{ "packVersion": "2026.07.1", "formulas": [ { "id": "monthly-interest-accrual", "label": "Monthly interest accrual", "expression": "interest per month = balance x (APR / 12)", "katex": "I_m = B \\times \\frac{APR}{12}", "source": { "label": "CFPB — How does my credit card company calculate the amount of interest I owe?", "url": "https://www.consumerfinance.gov/ask-cfpb/how-does-my-credit-card-company-calculate-the-amount-of-interest-i-owe-en-51/", "citation": "Consumer Financial Protection Bureau, Ask CFPB answer 51: card companies calculate interest daily from a daily periodic rate on the average daily balance; the monthly convention shown here is the standard calculator approximation of that process." }, "verifiedDate": "2026-07-31", "status": "verified", "uncertainty": "issuers compound daily on the average daily balance, so a real statement can differ by a few dollars", "provenance": "convention", "notes": "Every schedule on this site accrues interest once per month and rounds to cents after each step. The approximation and its direction are disclosed on /methodology." }, { "id": "minimum-payment-formula", "label": "Issuer minimum-payment formula", "expression": "minimum = max(floor, pct x balance + interest and fees)", "katex": "M = \\max(F,\\; p \\times B + I)", "source": { "label": "CFPB — The Consumer Credit Card Market (December 2025), pp. 70, 76", "url": "https://files.consumerfinance.gov/f/documents/cfpb_consumer-credit-card-market-report_2025.pdf", "citation": "Consumer Financial Protection Bureau, The Consumer Credit Card Market (December 2025): most issuers set the minimum payment at 1 percent of the cycle-ending balance plus finance charges and fees; all reviewed issuers set fixed dollar floors, which ranged from $15 to $50, with $40 the most common." }, "verifiedDate": "2026-07-31", "status": "verified", "uncertainty": "your issuer's exact formula is in your cardholder agreement and may differ", "provenance": "primary", "notes": "The convention rows live in min-payment-conventions.json; the engine takes one row as a parameter and stays pure." }, { "id": "fixed-payment-amortization", "label": "Fixed-payment amortization", "expression": "each month: interest = balance x APR/12; principal = payment - interest; closed form n = -ln(1 - rP/M) / ln(1+r)", "katex": "n = \\frac{-\\ln(1 - rP/M)}{\\ln(1+r)}", "source": { "label": "CFPB — How does paying down a mortgage work?", "url": "https://www.consumerfinance.gov/ask-cfpb/how-does-paying-down-a-mortgage-work-en-1943/", "citation": "Consumer Financial Protection Bureau, Ask CFPB answer 1943: with a fixed payment, each month's interest is computed on the remaining balance and the rest of the payment reduces principal, so early payments are mostly interest and the split reverses over time — the standard amortization mechanics this engine implements." }, "verifiedDate": "2026-07-31", "status": "verified", "provenance": "primary", "notes": "The engine's simulated schedule and the closed-form month count are tested to agree (tests/debt/amortize.test.ts)." }, { "id": "snowball-method", "label": "Debt snowball ordering", "expression": "pay minimums on everything; direct all extra at the SMALLEST balance; roll freed minimums into the next debt", "source": { "label": "CFPB — How to reduce your debt", "url": "https://www.consumerfinance.gov/about-us/blog/how-reduce-your-debt/", "citation": "Consumer Financial Protection Bureau, How to reduce your debt: the snowball method focuses on the smallest debt first while making minimum payments on the rest, then moves to the next smallest once it is paid off." }, "verifiedDate": "2026-07-31", "status": "verified", "uncertainty": "snowball usually costs somewhat more interest than avalanche; the comparison tool shows the exact difference for your debts", "provenance": "primary", "notes": "The engine's rollover invariant (constant budget, freed minimums cascade) is what makes the method work and is pinned by tests/debt/rollover.test.ts." }, { "id": "avalanche-method", "label": "Debt avalanche ordering", "expression": "pay minimums on everything; direct all extra at the HIGHEST APR; roll freed minimums into the next debt", "source": { "label": "CFPB — How to reduce your debt", "url": "https://www.consumerfinance.gov/about-us/blog/how-reduce-your-debt/", "citation": "Consumer Financial Protection Bureau, How to reduce your debt: the highest-interest-rate method focuses extra payments on the debts with the highest rate of interest, which minimizes total interest paid." }, "verifiedDate": "2026-07-31", "status": "verified", "provenance": "primary", "notes": "Mathematically the interest-minimizing order among constant-budget plans; the site states the trade-off against snowball rather than declaring a winner." }, { "id": "balance-transfer-fee", "label": "Balance-transfer fee and promo math", "expression": "fee = max(feeMin, feePct x balance); transferred balance = balance + fee; compare stay vs transfer at the same monthly payment", "source": { "label": "CFPB — What is a balance transfer fee?", "url": "https://www.consumerfinance.gov/ask-cfpb/what-is-a-balance-transfer-fee-can-a-balance-transfer-fee-be-charged-on-a-zero-percent-interest-rate-offer-en-53/", "citation": "Consumer Financial Protection Bureau, Ask CFPB answer 53: a balance transfer fee is charged to move an outstanding balance to a different card, and it may be charged even on a zero percent interest rate offer." }, "verifiedDate": "2026-07-31", "status": "verified", "uncertainty": "the fee is usually capitalized into the transferred balance, which this tool models and states on-page", "provenance": "primary", "notes": "The comparison output is 'the interest difference in this scenario' — never a savings promise (TSR posture)." }, { "id": "deferred-interest-retroactive", "label": "Deferred-interest retroactive charge", "expression": "interest accrues silently from day one; if any balance remains at window close, ALL accrued interest is charged at once", "source": { "label": "CFPB — No interest if paid in full within 12 months: how does this work?", "url": "https://www.consumerfinance.gov/ask-cfpb/i-got-a-credit-card-promising-no-interest-for-a-purchase-if-i-pay-in-full-within-12-months-how-does-this-work-en-40/", "citation": "Consumer Financial Protection Bureau, Ask CFPB answer 40: on a deferred interest plan, if you do not pay off the entire promotional balance in time, interest going back to the date of purchase is added on top of the remaining balance." }, "verifiedDate": "2026-07-31", "status": "verified", "uncertainty": "accrual is modelled monthly on the declining balance; issuers compute it daily on average daily balance", "provenance": "primary", "notes": "This is NOT the same as a true 0% intro APR — the distinction is the whole point of the tool and is stated in its first sentence." }, { "id": "credit-utilization", "label": "Credit utilization", "expression": "utilization = balance / limit x 100, per card and across all cards", "katex": "U = \\frac{B}{L} \\times 100", "source": { "label": "CFPB — How do I get and keep a good credit score?", "url": "https://www.consumerfinance.gov/ask-cfpb/how-do-i-get-and-keep-a-good-credit-score-en-318/", "citation": "Consumer Financial Protection Bureau, Ask CFPB answer 318: keeping credit use below about 30 percent of total limits is the commonly cited guidance, and paying balances in full each month earns the best scores; utilization is reported both per card and overall." }, "verifiedDate": "2026-07-31", "status": "verified", "uncertainty": "the 30% figure is guidance widely cited by the CFPB, not a scoring-model constant; this site never simulates a score", "provenance": "primary", "notes": "Education-side only: the tool computes the ratio and the paydown needed to reach a target ratio, and stops there." } ] }