How these numbers are calculated

The engine's conventions

1. Interest accrues monthly. Each month the engine adds balance × APR / 12, then applies the payment. Card issuers actually compound daily on your average daily balance, which makes real statements slightly worse than this convention shows — the approximation is disclosed on every tool rather than hidden, and the month counts it produces are the standard ones every payoff calculator quotes.
2. Money is rounded to cents after every step. Schedules sum exactly: the rows of any table on this site add up to its totals, and a final balance is exactly zero, never −0.004. Floating-point arithmetic is nudged before rounding so a value that is mathematically at half a cent rounds the way arithmetic says it should.
3. The rollover invariant. In snowball and avalanche plans the monthly budget is constant: the sum of every debt's original minimum payment plus your extra. When a debt clears, its minimum does not vanish — it rolls into the next debt the same month, including any leftover from the clearing payment. This is the entire mechanism of the method, and the engine's tests pin it: every month, payments across all debts sum to that constant budget until the last debt ends.
4. Minimum payments are recomputed monthly. A minimum-payment schedule recalculates the minimum from each month's new balance using an issuer convention from the CFPB's December 2025 market report — the default is 1% of balance + interest, $40 floor. Your card's exact formula is in your cardholder agreement, and the tool lets you switch conventions.
5. Dates clamp, not slide. A schedule that starts January 31 has its February payment on February 28 (or 29), not March 3. Month n of a schedule is always the start date plus n months, clamped to the end of shorter months.
6. Honest failure states. A payment that does not cover the month's interest is reported as "never pays off", with the interest-only threshold shown — never a 100-year schedule quietly truncated. Schedules are capped at 1,200 months and say so when the cap bites.
7. Reference APRs are context, not inputs. Where a page cites an average card rate (22.15% on accounts assessed interest, May 2026), it comes from the Federal Reserve's G.19 release, is dated, and is never silently substituted for the rate you type.

The rules we hold ourselves to

1. Tools, not advice. These pages compute and cite. They do not tell you what to do about a balance, because that depends on the rest of your situation — which a website cannot see.
2. Every formula named inline. The formula and its source appear on the page that uses it, not buried here.
3. Figures belong to the bodies that publish them. We write "the CFPB documents this formula as…", never "your card works like this". No result is styled as an alarm, and no page uses guilt as a design element.
4. No changing number is hardcoded. Reference values live in versioned data files with a source URL and a check date; the build fails if any row is missing one.
5. We defer to the source of record. Every tool links the official document it derives from — the CFPB, the Federal Reserve, CMS, the IRS — and where an official calculator exists, we link that too.
6. Nothing you type is transmitted. All calculation is client-side; there is no account, and no balance, debt list, or letter draft is ever sent to a server.

Formula registry (8 entries)

• Monthly interest accrualconvention

  interest per month = balance x (APR / 12)

  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. Source ↗

  Known gap: issuers compound daily on the average daily balance, so a real statement can differ by a few dollars

  Last checked 2026-07-31

• Issuer minimum-payment formula

  minimum = max(floor, pct x balance + interest and fees)

  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. Source ↗

  Known gap: your issuer's exact formula is in your cardholder agreement and may differ

  Last checked 2026-07-31

• Fixed-payment amortization

  each month: interest = balance x APR/12; principal = payment - interest; closed form n = -ln(1 - rP/M) / ln(1+r)

  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. Source ↗

  Last checked 2026-07-31

• Debt snowball ordering

  pay minimums on everything; direct all extra at the SMALLEST balance; roll freed minimums into the next debt

  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. Source ↗

  Known gap: snowball usually costs somewhat more interest than avalanche; the comparison tool shows the exact difference for your debts

  Last checked 2026-07-31

• Debt avalanche ordering

  pay minimums on everything; direct all extra at the HIGHEST APR; roll freed minimums into the next debt

  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. Source ↗

  Last checked 2026-07-31

• Balance-transfer fee and promo math

  fee = max(feeMin, feePct x balance); transferred balance = balance + fee; compare stay vs transfer at the same monthly payment

  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. Source ↗

  Known gap: the fee is usually capitalized into the transferred balance, which this tool models and states on-page

  Last checked 2026-07-31

• Deferred-interest retroactive charge

  interest accrues silently from day one; if any balance remains at window close, ALL accrued interest is charged at once

  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. Source ↗

  Known gap: accrual is modelled monthly on the declining balance; issuers compute it daily on average daily balance

  Last checked 2026-07-31

• Credit utilization

  utilization = balance / limit x 100, per card and across all cards

  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. Source ↗

  Known gap: the 30% figure is guidance widely cited by the CFPB, not a scoring-model constant; this site never simulates a score

  Last checked 2026-07-31

Expert review

The highest-stakes pages carry a reviewer byline slot. It renders only when a licensed professional — a CPA or an attorney, depending on the page — has actually reviewed it, with their name, credential and the date. Where you see no byline, no review has happened yet, and we would rather leave the space empty than fill it with something reassuring and untrue.

Data updates

Reference values get re-checked against their sources on a schedule — card-rate figures each G.19 release, issuer conventions against each CFPB market report — and every change is dated in the changelog. The most recent entry is 2026-07-31. The verified data files are also served as plain JSON for anyone who wants to check our arithmetic or cite the same sources — see the developers page.

Coverage

This site currently publishes 14 calculators. Some things are missing by design: there are no student-loan tools, no debt-to-income calculator, no consolidation or settlement content, and no credit-repair content here — and there never will be. HSA and FSA calculators live at budgetbuildinvest.com, their canonical home in this network.

Canonical: https://debtandbills.com/methodology · plain-text summary at https://debtandbills.com/llms.txt