Methodology

Every figure on this site is produced by a single payoff engine — the same engine the Dang! Payoff app uses. The public pages do not carry their own arithmetic: the numbers in the static HTML are generated by calling that engine at build time, and the calculators call the identical module in your browser when you change an input.

This page states the conventions, so that any number here can be checked by hand.

The monthly loop

Each month, in this order: interest accrues on every live debt at its balance times the monthly share of its APR; every live debt is paid its minimum, capped at what is owed; and the whole remaining monthly budget is applied to the priority debt, rolling on to the next if that debt is cleared mid-month.

The opening month is not special. Interest accrues before the earliest payment, the same as every other month.

The budget never shrinks

The total monthly budget is the sum of all minimums plus any extra, and it is held constant for the whole plan. When a debt is cleared, its minimum stays in the budget and joins the attack on the next debt. This is what makes both snowball and avalanche accelerate.

Priority order

Snowball ranks by lowest balance, then highest APR, then the order you entered them. Avalanche ranks by highest APR, then lowest balance, then input order. The ranking is recomputed every month, so a debt that becomes the smallest mid-plan takes over.

When a payoff is impossible

Before simulating anything, the engine checks whether the monthly budget exceeds the opening month's total interest. If it does not, the balance can never fall and the engine says so, rather than returning a date impossibly far out.

Minimum payments

The payoff engine treats a minimum payment as an input you supply. For the minimum-payment pages we model the minimum itself, recomputing it from the balance each month under a stated rule — because a real minimum shrinks as the balance does, and holding it fixed would understate the term.

Both common issuer shapes are modelled: this month's interest plus a percentage of the balance with a dollar floor, and a flat percentage of the balance with the same kind of floor. Minimum-payment formulas differ between issuers and yours is in your cardholder agreement; we do not claim either shape is universal.

Precision and rounding

Full floating-point precision is kept throughout the simulation and rounding happens only at output, with total interest rounded to cents. Values within a vanishing fraction of nothing are treated as nothing, so floating-point dust never shows up as a lingering balance.

Dates are estimates

Month arithmetic is integer year-and-month arithmetic with no timezone involved, so a payoff month does not drift. But the month itself is only as good as the assumptions: a constant rate, no new spending, no fees, no missed payments. Every calculator lists its assumptions beside its answer. We describe these as estimates, never as an exact day.

What the engine does not model

Average-daily-balance interest computation, cash-advance and balance-transfer rates, promotional rates expiring, annual fees, late fees, penalty APRs, variable rates changing, and anything about your credit report. A statement from your issuer is about your actual account; this is a general model.

Related

• Corrections and source policy
• About Dang! Payoff
• Debt payoff calculator