Calculation details
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Use this result
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What does this calculator estimate?
Mortgage overpayments are extra principal payments that reduce your loan balance faster. Because the balance drops sooner, you pay less interest over the life of the loan and finish years earlier. Enter your loan, rate, term, and monthly overpayment to see the savings.
- Overpayments go straight to principal
- Lower principal = less future interest
- Shortens the term and can cut interest by tens of thousands
How overpayments work
Every extra dollar above the scheduled payment reduces principal directly — and since interest is charged on the balance, less principal means less interest for the remaining life of the loan. The calculator compares the standard plan against your overpayment.
Limitations to watch for
Prepayment penalties are rare on mortgages but check your note. The extra payment is assumed monthly and constant. Consider opportunity cost: money invested may earn more than the mortgage rate — the tool shows the guaranteed interest saving, not the investment alternative.
How to use it in practice
Enter your balance, rate, term, and a realistic extra amount. Use the savings to decide between paying down the mortgage and investing. Round up your payment to the nearest $50 — the calculator shows exactly what that does.
['Enter the balance, rate, and term.', 'Enter the extra monthly payment.', 'Read the time and interest saved.']
How this calculator works
Formula
Simulates the loan with the standard payment and with an extra monthly amount, comparing term length and total interest. Extra payments go to principal, cutting both.
Worked example
A $300,000 loan at 6.5% for 30 years pays $1,896/month; adding $200/month clears it in ~23 years and saves roughly $103,000 in interest.
Assumptions to verify
- A fixed-rate loan with level payments.
- The extra payment is constant monthly.
- No prepayment penalties.
Frequently asked questions
How much do overpayments save?
On $300k at 6.5%, $200/month extra saves about $103,000 in interest and ~7 years.
How does it work?
Extra payments reduce principal, which reduces the interest charged on every future payment.
Should I overpay or invest?
Compare the guaranteed mortgage rate saving against expected investment returns — overpayment wins when the mortgage rate is higher.
Can I make lump-sum overpayments?
Yes — a yearly lump sum has the same effect, prorated by when it's applied.
Are there penalties?
Prepayment penalties are uncommon on residential mortgages, but check your note.
How much extra should I pay?
Even $50–100/month cuts years off the term — the calculator shows your exact number.
Does it help with an ARM?
The tool models fixed rates; ARMs change the math when the rate adjusts.
Cite this tool
BoringToolsKit. “Mortgage Overpayment Calculator.” boringtoolskit.com/mortgage-overpayment-calculator/ (reviewed 2026-08-25). Free to reference in articles, syllabi, and answer posts with a link.
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