Reading your mortgage payoff numbers
The interest-savings figure above looks dramatic on a 30-year mortgage because compounding works in reverse — even modest extras shave years off and save tens of thousands. But that comparison is incomplete without the alternative-use cost of those dollars.
Standard rule: if your mortgage rate is below your expected long-run after-tax return on retirement investments (typically 4-5% for a balanced portfolio), prepaying loses you money in the long run. If your rate is above that, prepay has a fair argument. Above 7%, prepay almost always wins.
There's a separate behavioral case for prepaying — being mortgage-free at retirement is psychologically valuable in a way the math underprices. If certainty matters to you more than maximizing expected value, prepayment is reasonable even at lower rates.
Worked example: $400k mortgage at 7% over 30 years
Base monthly payment (P&I only) ~$2,661. Total interest paid over 30 years: about $558,000. That's the no-extras baseline — interest is roughly 1.4× the principal because of compounding over a 360-month term.
Add $200/month from month one onward and the mortgage pays off about 5 years and 7 months early (293 months instead of 360). Total interest drops to roughly $445,000. Savings: about $113,000 nominal. After applying a 24% marginal-bracket interest deduction on the lost write-off, the post-tax savings are closer to $86,000.
Same $200/month, but starting at month 120 (year 10) instead of month 1: pays off about 3 years 4 months early, saves around $53,000 nominal. Same dollar amount per month, same loan, less than half the interest savings. The extra-payments calculator above lets you swap the start month so you can see this on your own loan.
Want to finish the 30-year mortgage in 15 years instead? Same loan, same 7% rate. The 15-year amortizing payment on $400k is about $3,595/month, which is roughly $934/month above the 30-year P&I of $2,661. Add that $934 every month from month one and the loan pays off at month 180 — half the original term. Total interest drops from about $558,000 to about $247,000, a savings of roughly $311,000 nominal. The math is just: pay what a 15-year loan would have cost, on a 30-year contract you keep for the optionality. Drop the extras any month if cash flow tightens.
Mortgage prepayment reality checks
PMI: borrower-requested vs automatic termination (HPA)
Two distinct thresholds under the Homeowners Protection Act. Borrower-requested cancellation: at 80% LTV based on original purchase price, you can submit a written request — the servicer must cancel if conditions are met (current on payments, no junior liens, sometimes a fresh appraisal if home value matters). Automatic termination: at 78% LTV based on the original amortization schedule, on the date the loan was scheduled to reach 78%, provided you're current. Extra principal payments can let you request cancellation earlier at 80% actual balance, subject to servicer conditions — but they don't accelerate automatic termination, which is tied to the scheduled date.
Recast vs prepay vs refi — three different tools
Recast: lump sum lowers your monthly payment, keeps the same term. Prepay: keep the same monthly, shortens the term. Refi: new loan, new rate. Recast is best when you have a large lump and want cash-flow relief; prepay when you want to pay off faster; refi when rates drop more than ~0.75% from your current.
Watch for prepayment penalties on non-conforming loans
Conforming Fannie/Freddie loans (the typical 30-year fixed) cannot have prepayment penalties. Non-conforming, jumbo, or hard-money loans sometimes do. Read the note's prepayment clause; ask the lender for the actual penalty schedule before any lump sum.
The interest deduction shrinks the effective rate
If you itemize and deduct mortgage interest (limited to the first $750k of mortgage debt for loans after 2017), your effective rate is about 75-80% of the nominal rate at typical brackets. This narrows the gap with stock-market expected returns — a 7% mortgage acts like ~5.5% after taxes.
Check the Fed before refinancing
30-year mortgage rates correlate with the 10-year Treasury yield, not directly with Fed Funds. See /rate-outlook for current spreads — a Fed cut doesn't always translate to lower mortgage rates immediately.
What most mortgage payoff calculators don't tell you
Three things competitors typically miss. First: the prepay-vs-invest comparison should use AFTER-TAX rates on both sides. If you itemize and deduct mortgage interest, your effective rate is roughly 75-80% of the nominal rate at typical brackets — a 7% mortgage acts like 5.3% at a 24% marginal. Comparing nominal mortgage APR to gross expected stock returns inflates the prepay case.
Second: recasting is a third option that almost no calculator surfaces. A recast lets you apply a lump sum to principal AND lower your monthly minimum by re-amortizing the remaining balance over the original term. Total interest savings are similar to a straight prepayment, but cash-flow flexibility is much better. Most lenders charge $150-500; many borrowers don't know it exists.
Third: the 30-year-with-extras vs 15-year debate. A 15-year mortgage typically gets you 0.5-0.75% lower rate, but locks you into the higher minimum payment. A 30-year + matching extras gets the same payoff date with slightly higher total interest paid (because of the rate spread) but full optionality to drop the extras any month. For most borrowers, the optionality is worth more than the rate spread.
Frequently asked questions
Should I pay off my mortgage early?
At 7%+, paying down usually beats investing on an after-tax basis. Below 5%, a broad stock index has historically outperformed prepayment over 20-year horizons. Between 5–7%, it comes down to one thing: would you actually invest the freed-up cash, or would it disappear into spending? If the latter, pay the mortgage.
Does paying off a mortgage hurt my credit score?
Slightly and temporarily. Closing an installment loan removes a positive open account and changes the mix of credit on your file. Typical drop is 5-20 points, recovers within 3-6 months. Not a reason to keep a mortgage, but worth knowing if you have a credit-sensitive event (lease, refi, business loan) within 6 months.
Recast or refinance — which saves more?
Refinance saves more if rates have dropped meaningfully (>0.75% from your current). Recast saves more if you have a lump sum and your current rate is already competitive. Recast also has a much smaller fee ($150-500) vs refinance closing costs (2-5% of loan). Plug your loan into both calculators side by side.
How much extra do I need to pay to finish a 30-year mortgage in 15 years?
Pay the equivalent of the 15-year amortizing payment. On a $400,000 mortgage at 7%, the 30-year P&I is about $2,661/month and the 15-year P&I at the same rate is about $3,595/month — so you'd add roughly $934/month in extras to your 30-year loan to finish it in 15 years. That cuts total interest from about $558,000 to about $247,000 — saving roughly $311,000 over the life of the loan. The shortcut: ask your servicer for the 15-year payment quote on the same balance and rate, subtract your current 30-year payment, and that's the monthly extra you need. Confirm the extras flow to principal (not 'paid ahead') — see /guides/principal-only-payments for the routing fix.
Is a 15-year mortgage better than a 30-year with extras?
A 15-year loan typically gets you 0.5-0.75% lower rate than a 30-year. A 30-year + extras matches the same payoff date with HIGHER interest paid (because the rate is higher) but more flexibility (you can drop the extras any month). Most borrowers should take the 30-year and self-discipline the extras — flexibility tends to outweigh the rate spread.
Where this mortgage calculator's math stops being honest
The math is exact for fixed-rate amortizing loans with monthly payments, but it doesn't capture every situation. Cases where the output above will mislead you:
- Mortgage-interest deduction (where it applies) effectively lowers your real rate by your marginal tax rate — at 24% marginal, a 7% mortgage is a 5.3% effective rate. The calculator doesn't apply that adjustment; tax savings reduce the true cost of carrying the mortgage.
- Adjustable-rate mortgages will reset; the model assumes your rate stays fixed. For ARMs, run scenarios with the projected reset rate to see actual exposure.
- Mortgage insurance (PMI/MIP) often drops at a specific LTV — prepaying to that LTV faster has separate value not captured in the interest savings. See the FHA / conventional guides for those thresholds.
- Recasting (re-amortizing) after a lump sum is a separate option some lenders offer — same interest savings but lower minimum payments going forward. Ask your servicer before sending a large prepayment.
Sources and references
- Freddie Mac Primary Mortgage Market Survey (PMMS) — national average 30-year and 15-year fixed rates
- CFPB on mortgage prepayment penalties — regulatory guidance on conforming vs non-conforming loans
- IRS Publication 936 — home mortgage interest deduction — current deduction rules and the $750k debt cap
- PayoffMath rate-outlook page — how 30-year mortgage rates relate to the 10-year Treasury
Related guides
- Refinance vs. prepay — which saves more?
- Refinance vs. purchase — which move wins?
- Lump sum vs. extra monthly — which pays off faster?
Related calculators
Other loan-type calculators
Same math, tuned to typical numbers and rules for each loan type.
Home loan calculators
Consumer loan calculators
Business loan calculators
Calculator and notes maintained by James L. Wu. The amortization math is the easy part. PMI cancellation under the Homeowners Protection Act, conforming-loan limits, and the real cost of unwinding a refinance are the details that decide whether prepaying actually pays off. Not financial advice — confirm specifics against your loan documents. See methodology for the formulas + assumptions and the editorial policy for sourcing. Last refreshed April 2026.