Methodology

By the site owner. Last reviewed 2026-10-18.

This page explains exactly how the calculators work, so you can check the logic or reproduce a result in a spreadsheet. The same rules apply to the debt payoff calculators and the credit card calculator.

How is each month calculated?

Each month, in this order:

1. Interest is added. For every debt with a balance, interest = balance x APR / 12, rounded to the nearest cent, and added to the balance.
2. Minimums are paid. Every active debt receives its minimum payment, or its whole balance if that is smaller.
3. The rest goes to the target. The monthly budget is the sum of all starting minimums plus your extra payment. After the minimums are paid, what is left goes to the first unpaid debt in the target order, then spills to the next if that debt is cleared.
4. Cleared debts roll over. When a debt is paid off its minimum is not released into your spending: it stays in the budget (the "snowball effect").

The budget therefore stays constant until the last month, which is usually smaller because it only needs to cover what remains.

Target order

• Snowball: smallest starting balance first. Ties go to the higher APR.
• Avalanche: highest APR first. Ties go to the smaller balance.
• Minimums only: the baseline we compare against. Each debt is paid at its own minimum with no extra and no rollover.

The order is set once at the start and not re-sorted as balances change.

Rounding and arithmetic

All money is held as whole cents internally, so no floating-point drift builds up. Interest is rounded to the nearest cent every month. Results shown in whole dollars are rounded only at display time. Totals add the cent-rounded monthly figures.

Limits and what happens at them

• 600 months. The simulation stops at 50 years. If debts remain, the calculator says so and shows no debt-free date.
• Minimum below interest. Flagged for each debt. If the total payment cannot get ahead of the interest, the plan is reported as not shrinking.
• Zero APR. Supported. Interest is zero.
• Payments never exceed what is owed. The last payment on a debt is smaller.
• Input ranges. Up to 15 debts, balances up to 10 million, APR from 0 to 100%.

Credit card calculator

A single debt using the same monthly cycle. Three variations: a fixed monthly payment; the payment needed to clear the card in a given number of months; and an issuer-style minimum that is recomputed each month (a percentage of the balance, optionally plus that month's interest, never below a floor). The needed payment starts from the annuity formula P = r x B / (1 - (1 + r)^-n), with r = APR / 12, then is rounded up to the cent and checked against the month-by-month simulation, because cent rounding can push the last payment past the target month by a few cents.

How many months does it take to pay off a debt? A hand-checked example

A balance of $1,000 at 12% APR (1% a month) with a $100 payment. The closed-form number of months is n = -ln(1 - 0.01 x 1000 / 100) / ln(1.01) = 10.59, so 11 payments. After 10 payments the balance is 1,000 x 1.01^10 - 100 x (1.01^10 - 1) / 0.01 = $58.40. Month 11 adds $0.58 of interest, so the final payment is $58.98 and total interest is $58.98. The calculator produces exactly this: 11 months, $58.98 interest. This case, and several others, run as automated tests.

What is not modeled

• Variable rates, introductory or promotional rates that end, and fees (annual, late, transfer or origination).
• Daily compounding and the exact day count in each billing cycle.
• New borrowing, skipped payments, grace periods or deferment.
• Tax effects such as interest deductibility.
• Which payments your lender applies to interest or principal first when they differ from the model.

Assumptions about dates

The first payment is assumed to be next month. The debt-free date is the month of the last payment.