Calculator Methodology, ccpayoffcalc.com

Calculator Methodology

Last updated: May 2, 2026.

This page documents the math, assumptions, and limitations of the calculators on ccpayoffcalc.com. Reading it should let any user replicate the math in a spreadsheet and verify the calculator’s outputs.

Interest accrual

We use the average daily balance method, which is what nearly all U.S. credit card issuers use.

daily rate = APR / 365
daily interest = current balance × daily rate
monthly interest = sum of daily interest over the billing cycle

For example, on a $5,000 balance at 22.30% APR:

• Daily rate = 22.30% / 365 = 0.0611%
• Daily interest on $5,000 = $5,000 × 0.0611% = $3.06
• Monthly interest (30-day cycle, balance constant): $3.06 × 30 = $91.80

When you make a payment mid-cycle, the balance for the rest of the cycle is lower, so daily interest accrues at a lower rate for those days. This is why earlier-in-cycle payments save more interest than later-in-cycle payments.

Some retail and store cards use a 360-day basis instead of 365 (slightly more aggressive). We assume 365-day basis as the default; specific card pages may note when 360-day applies.

Minimum payment formula

We use the CFPB-typical formula:

minimum payment = max($25, 1% of principal + current month interest)

This is the formula most major U.S. issuers use to comply with CARD Act of 2009 requirements.

Specific issuers may vary slightly (some use 2% of principal + interest with a $35 floor; some apply different formulas in specific situations). The variations typically produce minimum payments within 5-10% of our model’s output. For multi-year payoff projections, the cumulative impact of formula variation is small.

If your specific minimum payment differs noticeably from our calculator’s output, the difference is usually:

1. Your issuer uses a 2%-of-principal floor (vs our 1%)
2. Your issuer charges fees that we don’t model (over-limit fees, late fees from previous cycles)
3. Your card’s APR has changed since you applied (variable APRs move with prime rate)

Strategy execution logic

For multi-card calculations, the calculator distributes your monthly payment across cards using the selected strategy.

Avalanche logic:

1. For each card: compute minimum payment via formula above. Apply minimum.
2. Compute "extra" budget = total monthly amount available - sum of minimums.
3. Apply entire "extra" budget to the card with the highest APR.
4. After each cycle, recalculate minimums and repeat.

Snowball logic:

Same as avalanche, but step 3 applies "extra" budget to the smallest-balance card.

Hybrid logic:

For the first 1-2 cards (configurable):
  Apply "extra" budget to the smallest-balance card.
After 1-2 cards are paid off:
  Apply "extra" budget to the highest-APR remaining card.

Balance transfer logic:

1. Compute transfer fee = transfer amount × fee%.
2. Add fee to transferred balance.
3. During promo (0% APR for X months): apply payments to principal only.
4. After promo: apply normal interest accrual at post-promo APR.
5. Track break-even: month at which cumulative interest savings exceed fee.

Inputs and outputs

Inputs:

• Per-card balance (dollar amount)
• Per-card APR (percentage; convertible from “interest rate” , they are the same on credit cards)
• Per-card minimum payment (optional; we compute via formula if not provided)
• Total monthly payment available (dollar amount)
• Strategy: avalanche, snowball, hybrid, or balance transfer
• Optional balance transfer parameters: transfer fee, promo length, post-promo APR

Outputs:

• Months to payoff
• Total interest paid
• Total amount paid (principal + interest)
• Per-month breakdown (interest, principal, balance) for each card
• Side-by-side comparison if multiple strategies selected

Assumptions and limitations

Fixed APR. We assume the APR you enter is constant throughout the payoff. Real-world APRs are typically variable (move with prime rate). If prime rises during your payoff, actual interest will be slightly higher than projected; if prime falls, slightly lower.

No new spending. We assume you do not make new purchases on the cards being paid off. New spending changes the balance and the strategy logic.

No fees beyond what is modeled. We model the transfer fee for balance transfers. We do not model annual fees, late fees, over-limit fees, foreign transaction fees, or cash advance fees. If your card has an annual fee, mentally add it to total cost per year.

No prepayment penalty. We assume payments above the minimum reduce balance immediately. This is true of credit cards. Personal loans and HELOCs may have different behaviors, especially if the loan does not re-amortize.

No partial-cycle precision. The calculator computes monthly snapshots. If you make payments at irregular intervals, the actual interest accrual differs slightly from the monthly model.

No grace period. We assume any carried balance forfeits the grace period on new purchases (which is true on most cards). If your card has unusual grace period rules, results may vary.

No penalty APR. We do not model the penalty APR that some cards apply after late payments. If you trigger a penalty APR, your actual interest will be substantially higher than the calculator’s projection.

Federal Reserve and CFPB data integration

For “average APR” claims on this site, we cite Federal Reserve G.19 for current commercial bank rates and the CFPB Consumer Credit Card Market Report for issuer-reported survey rates.

The two sources sometimes produce slightly different numbers because they survey different cuts of the market:

• G.19 measures interest rates on credit card accounts at commercial banks, including the rate on accounts that pay no interest (transactors)
• CFPB report surveys card issuers directly and reports rates on actual carried balances

Both are accurate; we cite each in context. When we say “average APR” we typically mean the CFPB issuer-survey number (currently 22.30% for general purpose, 28.93% for store cards). When we say “Q1 2026 average all-account rate” we mean G.19’s number (currently 21.00%).

Updating the model

The calculator math is stable; the input data updates on these cadences:

• G.19 rates: monthly (first business week of each month)
• CFPB report rates: annually (typically December)
• Issuer-specific APRs (programmatic pages): verified daily by automated checker against issuer pricing pages
• Personal loan rates: updated quarterly per Federal Reserve H.15 and major lender disclosure pages

The “Last verified” date at the top of each calculator page reflects the most recent data verification.

Replicating our findings

To replicate any specific calculator result:

1. Use the same input balance, APR, and monthly payment
2. Apply the daily-balance method as documented above
3. Apply the CFPB-typical minimum payment formula
4. Run forward in monthly increments

Open Excel, Google Sheets, or any standard finance calculator. The math is replicable.

Software and infrastructure

The calculator runs entirely in your browser. Inputs do not leave your device. No backend storage, no analytics on input values, no email gates.

The Behavior-aware Payoff Coach (AI feature) sends only summary statistics (computed totals, not your card data) to our Vertex AI integration on Google Cloud. The AI returns coaching steps; the AI’s responses are not stored.

Not financial advice. Calculations are estimates based on the inputs you provide. Consult a non-profit credit counselor (NFCC member) or licensed financial advisor before making major debt-management decisions.

Quick answers

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