Why mortgage prepayment is so powerful — and so underused
A 30-year mortgage is structured so that the early years are almost pure interest. On a $320,000 loan at 6.75%, your first monthly payment of ~$2,075 splits roughly $1,800 to interest and only $275 to principal. The bank gets rich in the opening innings. Any extra dollar you route to principal permanently eliminates every future interest charge that dollar would have generated — a small prepayment today is worth many times more than the same prepayment late in the loan.
A real example: the $280,000 mortgage
Dan and Carla, both 38, have a $280,000 mortgage at 6.5% with 27 years remaining. Their standard payment is $1,770/month. Here's what different extra payment amounts do:
- No extra: Paid off in 27 years (age 65), $326,000 in remaining interest
- $100 extra/month: Paid off in 23.5 years (age 61.5), saves approximately $38,000 in interest
- $300 extra/month: Paid off in 19.5 years (age 57.5), saves approximately $80,000 in interest
- $500 extra/month: Paid off in 17 years (age 55), saves approximately $105,000 in interest
An extra $300/month saves $80,000 in interest and means Dan and Carla own their home outright 7.5 years sooner. That $300/month paid in for 7.5 fewer years — 90 payments of $1,770 not made — is worth more than $159,000 in avoided future payments.
How to use this calculator
- Find your loan details: Log into your mortgage servicer's portal and find your current principal balance (not the payoff amount, which includes accrued interest), your interest rate, and your remaining term in months.
- Enter your current payment: The principal + interest portion of your monthly payment (exclude escrow for taxes and insurance if your servicer bundles them — those don't affect the loan payoff math).
- Enter an extra amount: Start with a realistic number — what could you sustainably add every month? Even $50 matters over 20+ years.
- Read the results: Months saved, interest saved, new payoff date. The balance curve shows graphically where your standard payoff line crosses zero vs the accelerated payoff line.
The "one extra payment a year" trick
The most famous mortgage trick is making 13 payments a year instead of 12 (either as one lump extra payment, or by paying biweekly so you sneak in an extra half-payment twice a year). On a typical 30-year mortgage, this alone shaves about 4 years off the term and saves tens of thousands in interest. The calculator above lets you test any extra amount — if "one extra payment a year" is ~$175/month on your loan, try it.
Should I prepay the mortgage at all?
This is the big debate. The case for prepaying: it is a guaranteed, tax-free return equal to your mortgage rate. At 6.75%, that is hard to beat in bonds or savings. The case against: stock market returns have historically averaged ~10%, mortgage interest may be deductible, and liquidity matters (a prepaid mortgage is money you cannot easily get back). Most financial planners split the difference: fund retirement accounts to the employer match, keep a real emergency fund, then prepay the mortgage with whatever is left.
Recasting vs refinancing vs prepaying
Three different tools, often confused. Prepaying shortens the term while keeping the same monthly payment. Recasting (a formal lender-permitted re-amortization after a lump sum) keeps the term but lowers the monthly payment. Refinancing replaces the loan entirely — new rate, new term, closing costs. Prepaying is almost always the right default because it has zero cost and requires no paperwork.
When not to prepay
If you have any credit card debt, stop reading this and go to our credit card payoff calculator— a 22% APR destroys a 6.75% mortgage in importance. Same logic applies to auto loans above 8% and personal loans above 10%. The mortgage is usually the last debt to attack.
The write-it-on-the-check trick
When you send extra money to your mortgage, write "apply to principal" in the memo line or use the dedicated principal-only payment option in your lender's portal. Otherwise the servicer may credit it as a future payment and you will get no early payoff benefit at all. This is the single most common mistake mortgage prepayers make.
Frequently asked questions
Is there a prepayment penalty on my mortgage?
Under the Dodd-Frank Act, prepayment penalties on most residential mortgages issued after January 2014 are severely restricted and largely prohibited on qualified mortgages. If your mortgage was issued before 2014 or is non-qualified, check your loan agreement or call your servicer. Most modern mortgages have no prepayment penalty.
Should I recast instead of prepay to lower my monthly payment?
Recasting re-amortizes your loan over the remaining term with the reduced principal after a lump-sum payment. This lowers your required monthly payment but doesn't shorten the loan unless you continue making the same or higher payment. Recasting makes sense if you want to lower your minimum payment for cash flow flexibility — for example, if you're approaching a period of lower income. Most servicers charge $200–$500 for a recast.
How does a biweekly payment schedule work, and is it worth it?
Biweekly payments split your monthly payment in half and you pay every two weeks. Because there are 52 weeks in a year, you end up making 26 half-payments = 13 full payments instead of 12. That extra payment each year goes entirely to principal. On a $280,000 30-year mortgage at 6.5%, this alone saves about $42,000 in interest and cuts 4 years off the loan. Beware: some servicers charge fees for biweekly setups. You can replicate the effect for free by adding 1/12 of your monthly payment as extra principal each month.
My rate is 3.25% (locked in 2021). Should I still prepay?
At 3.25%, the math case for prepaying is weak. A guaranteed 3.25% return (by paying down the mortgage) competes poorly against even conservative investments. I-bonds, money market funds, and index funds all have realistic cases for beating 3.25% after tax. For most households with a sub-4% mortgage, prioritize maxing tax-advantaged retirement accounts and building investment assets before prepaying the mortgage.
What's the difference between interest saved and total cost reduction?
Interest saved is the interest you avoid by paying off early. Total cost reduction is interest saved minus the extra principal you paid in ahead of schedule. Example: if you pay $300 extra for 5 years ($18,000 extra principal) and save $60,000 in interest, your net benefit is $42,000 — but your actual out-of-pocket over the loan life is still lower by $60,000 because those 60 months of future payments don't happen. Both numbers matter depending on how you frame the question.
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For education only. Not financial advice.