Benefits of Using This Tool
Saves time — no manual spreadsheet iterations needed.
Reduces manual errors — automated month-by-month calculation.
Instant results — see standard vs. extra comparison immediately.
Free — no subscription or payment required.
Private — all calculations happen in your browser; no data is sent to any server.
Accessible on any device — works on desktop, tablet, and mobile.
User-focused — clear side-by-side metrics and a full amortization schedule.
FAQs
How accurate is this mortgage extra payment calculator?
It is mathematically precise based on the inputs you provide. It uses the standard amortization formula and iterates month by month, applying extra payments exactly as described. However, actual lender practices may vary (e.g., exact day of payment application), so use it as an estimation tool.
Can I calculate extra payment savings manually?
Yes, you can replicate the process using the formula shown above. But doing it month by month for 30 years is tedious. This calculator automates that work, giving you instant, error-free results.
What causes the interest saved to change when I adjust the start month?
Starting extra payments later gives the loan balance more months to accrue interest at the original, higher balance. Starting earlier captures more compound interest savings. The calculator shows the exact difference.
Is this tool safe to use?
Yes. Everything runs in your browser using JavaScript. No loan data is sent to any server. You can use it offline once the page loads.
What is the difference between a monthly extra payment and a one-time lump sum?
A monthly extra payment repeats every month starting from the chosen start month. A one-time lump sum applies only once. Both reduce principal, but a recurring monthly payment continues saving interest over many months, while a lump sum provides an immediate but one-time reduction. You can use both together.
Does making extra payments affect my credit score?
Not directly. Extra payments reduce your loan balance faster, which lowers credit utilization for revolving credit. But the act of paying extra does not inherently harm or boost your score. The main benefit is financial savings.
What if my loan has a prepayment penalty?
Some mortgages charge a fee for paying off principal faster than scheduled. This calculator does not account for penalties. Check your loan documents or ask your lender before making large extra payments.
How does the extra payment start month work?
Extra payments begin on the month number you specify. For example, if you enter “1,” the first payment includes the extra amount. If you enter “6,” the first five payments are standard, and the sixth includes the extra monthly amount and any lump sum.
Can I see the full amortization schedule with extra payments?
Yes. Click the “Amortization Schedule” tab after calculating. It shows month-by-month: payment amount, principal portion, interest portion, extra paid, and remaining balance.
What is a typical interest savings percentage from extra payments?
There is no fixed percentage. It depends on your rate, term, and extra payment amount. A common result: adding 10% to your monthly payment often saves 15–25% of total interest and cuts 4–6 years off a 30-year loan. Use the calculator to see your exact numbers.
Is it better to make extra mortgage payments or invest?
This is a personal finance question. Compare the mortgage interest rate (your guaranteed savings) against expected after-tax investment returns. If your mortgage rate is high (e.g., 6%+), paying it down often wins. If your rate is very low (e.g., 3%), investing may outperform. This calculator gives you the mortgage-side numbers to inform your decision.
What happens if I stop making extra payments after starting?
The calculator assumes you continue the extra monthly payment once started. If you stop in real life, your payoff timeline would revert closer to the standard schedule, but you would still keep any progress already made. For precise planning, run a new calculation with your updated extra payment plan.