interest. The same $10,000 applied in year 15 saves far less, because there is less future interest left to kill.

The rule: prepay as early as you can. Bonuses, tax refunds and windfalls do their best work in the first third of the loan.

If you expect a lump sum later, like a maturing deposit, you can still plan around it: even scheduled future lump sums dramatically change the payoff date, which the calculator below will show you.

Biweekly and round-up tricks

Paying half your monthly amount every two weeks creates 26 half-payments a year, equal to 13 full monthly payments instead of 12. That one extra payment a year, applied to principal, typically shaves 4 to 6 years off a 30-year loan.

Set it up yourself rather than paying a company for a "biweekly program." Just divide your monthly payment by 12 and add that amount to each monthly payment; the effect is identical and free.

Rounding up works the same way on small loans. A $383 car payment rounded to $400 sends $17 extra monthly to principal, quietly cutting months off the term with money you never miss.

Prepay versus invest: the real comparison

Prepaying a 7% loan is a guaranteed, risk-free 7% return on that money, and it is after-tax, since you pay with after-tax dollars and the "return" is interest you no longer owe. Guaranteed 7% does not exist anywhere else.

Investing might earn 8 to 10% long-term, but with volatility and no guarantee. The honest comparison is risk-adjusted: would you borrow at 7% to invest in stocks? If not, prepaying beats investing for that dollar.

The crossover most planners use: prepay debt above roughly 6 to 7% before taxable investing, beyond capturing employer retirement matches. Below 4 to 5%, investing usually wins for long horizons.

When prepayment is the wrong move

Check for prepayment penalties first. Some loans charge fees for early payoff, especially in the first years. A penalty can erase the benefit, so read your note or ask your servicer.

Do not prepay while carrying higher-rate debt. Extra money belongs on the 22% credit card before the 7% mortgage, every time. Attack debts in descending rate order.

And do not drain your emergency fund to prepay. An illiquid extra principal payment will not help when the roof leaks or the job disappears. Keep 3 to 6 months of expenses liquid first.

How to prepay correctly

Tell your lender explicitly, in writing, that extra amounts apply to principal. Some servicers otherwise treat extras as early future payments, which does not reduce interest the same way.

Confirm there is no penalty and no minimum extra amount. Then automate the extra payment so it happens without monthly willpower.

Once a year, request an updated amortization schedule or check your payoff date online. Watching the date march earlier is the motivation that keeps the habit alive.

The crossover point: when principal takes the lead

On a standard 30-year loan at 7%, your early payments are mostly interest and the principal slice grows slowly. The crossover, where principal finally exceeds interest in a payment, arrives around year 22. Before that point you are mainly renting money; after it, you are mainly buying the house.

Extra payments drag the crossover earlier, and early extras move it the most. Every dollar of prepayment in year two does more work than the same dollar in year twenty, because it kills interest across more remaining years.

Ask your servicer for an updated amortization schedule once a year and watch the crossover approach. Seeing the principal column grow is the feedback loop that keeps prepayment habits alive through the boring middle years.

Refinance vs prepay: pick your weapon

Sometimes the best prepayment is a new loan. Refinancing that $250,000 balance from 7.5% to 6.25% five years in might drop the payment by roughly $190 a month. With $5,000 in closing costs, the breakeven is about 26 months; if you will stay longer, refinancing beats prepaying at the old rate.

Watch the term-reset trap: refinancing into a fresh 30-year loan restarts the amortization clock, which can erase the rate benefit. Refinancing into a shorter term, or keeping the old payment as a built-in prepayment, avoids this.

The power move is combining both: refinance to the lower rate, then keep paying your old higher payment. The difference becomes automatic principal prepayment on cheaper debt, and the loan collapses years ahead of schedule.

The recast option nobody mentions

A recast lets you make a large lump-sum payment and have the lender re-amortize the loan over the remaining term at the same rate, lowering your required monthly payment. A $50,000 recast on a $250,000 loan at 7% cuts the payment by roughly 20%, for a fee of only a few hundred dollars.

Unlike refinancing, there is no new loan, no appraisal and no rate change, so it is fast and cheap. It suits windfalls when your goal is breathing room in the monthly budget rather than the fastest payoff.

Choose based on your goal: straight prepayment shortens the term and saves the most interest, while recasting lowers the required payment and buys flexibility. If cash flow is tight, recast; if the rate is high and cash flow is fine, prepay straight.

One extra payment a year: the lazy prepayment

If monthly extras feel like a commitment, try the annual version: make one additional full payment each year, applied to principal. On the $250,000, 7%, 30-year example, that single extra $1,663 yearly payment cuts roughly 6 years off the loan and saves about $75,000 in interest.

The easiest funding source is a tax refund or annual bonus, money that arrives once and is painless to redirect before lifestyle absorbs it. Set a standing instruction with your lender each spring and the habit runs itself.

This is also the most flexible prepayment: skip it in a tight year with no penalty and no catch-up needed. Flexibility is why many borrowers sustain the annual extra for decades while monthly extras get abandoned.

Frequently asked questions