What the two orders actually are
Both strategies pay the minimum on everything. The only question is where the money above the minimums goes. The avalanche sends it to the highest interest rate. The snowball sends it to the smallest balance. Everything else about the two plans is identical, including the total monthly outlay, which is why the comparison is clean: the same dollars, in a different order, produce a different total.
The avalanche cannot lose on interest. Interest accrues on balances at their own rates, so removing a dollar from a 22.9 percent balance stops more future interest than removing a dollar from a 4.5 percent balance, every time. There is no rate structure where reordering beats that. If a comparison ever shows the snowball costing less, the two runs were not using the same payment, or a fee is being modelled on one side and not the other.
Where the snowball earns its reputation
The snowball clears whole accounts sooner when the small balances are not the expensive ones. That is a real effect and it is not a financial one. A cleared account is one fewer payment to track, one fewer minimum that has to be met in a bad month, and a visible finish line reached early. There is published work suggesting people stick with payoff plans longer when they get an early completion, and a plan you finish beats a cheaper plan you abandon in month nine.
The way to use both facts together is to price the difference and then decide. If the avalanche saves $80 over three years, take the version you will actually follow. If it saves $2,400, that is a real amount of money and it is worth some discomfort. The table above gives you that number rather than a recommendation, because the recommendation depends on something the calculator cannot see.
Reading the payoff order table
Each row shows when a debt is cleared and how much interest it cost along the way. The interest column is per debt, so it tells you where the money went rather than just how much of it went. A small balance at a high rate often produces a surprisingly large figure, and a large balance at a low rate often produces a smaller one than expected, which is the whole argument for sorting by rate rather than by size.
The reconciliation block underneath is a check, not decoration. Total repaid always equals the balances you entered plus the interest, to the cent, because the simulation works in whole cents throughout. If those two figures did not agree, something in the input would be being silently dropped.
The things this does not model
Fixed rates. Real card rates move with the prime rate, and a promotional rate expiring mid-plan can change the ordering entirely. Flat minimums. Card minimums usually fall as the balance falls, which frees cash a little faster than shown, in the plan's favour. No fees, no prepayment penalties, no late charges. And no new borrowing, which is the assumption most likely to fail, since the plan only works if the balances go in one direction.
Two structural options sit outside this arithmetic entirely and can beat both orderings. A balance transfer or a consolidation loan changes the rates themselves rather than the order you attack them in, and a lower rate helps every future month at once. So does anything that raises the monthly payment. Before optimising the sequence, check whether the rate or the payment can move, because both have larger effects than the ordering does. If you want to see what a consolidation loan would actually cost, the loan calculator gives the payment and the total interest for a single fixed loan.
Questions people ask
Is the avalanche always cheaper?
In interest, yes, or equal. Sending every spare dollar to the highest rate removes the most expensive debt first, and no other ordering of the same payments can produce less total interest. The two come out equal when all the rates are the same, or when the payment is large enough to clear everything so quickly that the ordering barely gets a chance to matter. It is never the case that the snowball is arithmetically cheaper, so if you see that result, check that both runs used the same monthly payment and the same minimums.
Then why does anyone recommend the snowball?
Because completion rates are a real variable and arithmetic is not the only thing that decides how much you pay. Clearing an account early removes a payment and a due date and gives a plan a visible milestone, and people who get an early win are more likely to still be following the plan a year later. The honest framing is that the snowball costs a known amount of extra interest, shown above, in exchange for an unknown improvement in the odds you finish. If the extra interest is small, that is often a good trade. If it is large, it is a worse one.
What happens if my payment does not cover the interest?
The balances grow and no ordering helps, which is why the page stops and says so rather than producing a payoff date. When the monthly payment is below the monthly interest, the arithmetic is not a sequencing problem any more. The things that change it are a larger payment, a lower rate through a transfer or consolidation, or in serious cases a formal arrangement with the creditors. Reordering the payments in that situation just rearranges which balance grows fastest.
Should I include a mortgage in the list?
Usually not, for two reasons. It is generally the lowest rate you hold, so it sorts to the bottom of both orderings and never receives a spare dollar during the plan anyway, and its size distorts the totals so much that the numbers for the debts you are actually working on become hard to read. Run this page on the debts you intend to clear in the next few years, and treat the mortgage as a separate question with the loan calculator.
Does the lump sum go in before or after the first month of interest?
Before. A lump sum you have today is applied immediately, in whichever order the strategy would choose, and only then does the first month of interest accrue on what is left. That is the correct treatment for money you actually hold, and it is also the favourable one, so if the cash will not really arrive until later the payoff date shown is slightly optimistic. Anything left over after clearing a debt with the lump sum rolls into the next one in the same order.