What Happens If I Pay an Extra $100 a Month?

Paying an extra $100 a month toward principal is one of the most painless wealth moves a homeowner can make. It is a small, forgettable amount — roughly the cost of a couple of streaming subscriptions — yet on a $300,000 loan at 6.5% it cuts about 4 years and roughly $57,000 in interest off a 30-year mortgage. Here is the exact math, how the savings scale, and the one mistake that erases it all.

The math on $100 a month

On a $300,000, 30-year loan at 6.5%:

• Base payment: $1,896 a month, with $382,600 in total interest over 30 years.
• With an extra $100/month: the loan pays off about 4 years early (around month 312 instead of 360), saving roughly $57,000 in interest.

That $57,000 comes from a total outlay of just $31,200 in extra payments ($100 × 312 months). You turn $31,200 into $57,000 in savings — effectively a guaranteed, tax-free 6.5% return on every extra dollar, because each one eliminates interest that would have accrued for the remaining life of the loan.

Why small amounts add up

The reason is how amortization works. Interest is charged every month on the remaining balance. Every extra dollar you send to principal shrinks that balance permanently, so every future month's interest is calculated on a smaller number — and a little more of each future payment goes to principal instead of interest. It is a compounding loop that quietly accelerates. The earlier you start, the bigger the effect, because an extra dollar in year one eliminates up to 30 years of interest on that dollar.

How the savings scale

The relationship is not linear — bigger extra payments save disproportionately more:

Notice that doubling $100 to $200 more than doubles the savings, and $500 a month saves nearly three times what $100 does. The more you can comfortably add, the faster the payoff. But even $50 a month is worth doing — it is not an all-or-nothing decision.

The one mistake that erases it

Extra payments only save money if they are applied to principal. Many servicers default to treating extra money as a prepayment of next month's bill — which saves you nothing, because the interest still accrues on schedule. Always write "apply to principal" or select "principal-only" when you send extra money, and check your next statement to confirm the balance dropped by more than the scheduled principal amount.

When NOT to pay extra

Before you automate the extra $100, make sure these are covered:

• Higher-interest debt first. Credit card debt at 20%+ APR should be paid off before prepaying a 6.5% mortgage. Always pay the highest rate first.
• An emergency fund. Extra principal is locked in home equity and hard to access. A 3-to-6-month cash cushion comes first.
• A low mortgage rate. If your loan is at 3% or 4%, investing the extra money may beat prepaying. See how to pay off your mortgage early for the full trade-off.

Frequently asked questions

Is it better to pay $100 extra monthly or $1,200 once a year?

Monthly wins slightly, because it reduces the balance sooner each month. But the difference is small — pick whichever you will actually stick with.

Does paying extra reduce my monthly payment?

No. It shortens the loan term and cuts total interest, but your required payment stays the same. (A recast is the way to lower the payment instead.)

Can I stop the extra payments anytime?

Yes. There is no contract for extra payments — add them or stop them whenever your budget changes.

Will my lender charge me for extra payments?

Most modern mortgages have no prepayment penalty, but check your note to be sure.

See exactly what any extra amount saves you with the HomeMath calculator's extra-payment field — it shows the new payoff date and total interest saved instantly.