Two costs, pulling in opposite directions
The comparison confuses people because there are two separate things to weigh and only one of them is stated as a rate. The first is what the plan charges: interest, and sometimes a setup fee. The second is what you give up by not paying in full, which is usually a discount, a rewards rate, or both. Zero percent financing sets the first to nothing and often leaves the second untouched, which is how a free plan ends up being the expensive choice.
On a $1,200 purchase with a 5 percent discount for paying outright, choosing an interest-free plan costs $60. Nobody charged it and it is a real cost, because it is money that would have stayed in your account. Against that sits the interest the deferred money earns while the plan runs, which on a declining balance averaging about $650 at 4 percent over a year is roughly $26. The discount wins, and it usually does, because a discount is a percentage of the whole price paid immediately while savings interest is a percentage of a falling balance earned over time.
How the plan interest is worked out
Interest-bearing plans here are amortised: a level monthly payment, interest charged each month on what is still owed, and the remainder reducing the balance. That is the same structure as an ordinary instalment loan, and it means the total interest is far below the headline rate applied to the full price. $1,200 over 12 months at 18 percent APR costs about $120 in interest, which is 10 percent of the price rather than 18, because you only owe the full $1,200 in the first month.
Partly interest-free offers are modelled as stated: the free months apply a plain instalment of the price divided by the term with no interest, and the remaining balance is then amortised over the months that are left. Providers implement these differently, some waiving the earliest months and some the latest, some reducing the rate instead of waiving it, and the totals differ accordingly. The agreement, or the first statement, is the thing to check against.
The setup fee field exists because it is the most common cost on an offer that advertises no interest at all. A fee of $39 on a $1,200 twelve-month plan is the equivalent of about 6 percent APR, quietly.
The break-even rate, and the thing not to put in it
The break-even figure is the savings rate at which the interest earned on the deferred money exactly cancels what the plan costs. Above it, the plan is cheaper; below it, paying in full is. It is a clean answer as long as the rate you compare against is a rate you are actually contracted to receive.
The failure is entering an expected investment return. The plan interest is certain and dated; an investment return is neither. If the investment falls, the payments still arrive on schedule and the principal has shrunk as well. The comparison is only meaningful between a fixed cost and a fixed return, which means a deposit rate. That is why the field asks for a savings APY and why the output says so when the break-even rate has climbed somewhere no deposit account goes.
What the arithmetic is not covering
A plan converts a single decision into a monthly obligation. For as long as it runs, that payment is claimed before anything else those months might need, and several overlapping plans make the month genuinely hard to see. That is a real cost and it does not appear in any total.
The other direction is equally real. Paying in full out of thin reserves can leave you borrowing at a much worse rate a month later for something unavoidable, which is a way of paying far more for the same purchase by a longer route. If the cash is there and the emergency reserve survives the purchase, paying in full is a straightforward arithmetic question. If it does not survive, the arithmetic on this page is answering a question you are not actually facing; the emergency fund calculator is the more relevant page, and it comes first.
For the plan cost on its own, without the discount comparison, the card installment fee calculator gives the monthly payment and total interest directly and covers deferred-interest promotions, which behave very differently from the genuine 0 percent modelled here. For rewards comparisons the cashback comparison is the right tool, and for whether the purchase makes sense at all, repair against replace and upgrade timing ask that question rather than this one.
Questions people ask
Is a genuine 0% plan ever worse than paying in full?
Yes, whenever taking it costs you something that paying in full would have given you. The usual case is a discount available only for immediate payment, and the second most common is rewards, since many issuers exclude promotional financing from cashback and points. A setup or account fee does the same job under a different name. Interest earned on the deferred cash offsets part of the loss but rarely all of it, because a discount is a percentage of the whole price now while savings interest accrues on a shrinking balance over months. Set the discount field to zero when both offers can be combined, and the plan wins as expected.
What counts as a discount for paying in full?
Only what you actually forfeit by taking the plan. A price cut for immediate payment, cashback or points your card pays on an ordinary purchase but not on promotional financing, a supplier discount for settling now. If the same discount applies whichever way you pay, enter zero, because it cancels out of the comparison and putting it in makes the full-payment side look better than it is. The commonest mistake here is counting rewards on both sides while the plan is in fact excluded from them, or the reverse.
The break-even rate came out above twenty percent. What does that mean?
It means the plan cannot be rescued by savings interest at any rate you could realistically get, so at those terms it is simply the more expensive side. The distance above a realistic deposit rate is a rough measure of how much more. It is also the point at which people start substituting an expected investment return to make the number reachable, which changes the comparison from a fixed cost against a fixed return into a fixed cost against a hope. The plan payments arrive whatever the investment does.
Should I take the plan if I do not have the cash?
Then this comparison is not the one you are making, and the checkbox turns off the interest credit to reflect that. What the page still tells you is what the plan costs in money, which is the input to the real question: whether the purchase is worth that cost now, or whether saving for it and buying later costs nothing extra. That question depends on how urgent the purchase is and on what else is competing for the same monthly payment, and if there is high-rate debt already running, the debt payoff planner shows what an added monthly commitment does to the payoff date.