How does the calculator measure what debt costs?
First it builds a payoff schedule. Each month it charges interest at your annual rate divided by 12 on the remaining balance, subtracts your fixed payment, and repeats until the balance is gone. Counting the months gives the payoff time; summing the interest charges gives total interest; adding total interest to the balance gives total paid.
Next it divides total interest by the starting balance to get interest as a percent of balance. That single ratio is useful because it combines the effect of rate and time. A low-rate debt paid slowly can end up costing a larger share than a higher-rate debt cleared quickly.
Finally it imagines a different path: the same monthly payment deposited into an investment earning the return you enter, compounded monthly, for the same number of months. The result is the value those dollars might reach. It is a way of seeing interest as money that could have been working for you, with the important caveat that investment returns are never certain.
Interest each month = Balance × (rate ÷ 12) Interest % of balance = Total interest ÷ Starting balance × 100 Value if invested = Payment × ((1 + i)^n − 1) ÷ i
- rate
- annual interest rate on the debt
- Payment
- fixed monthly payment
- i
- monthly investment return, annual return ÷ 12
- n
- months to pay off the debt
Example: the real price of $11,500 at 17.5%
Scenario: Imagine a $11,500 balance at 17.5%, paid down with $300 a month and no new borrowing. For comparison, assume an illustrative 6% annual return on money you invest instead.
- First month's interest$11,500 × (17.5% ÷ 12) = $167.71, so only $132.29 of the first payment reduces the balance
- Months to pay offRepeating the monthly cycle takes 57 payments (4 years 9 months); the last one is $166.36
- Total paid and interestTotal paid $16,966.36 − balance $11,500 = $5,466.36 of interest
- Interest as a percent of balance$5,466.36 ÷ $11,500 = 47.5%
- Value if invested$300 a month for 57 months at 6% ÷ 12 monthly = $19,729.08, of which $17,100.00 is deposits and $2,629.08 is assumed growth
Carrying this debt costs $5,466.36 in interest, 47.5% of the balance, while the same payments invested at an assumed 6% could have grown to about $19,729.08.
Raising the payment to $400 would cut the interest to $3,515.76 and the payoff to 38 months, which changes every figure here.
When is it worth calculating the true cost of debt?
Deciding which debt deserves extra money
Run each balance separately and compare interest as a percent of balance. The debt with the highest share is usually where an extra dollar does the most work.
Before taking on new debt
Enter the amount you are thinking of borrowing, the likely rate and the payment you could afford. Seeing the full interest bill up front makes it easier to judge whether the purchase is worth it.
Weighing debt payoff against investing
The invested-value result gives a rough sense of the trade-off. Comparing your debt's rate with a realistic, uncertain investment return helps frame which use of spare cash is likely to come out ahead.
Motivating a payoff plan
A single number that shows what the debt is taking from your future can be clarifying. Some people find that seeing interest as lost savings makes it easier to cut spending elsewhere.
What mistakes skew a cost-of-debt estimate?
Treating the investment figure as a sure thing
The calculator holds the return constant every month, but real investments rise and fall and can lose money. Use the invested value as a comparison, not as an amount you would definitely have had.
Comparing debt rates with pre-tax, pre-fee returns
An investment's headline return may be reduced by fees and taxes. Interest on personal debt such as credit cards is generally not deductible, so a dollar of interest saved is a full dollar kept.
Using a payment that barely covers interest
If your payment is close to the monthly interest charge, the payoff can stretch for decades and total interest can exceed the balance several times over. Check the first month's interest before entering a payment.
Leaving out new charges
The estimate assumes the balance only goes down. If you keep adding to the debt, especially on a card, the true cost is higher than shown.
Ignoring rate changes
Variable-rate debts can get more expensive. If your rate is tied to an index, test a higher rate to see how sensitive your total cost is.
How much does the interest rate change the total cost?
Rate does double damage. A higher rate means more interest each month, and because more of every payment goes to interest, the balance shrinks more slowly, so you pay that higher rate for more months. The result is that total interest climbs much faster than the rate itself.
The table keeps the balance at $11,500 and the payment at $300, and changes only the rate. Going from 8% to 24% triples the rate but multiplies interest by about 5.8 times. At 29.99%, the same payment takes 10 years 9 months and interest is more than double the original balance.
| Interest rate | Months to pay off | Total interest | Interest % of balance |
|---|---|---|---|
| 8% | 45 months | $1,824.69 | 15.9% |
| 12% | 49 months | $3,075.53 | 26.7% |
| 17.5% | 57 months | $5,466.36 | 47.5% |
| 24% | 74 months | $10,547.60 | 91.7% |
| 29.99% | 129 months | $27,032.23 | 235.1% |
Is paying off debt or investing the better return?
Paying down a debt earns a guaranteed return equal to its interest rate: every dollar that cuts a 17.5% balance saves 17.5 cents a year in interest, with no market risk. An investment might earn more or less, and you will not know which until later.
That is why many people prioritize high-rate debt such as credit cards before investing extra cash, while being more relaxed about low-rate debt. Two exceptions are common: building a small emergency fund first so a surprise expense does not land back on a card, and capturing any employer retirement match, which can be an immediate return. The calculator's two results let you see both sides for your own numbers.
Terms used on this page
- Interest Rate
- An interest rate is the price of money, stated as a percentage of the balance per year. Borrowers pay it to lenders, and savers earn it from banks. The rate alone does not include fees or compounding, so it can understate the true yearly cost of a loan or overstate a simple comparison of savings accounts.
- Compound Interest
- Compound interest is interest earned or charged on both the original principal and the interest that has already built up. Each period's interest is added to the balance, so the next period's interest is a little larger. Over long stretches, compounding makes savings grow faster and unpaid debts grow faster too.
- Amortization
- Amortization is the process of paying off a loan with equal, regular payments that each cover the interest due for the period plus a slice of principal. Early payments are mostly interest; later ones are mostly principal. By the final payment the balance reaches exactly zero, which is how most car, personal and student loans work.
- Emergency Fund
- An emergency fund is cash set aside for unexpected costs or income loss, such as a car repair, medical bill or job gap. It is usually kept in an insured, easy-to-reach savings account. A common target is three to six months of essential expenses, though any starting amount reduces the need to borrow at high rates.
- Prepayment Penalty
- A prepayment penalty is a fee some lenders charge if you pay off a loan early or pay down a large part of it ahead of schedule. It compensates the lender for interest it will no longer receive. Many consumer loans have no such penalty, but if yours does, the terms must appear in your loan disclosures.
Cost of Debt Calculator: common questions
What does interest as a percent of balance mean?
It is total interest divided by what you owe today. A result of 47.5% means that for every $100 of balance, about $47.53 more goes to the lender before the debt is gone. It folds rate and payoff time into one number, which makes it handy for comparing debts with different terms.
Can I pay more interest than I originally borrowed?
Yes. When the rate is high and the payment is modest, interest can exceed the balance. In this page's table, $11,500 at 29.99% with $300 payments costs $27,032.23 in interest. Raising the payment is the most direct way to bring that number down. Even a modest increase in the payment narrows the gap quickly when the rate is high.
Is the value-if-invested number realistic?
It is a what-if, not a forecast. It assumes the same monthly amount is invested at a steady return for the payoff period. Real returns vary year to year and can be negative, and fees and taxes reduce them. Use a modest return to keep the comparison conservative. It is best read as a way to put a dollar value on what the interest is costing you.
What counts as an expensive debt?
There is no official cutoff, but a debt is expensive when its rate is well above what your savings can reliably earn. By that test, revolving credit card balances are usually the costliest, while low fixed-rate installment loans are cheaper. Interest as a percent of balance shows the cost for your actual payment.
Does a lower payment make my debt cheaper?
No, it makes it more expensive overall. A lower payment eases the monthly budget but leaves the balance outstanding longer, so interest accrues for more months. Unless the rate also drops, a smaller payment almost always raises total interest. If you need a lower payment for budget reasons, it can still be the right choice, but the calculator will show how much extra interest that flexibility costs over the life of the debt.
How do I find my debt's interest rate?
Check your latest statement or online account. Card statements show the APR for each balance type, such as purchases and cash advances. Loan statements or the original loan agreement list the interest rate and APR. If your rate is variable, use the current figure and re-run the numbers when it changes.
Why does the calculator ask for an investment return?
To show opportunity cost, the value of what you give up. Money sent to interest cannot also be saved. By projecting what the payments could grow to, the calculator puts a dollar figure on that trade-off. It does not suggest any particular investment. If you prefer, enter the rate an insured savings account might pay to keep the comparison conservative.
Should I include my mortgage in a cost-of-debt calculation?
You can, but run it on its own. A mortgage is usually a large, long-term, lower-rate debt, so it will dominate any combined figure. Looking at each debt separately makes it clearer which balance is the most expensive per dollar owed. Mortgage interest may also be treated differently for taxes than consumer debt, which is another reason to keep the comparison separate.
Does paying off debt early ever cost money?
Occasionally. Some installment loans carry a prepayment penalty, and paying early means giving up liquidity you might need. Credit cards do not charge prepayment penalties. Check your loan agreement, and keep some emergency savings so that paying early does not push new spending back onto credit. When there is no penalty, paying early simply removes future interest.
Sources
- Credit cards — Consumer Financial Protection Bureau. Accessed October 17, 2026.
- Compound Interest Calculator — U.S. Securities and Exchange Commission (Investor.gov). Accessed October 17, 2026.
- Topic no. 505, Interest expense — Internal Revenue Service. Accessed October 17, 2026.
- Survey of Household Economics and Decisionmaking (SHED) — Federal Reserve Board. Accessed October 17, 2026.
We cite primary sources: regulators, central-bank data and federal agencies. Worked examples on this page are our own calculations, last checked October 17, 2026. See our methodology.