Open the calculator: it is on our free tools page. It runs both methods across up to four debts and shows you months and total interest for each.
The full strategic argument is in our snowball versus avalanche guide. This article is about entering your numbers correctly, because four inputs are usually wrong and each one distorts the answer.
What the calculator does
It simulates your debt month by month:
- Interest accrues on every balance at its stated APR ÷ 12
- Minimum payments are applied to each debt
- Your extra payment is directed to one target debt — smallest balance for snowball, highest APR for avalanche
- When a debt clears, its minimum payment rolls onto the next target
- Repeat until everything reaches zero
It outputs months to debt-free, total interest paid, and total paid. Run it twice, once per method, and compare.
Input 1: the real APR, not an estimate
This is the input people guess at, and the guess is usually low.
Find the actual rate on your statement — not the promotional rate you signed up for, not the rate from two years ago, and not "about 20%". It is printed on every statement, and it changes if you have missed a payment, because most cards carry a penalty APR that triggers on delinquency and can be substantially higher.
| If your APR is... | Check this |
|---|---|
| 0% promotional | The expiry date. Enter the post-promotional rate for any debt you will not clear before it ends |
| Under 15% on a credit card | Whether it is actually a personal loan or a balance transfer still in promotion |
| Over 26% | Whether a penalty APR has been applied, and whether you can get it removed |
| Different across cards | Enter each separately — averaging destroys the comparison |
The 0% promotional case is the one that breaks people. If you have a $9,000 balance at 0% that reverts to 26% in eleven months, and your plan takes eighteen months to reach it, avalanche with the promotional rate will target it last — and you will pay three months of 26% on a $9,000 balance, roughly $630, that the model never showed you. Enter the rate that will apply when the plan reaches the debt.
Input 2: minimum payments, exactly
Most cards calculate the minimum as a percentage of balance plus interest, with a floor — commonly 1–3% plus interest, minimum $25–$40.
This matters because a declining minimum is far worse than a fixed one, and the calculator models it two ways depending on how you enter it. Two options:
- Enter the current minimum as a fixed figure. Conservative and safer — it assumes you keep paying the same amount as the balance falls, which is what you should do anyway.
- Enter it as declining. More accurate to how the card behaves if you only ever pay what is asked, and it produces much longer payoffs — which is the point.
We recommend entering the current minimum as fixed, then treating any amount the minimum would have dropped to as extra payment. That is the behaviour that produces the snowball/avalanche result. If you let the minimum decline as the balance falls, you are not running either method — you are running the thirteen-year minimum-payment outcome described in our card payoff guide.
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Input 3: extra payment, honestly
The single most influential input, and the one where optimism does the most damage.
Do not enter what you hope to find. Enter what you have actually saved, on average, over the last three months — or what you can set up as an automatic transfer on payday today.
The relationship is dramatic. On a $15,000 stack at 24% average APR:
| Extra payment | Months | Total interest |
|---|---|---|
| $0 | 17 yr+ (minimums only) | $32,000 |
| $200 | ~96 | $16,900 |
| $400 | ~55 | $9,600 |
| $600 | ~41 | $6,900 |
| $800 | ~33 | $5,400 |
Going from $200 to $400 saves more than the entire snowball-versus-avalanche choice, several times over. If you are spending twenty minutes choosing a method and zero minutes finding $200, you are optimising the wrong variable.
The extra payment is found in tier one and two of the saving levers — housing, transport, insurance, subscriptions — not in the method choice.
Input 4: leave out what does not belong
Do not include:
- A mortgage. It is secured, low-rate and long-dated. Including it in a snowball will make it the last target and distort everything. Mortgage overpayment is a separate decision with a separate analysis.
- A low-rate student loan unless you are specifically targeting it. Federal loans have income-driven options, forgiveness paths and rates often below 7% — see our student loan repayment guide before accelerating them.
- A 0% promotional balance you will clear inside the promotion. It has no cost and no urgency.
- Money owed to family. Not an interest-bearing debt and it does not behave like one.
- Debts you are disputing or that may be time-barred. Resolve the status first — see negotiating with collectors.
Do include every credit card, personal loan, medical bill on a payment plan, auto loan above roughly 7%, and any buy-now-pay-later balance still accruing.
Reading the output
Three numbers to compare, in order of importance:
1. Months to debt-free. The behavioural number. A plan that ends in month 34 is much more likely to be completed than one ending in month 62, regardless of the interest difference.
2. Total interest. The financial number. The difference between methods is usually 2–8% of total interest — meaningful but rarely decisive.
3. The difference between them. This is the actual decision input. If it is under $200, choose whichever you will follow. If it is over $1,000, choose avalanche unless you have abandoned plans before.
Look at whether your smallest balance is also your highest APR. Store cards and subprime cards frequently carry both low limits and high rates, which means snowball and avalanche produce the identical payment order — and you get the motivational benefit for free. If that is your situation, stop deliberating and use snowball.
What to do with the result
- Freeze the cards you are paying down. Not cut back — stop. A plan running alongside new spending does not complete.
- Set a fixed monthly payment, automated, for the day after payday. Not the day before the due date.
- Direct the entire amount according to your chosen method. Minimums on everything, extra on the target.
- Re-run the calculator every six months or after any rate change, payment change or new debt. Your optimal order can change.
- Bank windfalls into the target debt. Tax refunds alone commonly cut a year off the term.
- Do not close accounts as they clear, except cards with annual fees you will not use. Closing your oldest card hurts utilisation and average account age — see what your score controls.
When the calculator says something uncomfortable
If the months-to-debt-free figure is above roughly 60, one of three things is true:
- Your extra payment is too small. Go find $200 more per month before choosing a method.
- Your APR is too high. A balance transfer to 0% or a personal loan at 12% changes the answer more than any ordering rule. Call your issuer and ask for a rate reduction first — it costs eleven minutes.
- The balance is too large relative to income. At that point this is a structural problem and the options are credit counselling through a nonprofit agency, or a bankruptcy consultation. Neither is failure; both are tools, and both are better than a plan that runs for eleven years and does not finish.
Which method does the calculator say is better?
Avalanche almost always produces a lower total interest figure, because it targets the most expensive debt first. The gap ranges from zero — when your smallest balance is also your highest rate — to several thousand dollars when a small low-rate debt sits first in the snowball order. Run your own numbers; the generic comparisons use generic debts.
Should I include my mortgage or student loans?
Generally no. Mortgages are secured, low-rate and long-dated, and including one distorts the entire ordering. Federal student loans have income-driven and forgiveness options that change the analysis. Include credit cards, personal loans, auto loans above roughly 7%, and any high-cost consumer debt.
What if I get a windfall mid-plan?
Apply it to the current target debt and re-run the calculator — it may change which debt should be targeted next, particularly if the windfall clears one entirely. Keep a small portion for your emergency fund if that is not yet at tier one, since without it the next surprise goes straight back on a card.
How often should I re-run this?
Every six months, and immediately after any change in APR, minimum payment, extra payment capacity, or a new debt. Rate changes are the most common reason the optimal order shifts, especially when a 0% promotional period expires.
- Federal Reserve, Consumer Credit — revolving credit APR data.
- Consumer Financial Protection Bureau — credit card agreement database.
- CARD Act of 2009 — minimum payment disclosure requirements.
Reviewed for accuracy against our editorial guidelines. Figures quoted are illustrative and reflect publicly available rates at the time of the last update; always confirm current terms with the provider.