How our calculators work

Updated Nov 7, 2026

This page explains, step by step, how SteadyZero’s debt-payoff calculators turn your numbers into a payoff plan. We publish it so you can check our work. Every result is an estimate: your lender’s statement is the final word on what you owe.

On this page: Payoff orders · Interest · Minimum payments · 0% promos and balance transfers · Rounding · Assumptions and limitations · Test cases · Corrections and updates · AI and automation

1. The monthly cycle

For each month, the calculator does three things for every debt, in this order:

1. Add interest for the month to the current balance (section 3).
2. Pay the minimum on every debt that still has a balance (section 4).
3. Put the rest of your monthly budget toward debts in your chosen payoff order (section 2).

A month counts as one payment period, and payments are assumed to post at the end of the period. The plan ends when every balance reaches $0.00.

Your monthly debt budget is the total you’ll pay toward all debts each month. You enter your minimums plus any extra, so your budget always covers every minimum. If it doesn’t cover the interest, the calculator says the balances never reach zero instead of giving a date.

2. Payoff orders

Both orders pay every minimum first. They differ only in where the extra money goes.

• Rollover: when a debt is paid off, its minimum payment isn’t removed from your budget. It joins the extra money and goes to the next debt in your order. This “rolling” effect is what makes snowball and avalanche plans speed up over time.
• Same-month spillover: if the extra payment is more than what’s left on the target debt, the remainder goes to the next debt in the order that same month.
• The order is fixed when the plan starts. Snowball ranks debts by their starting balances and doesn’t re-sort as balances change. This matches the usual description of the method, and it keeps the plan predictable.
• How your lender applies payments may differ. On a single credit card with several balances (for example, purchases plus a balance transfer), federal rules generally require the issuer to apply the amount above the minimum to the highest-APR balance first (CFPB). Our calculators treat each debt you enter as one balance with one APR. If a card has several APR balances, enter each one as a separate line.

3. Interest: APR/12 monthly vs. daily balance

What we use: each month, interest = balance × (APR ÷ 12), rounded to the cent. This is a monthly-rate approximation.

What many card issuers actually do: many credit card issuers charge interest daily. They use a daily periodic rate (APR ÷ 365, or ÷ 360 at some issuers) applied to the average daily balance, often compounding daily (CFPB: how card interest is calculated; CFPB: daily periodic rate; CFPB: contract definitions). Installment loans (auto, personal, student) usually accrue simple daily interest on the principal balance.

Why we use APR/12:

• It doesn’t need your billing-cycle dates, your purchase activity, or the exact day your payments post, which most people don’t have handy.
• It’s the convention used by the standard loan formulas (Excel’s NPER and PMT), so you can reproduce our results.
• Over a whole year the difference is small, but it isn’t zero.

How big is the difference? For a $5,000 balance at 24% APR with no payments or new charges during the cycle:

The daily-compounding figures above are a worked comparison only: no live calculator uses daily compounding, so they aren’t covered by automated tests (the APR ÷ 12 figure is). Your actual monthly interest can run slightly above or below our figure, depending on cycle length and when you pay. Paying earlier in the cycle lowers interest under daily methods, and our calculators don’t model that benefit.

4. Minimum payments

Minimum payment formulas are set by each card agreement. Our calculators handle minimums in two ways:

• Snowball vs. Avalanche Calculator: you enter a fixed dollar minimum for each debt (the minimum on your statement). It stays the same every month until that debt is paid off.
• Credit Card Payoff Calculator, “paying only the minimum” comparison: an estimate that recalculates each month:
  • minimum = a percentage of the balance before interest (default 1%) + that month’s interest;
  • but never less than a dollar floor (default $25);
  • and never more than the remaining balance.

This estimate follows a common structure in published card agreements. For example, one large issuer’s agreement uses the greater of $25; 1% of the new balance plus interest and fees; or 1.5% of the new balance (example cardmember agreement, CFPB database). Our estimate uses only the percentage-plus-interest part and the dollar floor. Real formulas may also add fees and past-due amounts, and may round to the nearest dollar. Our estimate rounds to the cent and assumes no fees or past-due amounts.

In a multi-debt plan, each minimum stays fixed until its debt is paid off. Your monthly budget stays the same, so when a debt is paid off, its minimum rolls into the extra payment for the next debt (section 2).

5. 0% promotional APRs, balance transfers and deferred interest

• 0% (or reduced) intro APR: you enter the promo APR, the promo length in months, and the rate it reverts to. The calculator uses the promo APR for those months and the revert APR afterward. Interest at the revert APR applies only to the balance left when the promo ends.

• Balance transfer fee: entered as a percentage (common examples are 3% to 5%). The fee is added to the transferred balance on day one and accrues interest like the rest of the balance once the promo ends. The calculator also shows the payment needed to clear the balance before the promo ends: the smallest whole-cent monthly payment that reaches $0 by the last promo month (at a 0% promo, that’s (balance + fee) ÷ promo months, rounded up).

• Deferred interest (“no interest if paid in full,” common on store cards): this is not the same as 0% APR. If any promo balance remains when the period ends, or you fall more than 60 days late, the issuer can charge interest going back to the purchase date (CFPB: deferred interest).

  Our calculators don’t model deferred interest. Here’s how issuers typically calculate it (see Test case 6):

  • Interest builds up in the background each month at the standard APR while the promo runs.
  • If any promo balance is left when the promo ends, all of that built-up interest is added to the balance.
  • Your issuer’s exact method and figure may differ; your card agreement and statements show how it applies to you.

• The calculator doesn’t model losing a promo rate because of a late payment or a penalty APR. It assumes every payment is on time.

6. Rounding

• Money is calculated in cents. Each month’s interest is rounded to the nearest cent (half-cent rounds up) before it’s added to the balance.
• Payment-needed figures (such as “pay $X/month to finish the promo”) round up to the next cent, so following them never leaves a few cents unpaid.
• The final payment on each debt equals whatever is left, so it’s usually smaller than the regular payment.
• “Months to payoff” counts every month with a payment, including a small final one. Our month count can therefore be one higher than a fractional NPER result, which is the case in Test case 1.
• Totals (total interest, total paid) are sums of the rounded monthly amounts.

7. Assumptions and limitations

Our calculators assume:

• No new charges, cash advances or new loans after the start date.
• Every payment is made in full and on time, at the end of each monthly period, with no late fees, penalty APRs or lost promos.
• APRs stay fixed unless you enter a promo period. Variable-rate cards and loans will actually change with their index.
• No fees other than a balance transfer fee you enter: no annual fees, late fees or loan origination fees unless the calculator asks for them.
• One balance and one APR per line (section 2).
• Monthly interest of APR ÷ 12 (section 3), not your issuer’s exact daily method.

They do not:

• Account for taxes, deductions (for example, student loan interest), loan forgiveness programs, income-driven repayment, hardship plans, or anything your lender may offer.
• Know your credit report or credit score, and can’t predict how a plan affects either.
• Tell you which debts to pay or whether to borrow or transfer a balance. They show the math for the scenario you enter.

Your data: calculations run in your browser. Your calculator inputs never leave your browser, unless you choose to share a results link. We don’t collect email addresses on the site at launch: there’s no signup form, and “Email me my results” just opens your own email app (Opens your email app. We don’t store your address.) What we do store: (1) if you buy a template, the order record we receive from Stripe and Link (Sold through Link, LLC, a Stripe company), our merchant of record, so we can deliver your files (our Privacy Policy lists exactly what’s in it); and (2) cookieless Cloudflare Web Analytics for overall site traffic. We don’t use advertising pixels. See our Privacy Policy.

8. Test cases

We publish these so anyone (including us) can check the calculators. Every release of our calculators must pass automated tests that check them against the worked test cases on our methodology page; a mismatch blocks the release. Each case below says which calculator it checks. They were computed with the method above: APR ÷ 12, cents rounded half-up, payments at the end of each month.

Test case 1: one credit card, fixed payment

Automated test: Credit Card Payoff Calculator and Loan Payoff with Extra Payments. Inputs: $5,000 balance · 24.00% APR · $200 per month · no new charges.

• Monthly rate = 24% ÷ 12 = 2%.
• Month 1: interest = $5,000.00 × 0.02 = $100.00. Balance after the payment = 5,000 + 100 − 200 = $4,900.00.
• Month 2: interest = $4,900 × 0.02 = $98.00. Balance = $4,798.00.
• … this continues until month 36, when the final payment is $0.57.

Cross-check: the standard formula NPER(2%, −200, 5000) = 35.003 months, consistent with 35 full payments plus a tiny final one.

Test case 2: same card, smaller payment

Automated test: Credit Card Payoff Calculator and Loan Payoff with Extra Payments. Inputs: $5,000 · 24.00% APR · $150 per month. Result: 56 months, total interest $3,322.17. Cross-check: NPER = 55.48 months. So the extra $50 a month in Test case 1 saves 20 months and $1,321.60 in interest.

Test case 3: snowball vs. avalanche, four debts

Automated test: Snowball vs. Avalanche Calculator. Inputs: four debts below, $276 extra a month (a $750 monthly budget). Fixed minimums total $474.

In this example, avalanche costs $49.21 less in interest, and both finish in the same month. Snowball clears its first debt at month 5, while avalanche’s first payoff (Card C) comes at month 10. That illustrates the trade-off: avalanche is usually cheaper, while snowball gets you early wins. The size of the gap depends entirely on your numbers.

Test case 4: minimum payments only

Automated test: Credit Card Payoff Calculator (its “paying only the minimum” comparison). Inputs: $3,000 card · 22.00% APR · minimum estimated as 1% of the balance + interest, with a $25 floor · no extra payments.

• Month 1: interest = 3,000 × 0.22 ÷ 12 = $55.00. Minimum = 1% × $3,000 ($30.00) + $55.00 = $85.00.
• The minimum shrinks as the balance falls, until it hits the $25 floor.

Result: 180 months (15 years), total interest $4,433.02: more than the original balance.

Test case 5: balance transfer with a fee

Automated test: Balance Transfer Calculator. Inputs: transfer $6,000 · 3% fee · 0% for 15 months · reverts to 24.99%.

• Fee = $6,000 × 3% = $180.00. Starting balance = $6,180.00.
• Payment needed to clear it inside the promo = $6,180 ÷ 15 = $412.00 per month, with $0 interest.
• If you pay $300 a month instead: balance at promo end = 6,180 − (300 × 15) = $1,680.00. At 24.99% and $300 a month, that takes 7 more months (the last payment is $4.90), with $124.90 of interest.

Test case 6: deferred interest vs. paying in full

Worked example only: no live calculator models deferred interest, so there is no automated test for it. Inputs: $1,200 store-card purchase · “no interest if paid in full in 12 months” · standard APR 26.99%.

• Paying $100 a month: the balance is $0.00 after 12 payments, so no interest is charged.
• Paying $90 a month: $120.00 is left after 12 payments. The shadow interest tracked over those 12 months (each month’s starting balance × 26.99% ÷ 12) totals $190.28, and all of it is added. The new balance is $310.28, which is more than double the $120 left on the purchase.

9. Corrections and update policy

• Reporting an error: email support@steadyzero.com with the calculator URL, your inputs and the result you expected.
• What happens next:
  • If a calculator is wrong, we fix it, add your scenario as a new test case, and log the change on this page.
  • If an error could have materially misled people (for example, a wrong payoff date), we also add a dated note on the affected page.
• Scheduled review: we re-check every calculator against these test cases on each code release.
• Dates: the “Updated” date on a page changes only when its method, numbers or substance change, not for cosmetic edits.

10. AI and automation disclosure

We use software, including AI assistants, to help write code, draft explanations and check formatting. AI doesn’t decide how the math works. The method is documented on this page. Every release of our calculators must pass automated tests that check them against the worked test cases on our methodology page; a mismatch blocks the release. Pages are drafted with AI assistance. The operator of the Steady Zero website (steadyzero.com) is responsible for this page and fixes errors reported to support@steadyzero.com. See our Editorial Policy.

Educational estimates only. Not financial, legal, tax or credit advice. See our Disclaimer.