Loan Prepayment Savings Calculator
Find out how much interest you save — and how many months sooner you'll be debt-free — by making extra payments on your loan.
| Month | Payment | Principal | Interest | Balance (Base) | Balance (Extra) |
|---|
How to Use This Loan Prepayment Savings Calculator
Enter your current loan balance, annual interest rate, and original term in months. If you've already made some payments, enter how many months have elapsed. Then set your planned monthly extra payment using the slider or type it in directly. Hit Calculate Savings to instantly see your interest savings, new payoff date, and a full month-by-month amortization table comparing your base schedule to your prepayment schedule.
Why This Matters
Loan prepayments are one of the highest-guaranteed "returns" available to anyone with debt. On a $300,000 mortgage at 7% over 30 years, adding just $300 extra per month cuts about 8 years off the loan and saves over $90,000 in interest — money that never reaches the bank. That's nearly a third of the purchase price back in your pocket.
This matters most in a high-interest-rate environment. When mortgage rates are 6–7%+, prepaying is mathematically better than most bonds and competitive with conservative stock portfolios — and it's risk-free. It also helps people on adjustable-rate mortgages reduce principal faster before rate resets hit.
Car loans, student loans, and personal loans also benefit enormously. A $25,000 auto loan at 9% over 60 months? An extra $100/month shaves around 12 payments and saves ~$1,000. The shorter your remaining term, the more powerful each extra dollar becomes since you're eliminating high-interest early months.
How It's Calculated
The calculator first computes your standard monthly payment using the amortization formula:
M = P × [r(1+r)ⁿ] / [(1+r)ⁿ − 1]
Where P = principal, r = monthly rate (annual rate ÷ 12), n = remaining months. It then runs two parallel amortization schedules month by month — one at your base payment only, one adding your extra payment to principal each month. The interest saved is the difference in total interest paid across both schedules. The payoff date difference shows how many months (and years) earlier you'll be debt-free.
Tips & Common Mistakes
- Specify "principal-only" to your lender. Some lenders apply extra payments to future payments rather than reducing principal. Always designate extra amounts as principal-only in writing or through your lender's payment portal.
- Check for prepayment penalties. A small number of loans (particularly some personal loans and older mortgages) include prepayment penalties. Read your loan docs or call your servicer before making extra payments.
- Emergency fund first. Before prepaying a low-interest loan (under 4–5%), ensure you have 3–6 months of expenses saved. Liquidity matters — extra loan payments are irreversible.
- Biweekly payments are a simple hack. Paying half your monthly payment every two weeks results in 26 half-payments (13 full payments) per year instead of 12 — effectively one free extra payment annually.
- Lump-sum vs. monthly extra. A one-time lump sum at the start of a loan saves more total interest than the same amount spread as monthly extras, because it reduces the base principal immediately.
Frequently Asked Questions
Does prepaying always save money?
Yes, mathematically — prepaying always reduces the interest you'll pay. However, from a financial planning standpoint, if your loan rate is 3% and you could earn 8% investing that money in index funds, investing may be the better choice. The break-even point depends on your loan rate vs. your expected investment return, adjusted for risk and taxes.
What if my lender charges a prepayment penalty?
Some loans charge a fee (often 1–3% of the remaining balance or several months' interest) if you pay off the loan early. Check your loan agreement or call your servicer. If a penalty exists, factor it into your savings calculation — though most modern mortgages and federal student loans have no prepayment penalties.
Should I prepay my mortgage or invest in my 401(k)?
If your employer offers a 401(k) match, always contribute enough to get the full match first — that's an instant 50–100% return. Beyond that, compare your mortgage rate to your expected investment return. For most people with mortgages above 6%, prepaying is competitive with conservative investing. Below 4%, most financial advisors suggest investing the difference instead.
How accurate is the month-by-month table?
The table uses standard amortization math and is accurate for fixed-rate loans with no prepayment penalties. It assumes your extra payment is applied entirely to principal each month. Results may vary slightly if your lender calculates interest daily rather than monthly, or if your loan has escrow or other fees built in.