How mortgage prepayment works: $10K extra on a $100K loan
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By Editorial team
A borrower on r/Mortgages wanted to understand what actually happens to their amortization schedule when they make a large extra principal payment. Their example: a $100,000 loan at 6% fixed for 30 years, three payments already made, then $10,000 extra toward principal on Payment #4. Do they just jump down the amortization table to the row matching the new balance?
Short answer: yes. The extra $10,000 drops the balance to about $89,599. On the original schedule, that balance does not show up until around Payment #84 - roughly 80 payments skipped. Over the life of the loan, it saves about $38,125 in interest and retires the mortgage 80 months early. Here is how the math works.
The scenario modeled
We matched the Reddit poster's hypothetical exactly: a $100,000 US conventional loan at 6.00% fixed for 30 years ($125,000 home, 20% down). No taxes, insurance, or PMI - pure principal and interest. The $10K extra at payment 4 tab adds $10,000 as a one-time principal payment on the fourth scheduled payment. The Standard payments tab runs the same loan with no extras.
| Input | $10K extra at payment 4 | Standard payments |
|---|---|---|
| Home price | $125,000 | $125,000 |
| Down payment | $25,000 (20%) | $25,000 (20%) |
| Loan amount | $100,000 | $100,000 |
| Rate / term | 6.00% fixed / 30 years | 6.00% fixed / 30 years |
| Monthly P&I | $599.55 | $599.55 |
| One-time extra at Payment #4 | $10,000 | $0 |
The findings
After three payments the balance sits at $99,700. Payment #4 applies its regular $101 of principal plus the $10,000 one-time extra, dropping the balance to about $89,599. The monthly payment does not change - it stays at $599.55 - but the interest portion of each future payment falls sharply because there is $10,000 less principal accruing daily interest.
That is why the poster's intuition was right. On the original schedule, a balance near $89,599 does not appear until Payment #84. From Payment #5 onward, the interest and principal columns track closely to where Payment #85 would have been. The borrower has not literally moved to row 84, but the economics are the same: every payment from here carries less interest and more principal, and the loan ends 80 months sooner.
| $10K extra at payment 4 | Standard payments | Difference | |
|---|---|---|---|
| Monthly P&I | $599.55 | $599.55 | Unchanged after prepayment |
| Balance after Payment #4 | $89,599 | $99,599 | $10,000 lower on the OTP tab |
| Equivalent standard row | ~Payment #84 | Payment #4 | ~80 payments skipped |
| Modeled total interest (life of loan) | $77,713 | $115,838 | $38,125 less interest |
| Schedule length | 280 months | 360 months | 80 months shorter |
US context
US borrowers have strong prepayment rights. The Consumer Financial Protection Bureau explains that most residential mortgages originated after January 2014 cannot carry prepayment penalties under the Qualified Mortgage rule. When you send extra money, the CFPB recommends telling your servicer in writing to apply it to principal, then verifying on your next statement that the balance dropped by the full amount.
Source: Consumer Financial Protection Bureau, "What is a prepayment penalty?" accessed May 2026. URL: https://www.consumerfinance.gov/ask-cfpb/what-is-a-prepayment-penalty-en-1957/
When this applies - and when it doesn't
This applies when: you have a lump sum available (tax refund, bonus, inheritance), your loan has no prepayment penalty, and your servicer will apply the full amount to principal. The effect scales linearly: $5,000 extra would skip roughly 40 payments on this same loan.
It does not apply when: your servicer routes extra payments to escrow or future installments instead of principal, when a prepayment penalty exists (uncommon post-2014 but check your documents), or when the same dollars would pay off higher-rate debt first. Also keep in mind that this model assumes a fixed rate - on an adjustable-rate mortgage, the recalculated payment amount changes at each reset, so the "table skip" analogy breaks down.
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Frequently asked questions
- Does a large one-time payment reduce my required monthly payment?
- No. Your contractual payment stays at the amount calculated when the loan was originated. On a $100,000 loan at 6% over 30 years, the $599.55 monthly P&I does not drop after a $10,000 prepayment. What changes is the split: more of each future payment goes to principal and less to interest, and the loan ends sooner.
- Do I literally skip to a later row on the amortization table after a prepayment?
- Effectively yes. After a $10,000 extra payment at month 4 on this loan, the remaining balance drops to about $89,599. In the original schedule, that balance does not appear until around Payment #84. From that point the interest and principal columns track the original table closely, but the payment number on your statement stays at 5, not 84.
- Will my servicer apply a large extra payment entirely to principal?
- It depends on the servicer. Some apply extras to future payments (covering next month's interest first), others route part to escrow. The CFPB recommends confirming in writing how your servicer handles principal-only payments and checking your statement afterward to verify the balance dropped by the full amount.
- Is there a penalty for making a large prepayment on a US fixed-rate mortgage?
- Most conventional US mortgages originated after 2014 do not carry prepayment penalties, thanks to Ability-to-Repay rules. FHA and VA loans also prohibit them. Still, confirm with your loan documents before sending a large sum.