The power of an extra principal payment
A mortgage payment is really two payments bundled into one: interest, the rent you pay the bank for the money, and principal, the part that actually reduces what you owe. When you send extra money and mark it for principal, you're not prepaying next month's bill — you're permanently erasing a slice of the balance, along with every future month of interest that slice would have cost. That's the entire mechanism behind this calculator, and it's why a relatively small, consistent extra payment can knock years off a 30-year loan.
How paying extra works
Interest is charged each month on whatever balance remains — not on the original loan amount. On the worked example below, the very first month's interest is $1,950, because the full $360,000 balance is still outstanding. Pay that balance down faster, and every future interest calculation starts from a smaller number. This is also why timing matters: a dollar of extra principal paid in year one eliminates decades of future interest on that dollar, while the same dollar paid in year twenty-five only eliminates a few years' worth. Extra payments are most powerful early, but they still help at any point in the loan — there's just less remaining interest left to cancel out the later you start.
A worked example
Take a $360,000 loan at 6.5% for 30 years. Paid on schedule with no extras, it costs $459,160 in total interest over the full term. Add just $200 extra toward principal every month, and the loan pays off 6 yr 1 mo sooner while saving $108,917 in interest — money that never leaves your pocket in the first place, rather than a return you have to go earn somewhere else. Plug in your own loan amount, rate, and extra payment in the calculator above to see your exact numbers, including a target-date mode that works the problem backwards: tell it when you want to be mortgage-free, and it solves for the monthly extra payment required to get there.
Should you pay extra, or invest instead?
This is a genuine tradeoff, not a question with one right answer for everyone. Paying down your mortgage early is a guaranteed, risk-free return equal to your interest rate — every extra dollar you send against a 6.5% mortgage is a 6.5% return, locked in, with no volatility. Investing that same dollar in the market has historically returned more than that over long periods, but "historically" and "on average" hide a lot of variance: markets can be down for years at a stretch, and unlike your mortgage payoff, that risk is real and can't be diversified away entirely.
A few things worth weighing before you decide: tax treatment — money in a tax-advantaged retirement account grows differently than home equity, and mortgage interest may or may not be deductible for you; your rate — the case for paying extra is much stronger at 7% than at 3%, since the guaranteed return you're giving up by investing instead scales directly with your rate; liquidity — money paid into your home is much harder to get back out than money in a brokerage account, so extra principal payments are a one-way door until you sell or refinance; and risk tolerance — a guaranteed 6.5% return with zero volatility is worth more to some people than a probably higher but genuinely uncertain market return, purely for the peace of mind. Most financial planners suggest building a fully funded emergency fund and capturing any employer retirement match first, since those come before either option — an emergency fund because you can't easily pull equity back out of your house in a crisis, and a match because it's an immediate, guaranteed return that beats both alternatives. After that, splitting extra money between both goals is a perfectly reasonable way to hedge the decision rather than betting everything on one path.
If PMI is part of your payment, extra principal has a second benefit beyond interest savings: it pulls your loan-to-value ratio down faster, which can mean requesting PMI cancellation sooner too — see the PMI removal calculator to find your exact request-eligible date and how extra payments move it.
Watch out for
Prepayment penalties. Most conventional loans issued today don't have them, but some do — particularly certain non-QM, investment-property, or older loans. Check your note or call your servicer before sending a large extra payment.
Making sure extra goes to principal, not next month's payment. Some servicers default to applying an overpayment toward your next scheduled payment instead of the principal balance, which does nothing to shorten your loan. Confirm how to designate a payment as "extra principal" with your servicer, and check your next statement to make sure it landed correctly.
Keep an emergency fund first. Extra principal is effectively illiquid — once it's paid in, getting it back out means a sale, refinance, or home equity loan, none of which are fast or free. Build a cash cushion for real emergencies before directing extra money at the mortgage. If you haven't worked out your baseline payment yet, start with the mortgage payment calculator before layering extra payments on top here.
Frequently asked questions
How much does paying extra on my mortgage actually save?
It depends on your rate, balance, and how early you start — but the effect compounds. On a $360,000 loan at 6.5% for 30 years, an extra $200 a month saves about $108,917 in interest and pays the loan off 6 yr 1 mo sooner. Enter your own numbers in the calculator above to see your figures.
Why do extra payments made early save more than the same payments made later?
Interest each month is charged on your current balance. Early in the loan, the balance — and therefore the interest — is at its highest, so extra principal paid then eliminates the most future interest. The exact same extra dollar paid in year 25, when the balance is small, saves far less. This is also why refinancing into a new 30-year term can erase years of front-loaded interest progress even if the rate is lower.
Should I pay extra on my mortgage or invest the money instead?
There's no universally correct answer — it's a tradeoff between a guaranteed return (your mortgage rate) and a potential but uncertain return (the market). See the "Should you pay extra, or invest instead?" section below for the factors worth weighing before you decide.
Will my lender apply my extra payment to principal automatically?
Not always. Many servicers will apply an overpayment to next month's payment (pre-paying, not accelerating) or to escrow first, unless you explicitly mark it for principal. Always confirm with your servicer how to designate an extra payment, and check your next statement to make sure it landed as extra principal instead of an early next payment.
Does my loan have a prepayment penalty?
Most conventional mortgages issued today don't, but some loans — particularly certain non-QM, investment-property, or older loans — do. Check your loan note or ask your servicer before making large extra payments, since a penalty can offset some or all of the interest you'd otherwise save.
What's the difference between an extra payment and recasting?
An extra payment reduces your balance but keeps your required monthly payment the same — you keep paying the original amount, and the loan just finishes early, which is what this calculator models. Recasting uses a lump sum to re-amortize the loan at the same rate and remaining term, which lowers your required monthly payment instead of shortening the loan. Not all servicers offer recasting, and it usually comes with a small fee.
Is a biweekly payment plan the same as paying extra?
Functionally, yes. Paying half your monthly payment every two weeks works out to 26 half-payments a year — the equivalent of 13 full monthly payments instead of 12. That extra payment is pure principal, which is why the "pay biweekly instead" option in the calculator above models it as one extra monthly payment spread across the year. Some servicers charge a fee to enroll in a formal biweekly draft; you can get the identical result for free by just adding 1/12 of your payment to your principal each month yourself.
How should I use the "target payoff date" mode?
Switch to "Set a payoff goal" and enter how many years from now you want the loan gone. The calculator searches for the smallest extra monthly payment that hits that date, accounting for any lump sum or annual extra payment you've also entered — so you get one concrete number to actually budget for instead of guessing.