Mortgage math

What do extra payments actually do to your mortgage?

An extra $200 a month on a typical 30-year mortgage shaves roughly five to seven years off the term and saves $50,000-$80,000 in total interest. Biweekly payments — half the monthly amount every two weeks — produce a similar effect by adding one extra full payment per year. The numbers are surprising and the math is exact. This calculator runs both scenarios honestly.

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Loan inputs

Home price
$
Down payment
%
Mortgage rate
%
Term
yrs

Acceleration scenario

Extra monthly principal
$
Added to every monthly principal payment, starting month 1.
One-time extra principal
$
Single lump-sum applied at month 1.
How this is calculated

Standard amortization with extra principal. Each month: interest = balance × monthly rate; principal = (regular P&I − interest) + extras; new balance = old balance − principal. Loop terminates when balance reaches zero. The accelerated scenario applies your extras starting month 1.

Biweekly handling. Set as half the regular monthly payment, applied every 14 days. Over a year that's 26 half-payments (13 monthly equivalents), so you make one extra full monthly payment per year. The calculator translates this to monthly equivalents for the comparison table — the actual cash flow is biweekly.

What this assumes: a fixed-rate mortgage, no prepayment penalty, no missed payments, no rate changes. Most modern U.S. mortgages have no prepayment penalty, but check your loan documents to confirm. More on prepayment penalties.

Read the full methodology →Defaults reviewed May 2026

The acceleration result
—
years off the loan
—
Baseline (no extras)
Monthly payment—
Term—
Total interest—
Total of payments—
Accelerated
Effective monthly—
Payoff in—
Total interest—
Total of payments—
Practical note: these savings assume the extra amounts are applied every month for the full life of the loan. If you make extras for a few years and then stop, the savings will be smaller — but still meaningful, because early extras have compound effect on later interest.
Why extra payments work so well

Compound interest in reverse — that's what early payoff is.

Mortgages are amortizing loans where each month's interest is calculated on the remaining balance. When you make an extra principal payment, you eliminate that principal from the calculation forever — and you also eliminate the interest that would have accrued on it for every remaining month. An extra $200 in month 1 of a 30-year loan eliminates $200 of principal plus the interest that $200 would have generated across 359 future months. That's why even small extras compound into large total savings.

Where extras have the biggest effect

Early in the loan. The first extra dollar paid in month 1 saves more total interest than the last extra dollar paid in month 359. The compounding asymmetry means the strategic choice — when can you afford to start? — is more valuable than the size choice — how much extra? An extra $100/month starting today usually saves more total interest than an extra $300/month starting in five years.

Biweekly vs. monthly extras — what's the difference?

Mathematically, biweekly produces roughly one extra monthly payment per year (because there are 26 biweekly periods in a calendar year, vs. 24 if it were truly twice-monthly). On a typical mortgage, biweekly knocks 5-6 years off a 30-year term and saves a substantial chunk of interest. The same effect can be achieved by adding 1/12th of your monthly payment to each regular monthly payment — with no need for a special biweekly setup.

Some lenders charge a fee for biweekly enrollment ($300-$500). The fee is rarely worth it; you can replicate the strategy by manual extras. Some lenders require biweekly half-payments to be held in a suspense account until the second half arrives — confirm with your specific lender how the payments are applied.

When extras don't pencil out

Three cases where extra mortgage payments are usually a worse use of cash than the alternatives:

  • You don't have an emergency fund. Reserves come first. Mortgage extras are illiquid — you can't pull them back out without a refinance or HELOC, both of which take time and cost money. Cash in a high-yield savings account is liquid; cash paid into a mortgage isn't.
  • Higher-interest debt exists. A 22% credit card balance is a much better use of extra cash than a 6.75% mortgage. The math is straightforward: pay off the highest-rate debt first.
  • Tax-advantaged retirement accounts aren't being maxed. A 401(k) employer match is typically a 50-100% immediate return on your contribution — far better than any mortgage prepayment. IRA and 401(k) contributions also reduce taxable income now, which has compounding tax benefits over decades.

The right framing for early payoff: it's a low-risk, moderate-return use of cash that's only the best option once higher-priority allocations are handled. For households with strong emergency reserves, no high-interest debt, and maxed retirement contributions, mortgage prepayment is a reasonable use of marginal cash. For households without those foundations, prepayment is usually premature.

FAQ

Common early-payoff questions.

Will my lender accept extra principal payments?
Yes, in nearly all cases for U.S. residential mortgages originated after 2014. The Qualified Mortgage rule effectively prohibits prepayment penalties on most owner-occupied home loans. Check your loan documents to confirm; if you have an older loan or a non-QM loan, prepayment terms can vary. Some lenders require you to label extra payments as "principal only" — usually via a checkbox in their online portal — so they don't get applied as advance payment of next month's regular bill.
Should I refinance to a 15-year instead of paying extra on a 30-year?
Sometimes. A 15-year refinance typically gets a slightly lower rate (0.25-0.5%) and forces the discipline of higher payments. But it commits you to those higher payments — if you lose income, you can't dial back to the 30-year payment. Voluntary extras on a 30-year give you flexibility (skip extras during tight months, resume in good months) at a slightly higher rate. For households who value flexibility, the 30-year-with-extras approach is often the better fit. The Refinance vs. Keep calculator models the explicit refi math.
Does this account for tax effects?
No. Mortgage interest is potentially deductible on federal returns if you itemize, which complicates the after-tax cost of the loan. For households who itemize, the effective interest rate is lower than the stated rate, which slightly reduces the value of prepayment. Most U.S. households now take the standard deduction (the 2017 tax law raised the threshold), so itemization affects fewer borrowers than it used to. For tax-aware planning, work with a CPA.
Why is the "effective monthly" higher than my actual payment?
In biweekly mode, you're paying half your monthly amount every 14 days. Over a calendar year, that's 26 half-payments — equivalent to 13 monthly payments. The "effective monthly" figure is your total annual outflow divided by 12, which works out to about 8.3% higher than your nominal monthly payment. The 13th payment is the source of the extra principal that accelerates payoff.
What happens if I stop making extras after a few years?
The savings will be smaller than what this calculator shows, but still real. Each early extra payment permanently eliminates the interest that would have accrued on that principal for the rest of the loan — even after you stop adding new extras. If you make $200/month extras for 5 years and then stop, you'll still have shaved about 2-3 years off a 30-year term and saved a meaningful chunk of interest, though much less than the full-life-of-loan scenario. The asymmetry is why early extras matter most.
Is this investment advice?
No. OwningCost is not a financial advisor. Mortgage prepayment vs. investing the difference is a household-specific decision that depends on your interest rate, expected investment returns, tax situation, risk tolerance, and other priorities. For investment-allocation decisions, work with a CFP.
Related calculators & guides

Related tools.

Calculator

Amortization Schedule

The baseline schedule with no extras. See how each payment splits and where the principal-vs-interest crossover happens.

 Calculator

Refinance vs. Keep

The other path to a shorter term. Refi math: closing costs, break-even, total savings.

 Calculator

True Monthly Cost

Make sure the extra-payment cash isn't needed elsewhere first. Full monthly cost picture.

 Risk

How to Reduce Risk

Where mortgage prepayment fits in the cash-priorities stack: reserves first, high-interest debt next, then prepay.

 Reference

Glossary

Amortization, principal, prepayment penalty — every term defined plainly.
See the full schedule

Run the baseline amortization first.

Before deciding on extras, look at how the baseline loan amortizes. The Amortization Schedule calculator shows month-by-month and year-by-year — the same loan, just without the acceleration scenario.

Open Amortization Schedule Visit Financing hub