Months
Total Interest
Total Amount
Table of Contents
Credit cards in one sentence
A credit card is revolving, unsecured debt — you borrow up to a credit limit and pay interest only on the portion you don't repay each month. The flexibility is the feature; the variable APR (typically 18–25%+ in 2025) is the cost.
How payoff time is calculated
The months required to clear a credit card balance with a fixed monthly payment uses the inverse of the amortization formula:
N = −log(1 − (P × r) ÷ M) ÷ log(1 + r)
- N — number of months to pay off
- P — current balance
- r — monthly interest rate (APR ÷ 12)
- M — fixed monthly payment
Important constraint: M must exceed P × r (the monthly interest). Otherwise, your payment doesn't cover the interest accruing that month and the balance actually grows — the formula returns an error because no payoff is mathematically possible.
Worked example using the calculator's defaults ($1,000 balance, 1.3% APR, $100/month):
- r = 0.013 ÷ 12 ≈ 0.001083
- P × r = 1,000 × 0.001083 = $1.08 monthly interest (well under $100, so payoff is feasible)
- N = −log(1 − 0.1083) ÷ log(1.001083) ≈ about 11 months
- Total paid: about $1,007; interest: roughly $7
The 1.3% default APR is unusually low — most US credit cards in 2025 carry 18–25%+ APRs. Re-run the calculator at a realistic 22% on a $5,000 balance to see how interest dominates the math at typical rates.
How credit card interest is actually charged
Unlike installment loans, credit card interest is computed daily on the running balance. Most US issuers use the average daily balance method:
- Each day in the billing cycle, the issuer multiplies that day's balance by the daily periodic rate (APR ÷ 365).
- At cycle end, all daily interest charges are summed and added to your balance.
- If you pay the full statement balance by the due date, no interest is charged on purchases — this is the "grace period."
- If you carry a balance, the grace period is lost. Interest accrues on new purchases from the transaction date.
Cash advances and balance transfers usually have no grace period at all — interest starts accruing immediately, often at a higher APR than purchases.
The minimum payment trap
Credit card minimum payments are set to "interest plus 1–2% of principal." Sounds reasonable; in practice it's devastating. Same $5,000 balance at 22% APR with different payment strategies:
| Monthly payment | Months to pay off | Total interest |
|---|---|---|
| Minimum only (about $130, decreasing) | About 220 months (18+ years) | About $6,800 |
| $200 fixed | 32 months | About $1,500 |
| $300 fixed | 20 months | About $920 |
| $500 fixed | 11 months | About $500 |
| $1,000 fixed | 6 months | About $280 |
Doubling the payment more than halves both the time and total interest. Federal CARD Act rules now require credit card statements to display "how long to pay off at minimum payments" precisely because the minimum trap is so harmful to consumers.
Strategies to clear credit card debt
If you carry balances on multiple cards, two payoff strategies dominate the personal finance literature:
- Debt avalanche. Pay minimums on all cards; throw extra money at the highest-APR card. When it's clear, redirect that whole payment to the next-highest. Mathematically optimal — minimizes total interest paid.
- Debt snowball. Pay minimums on all cards; throw extra money at the smallest-balance card, regardless of rate. Closes accounts faster, creating psychological wins. Behavioral research (Gal & McShane, 2012) suggests higher follow-through rates.
A 0% balance transfer can accelerate either strategy — if you have the discipline to pay off the transferred balance before the promo ends. On a $5,000 balance, a 4% transfer fee costs $200, beating roughly $750+ in interest at 22% APR over 18 months. After the promo, the rate jumps to 20–25%, so it only works with a concrete payoff plan.
How credit card balances affect your credit score
FICO and VantageScore both weight credit card behavior heavily. Three behaviors that matter most:
- Utilization (about 30% of FICO). Total card balances divided by total credit limits. Keep this under 30%, ideally under 10%, for top scoring.
- Payment history (about 35% of FICO). A single 30-day-late payment can drop your score 50–100+ points and stays on your report for 7 years.
- Account age and mix. Older accounts help. Closing your oldest paid-off card can ding your score by lowering average account age — usually better to keep it open at $0.
Limitations of this calculator
- Fixed-payment assumption. The calculator assumes a constant monthly payment. Real-world minimum payments are recalculated each month and decline as the balance falls, which extends payoff dramatically.
- No new charges modeled. If you keep using the card while paying it down, payoff times and interest costs both increase. The simulation assumes zero new charges.
- Constant APR. Variable-rate cards adjust APRs as the prime rate changes; missed payments can trigger penalty APRs of 29.99%+ on the entire balance.
- No fees included. Annual fees, late fees, cash advance fees, and foreign transaction fees aren't part of this calculation.
- Average daily balance simplification. Real interest is compounded daily; the formula approximates this with a monthly compounding step, which is close but not identical to issuer statements.
Sources & references
- Consumer Financial Protection Bureau — Credit Cards — CFPB rules, consumer guidance, and Credit CARD Act enforcement.
- CFPB Consumer Credit Card Market Report — biennial industry analysis of APRs, fees, and balances.
- US Federal Reserve — Consumer Credit (G.19) — current US revolving credit rates and trends.
- Gal D, McShane BB (2012). "Can Small Victories Help Win the War? Evidence from Consumer Debt Management." Journal of Marketing Research 49(4): 487–501 — original snowball-method research.
- myFICO — What's in Your FICO Score — official breakdown of FICO scoring factors.
FAQs
The calculator inverts the standard amortization formula to solve for months: N = −log(1 − (P × r) ÷ M) ÷ log(1 + r), where P is the balance, r is the monthly rate (APR ÷ 12), and M is the fixed monthly payment. For the calculator's defaults ($1,000 balance, 1.3% APR, $100 per month), r = 0.001083, the monthly interest is only $1.08, and the loan is cleared in about 11 months with under $7 in total interest. The constraint is that M must exceed P × r, otherwise the balance grows faster than you pay.
Issuers calculate minimum payments as roughly 1–2% of the balance plus the month's interest. On a $5,000 balance at 22% APR, the minimum is around $130 — of which about $92 is interest and only $38 reduces the balance. As the balance shrinks, the minimum shrinks too, dragging payoff out to roughly 18 years and over $6,800 in total interest. Federal CARD Act rules require statements to disclose this, but the trap still catches millions of borrowers.
Almost always the opposite. Credit utilization (balance ÷ credit limit) is the second-largest factor in FICO scoring after payment history, so lower utilization means a higher score. The one caveat: closing the card after paying it off reduces your total available credit and can briefly raise utilization on your remaining cards. For score optimization, keep the paid-off card open with a $0 balance.
Often yes, if you can pay the balance off before the promo ends. Typical offers are 12–21 months at 0% APR with a 3–5% transfer fee. On a $5,000 balance, a 4% fee costs $200 — versus roughly $750+ in interest at 22% APR over the same window. The risk is the rate jumping to 20–25% after the promo, so the transfer is only smart if you have a concrete payoff plan.
Most US issuers use the average daily balance method. Each day, they compute a daily periodic rate (APR ÷ 365) and apply it to that day's balance. At month end, they sum the daily interest charges. This is why carrying a balance even for a few days costs you — and why paying the statement balance in full before the due date avoids interest entirely thanks to the grace period.