Most loan calculators answer the wrong question for someone who already owes money. They start from the amount you want to borrow and work forward to a monthly bill. But if you are sitting on an existing balance — a personal loan, an auto loan, a student loan, or a consolidated debt — the question you actually care about is the reverse: given what I still owe, my rate, and the payment I can afford, when exactly do I become debt-free? This loan payoff calculator answers that backward-looking question directly, and then shows you how much faster and how much cheaper the journey gets if you put a little extra toward principal each month.
Type in your remaining balance, the APR on your statement, and the payment you already make. The calculator runs the loan down month by month and reports the payoff date, the total interest you will pay, and — with a single slider — the savings from prepayment.
How to Use
- Enter the Current Loan Balance — the remaining principal on your statement, not the original amount you borrowed. If you took out a $25,000 loan three years ago and have paid part of it down, type what you owe today.
- Set the Interest Rate (APR) exactly as it appears on your statement. This is an annual figure; the calculator divides it by 12 to get the monthly rate at which interest accrues.
- Type your Current Monthly Payment — the minimum or scheduled payment you send every month. If you already pay more than the minimum, use the amount you actually send.
- Optionally slide the Extra Monthly Payment to an additional amount you could commit to. This money goes straight to principal and is the single lever that pulls your payoff date closer.
- Read the result card: it compares two plans side by side — paying only the required amount, versus adding your extra payment — and reports the months you cut off and the interest you save.
The engine charges interest on the outstanding balance at the start of each month, applies your payment to that interest first, and puts whatever is left toward principal. It repeats that loop until the balance reaches zero. That is the same math your lender uses, so the date you see is the date you actually own the loan free and clear.
The Runoff Math Behind the Number
Unlike a forward loan calculator, this tool does not need a loan term, because it is solving for the term itself. Each month the balance shrinks a little, and the interest charge shrinks with it. Given a balance B, a monthly rate r (the APR divided by 12), and a fixed monthly payment P, the number of months n to reach zero has a closed-form solution:
n = −ln(1 − r·B / P) / ln(1 + r)
Take a $20,000 balance at a 6.0% APR. The monthly rate is 0.005, and the first month's interest is $100. At a $400 payment, $100 covers interest and $300 attacks principal. Repeating that loop, the balance reaches zero after about 58 months, and the total interest paid is roughly $3,072. If the rate were 0%, there would be no interest at all, and the loan would simply divide evenly: $20,000 at $400 a month takes exactly 50 months.
The power of an extra payment shows up because every extra dollar permanently shrinks the balance that future interest is charged on. Raise the payment from $400 to $500 (just $100 more), and the same $20,000 loan at 6% clears in about 45 months instead of 58 — removing 13 months and saving roughly $702 in interest, even though you only added $100 per month.
When a Payment Is Too Small
There is one trap worth knowing about: if your monthly payment does not even cover the interest that month, the balance never shrinks — it grows. On a $20,000 loan at 6%, the monthly interest is $100, so any payment above $100 makes progress. But at a high rate on a large balance, a too-small minimum can leave you treading water. If you enter a payment that is below the monthly interest charge, the calculator flags it rather than printing a meaningless payoff date.
Examples
Example 1 — The headline comparison. You owe $20,000 at 6.0% APR and pay $400 each month. That takes 58 months (about 4.8 years) and costs about $3,072 in interest. Add an extra $100 per month — bringing your payment to $500 — and the loan disappears in 45 months, with total interest around $2,370. You save 13 months and roughly $702, for a commitment of just $100 more each month.
Example 2 — A modest balance, a big rate. A graduate carries a $15,000 private loan at 7.5% and pays $350 a month. The baseline plan needs about 51 months and accrues roughly $2,514 in interest. Bumping the payment by $50 to $400 trims it to about 43 months and cuts the interest to near $2,146 — saving around $369 and 8 months. Small balances reward proportionally strong prepayment.
Example 3 — A large balance stress test. On a $1,000,000 balance at 6.0% with a $6,000 monthly payment, the baseline runoff stretches to 360 months and piles up about $1,155,485 of interest. Adding $1,000 a month (payment $7,000) pulls the payoff in to roughly 252 months and saves about $397,233 in interest. On large balances, even a small percentage change in the payment moves the six-figure interest line dramatically.
Example 4 — Zero-rate grace period. If your rate is 0% (a promotional card or a 0% balance-transfer window), interest is zero and the math becomes trivial: $20,000 paid at $400 a month vanishes in exactly 50 months. Use that runway aggressively, because a 0% clock usually resets to a high rate once the promotion ends.
FAQ
How does this loan payoff calculator work?
It runs the balance down month by month. Each month interest is charged at the APR divided by 12, your payment covers that interest first, and the remainder reduces principal. The loop repeats until the balance hits zero, and the month count plus the accumulated interest are your payoff time and total interest. This is the same runoff method your lender uses.
What is the difference between this and a regular loan calculator?
A regular loan calculator takes the amount you want to borrow plus a loan term and works forward to tell you the required monthly payment. This calculator assumes you already have the loan: it takes your remaining balance and the payment you can make, and works backward to tell you when the loan will actually be gone. The direction of the math is reversed.
How much does an extra $100 a month save?
On a typical $20,000 balance at 6.0% APR, moving from a $400 payment to a $500 payment removes about 13 months and roughly $700 of interest. The savings grow with your balance and rate — on a six-figure loan the same $100 can save thousands, because every extra dollar permanently shrinks the base on which future interest is charged.
Why is my payoff date later than the lender promised?
The most common reason is that the "lender promised" date was based on paying the minimum, while your actual statement allocates part of each payment to fees or escrow. Another cause is a high APR: if your payment only barely clears the monthly interest, principal moves slowly. Check the current balance and APR on your latest statement, and make sure your monthly payment exceeds that month's interest charge.
What if my payment does not cover the interest?
Then the balance will not shrink — it will grow, which is the trap of revolving or negatively amortizing debt. The calculator detects this and warns you instead of showing a payoff date. In that situation you need to raise the payment above the monthly interest charge first, then build from there.
Should I pay extra every month or make one lump sum?
Both work, but they work slightly differently. A recurring extra payment compounds the benefit every single month, because it keeps reducing the balance early. A one-time lump sum has a strong effect only when you make it — the sooner the better. If you can afford a recurring extra, it usually beats a delayed lump sum because it shrinks interest for the maximum number of months.
Disclaimer
This loan payoff calculator is provided for informational purposes only and does not constitute financial, lending, or tax advice. Estimates assume a fixed interest rate and a consistent monthly payment applied at the end of each billing cycle; actual payoff dates, interest charges, and fees vary by lender, statement cutoff dates, credit profile, and loan program. Some loans impose prepayment penalties, which this tool does not subtract. Always confirm your payoff timeline and any prepayment terms directly with your lender before changing your payment plan.