Run the same balance through two payoff calculators and you will often get two different answers. Not slightly different: months apart, sometimes years.
The arithmetic is not in dispute. Compound interest has been settled for centuries and every tool implements it correctly. What differs is everything that has to be decided before the arithmetic can start, and most of those decisions are never shown to you.
Here are the four that account for nearly all of it.
1. Which minimum payment formula
This is the big one, and it is the one almost nobody discloses.
US issuers use two shapes:
- A percentage of the balance plus that month’s interest, typically 1% plus interest and fees.
- A flat percentage of the whole balance, typically 2% to 4%, with interest already inside that figure.
Both carry a floor, usually $25 to $35.
On an $8,400 balance at 22.9%, the first month’s interest is $160.30. Under 1% plus interest the minimum is $244.30, of which $84.00 reduces the debt. Under a flat 2% the minimum is $168.00, of which $7.70 reduces the debt.
The second payment is smaller and achieves roughly a tenth as much.
Project that forward and the two assumptions do not just disagree, they land in different categories of answer. At 22.9%, a flat 2% minimum leaves about 0.09% of the balance going to principal each month. Our own library refuses to return a payoff date for it, because the balance is still falling sixty years out, and a number measured in lifetimes is not an answer.
Same balance. Same rate. Both calculators doing correct arithmetic. One says a few decades and the other says effectively never.
2. When interest is applied
Every US issuer adds the month’s interest to the balance and then subtracts the payment. Some calculators do it the other way round, applying the payment to the balance first and charging interest on what is left.
Over one month the difference is small. Over 250 months it is not, and it runs in the direction that flatters the calculator: charging interest after the payment makes every projection look slightly better than the card will.
This is also why paying a balance in full still produces one final interest charge. The interest for that month was already added before your payment arrived.
3. Whether cents round as they post
A card issuer rounds interest to whole cents at the moment it posts it. Some calculators carry full floating point precision through the whole schedule and round only when printing.
There is a visible symptom. If a tool shows you a month-by-month table, add up the interest column. If it does not equal the total printed above it, the tool is rounding for display and calculating at a precision no account ever uses.
Our schedules round as a servicer posts, which is why the column adds up. That is a small thing, and it is the kind of small thing that tells you whether anybody checked.
4. Whether the last payment is a whole month
The final payment on almost any debt is smaller than the rest, because the balance left is less than a full payment.
Some tools count that partial month, some round it up to a whole one, and some have an off-by-one in the loop that records the payoff month. A difference of one month between two calculators is usually this, and it is harmless. A difference of several years is never this.
What to do with two different answers
Ask each tool what it assumed. If it does not say, that is the answer about which one to use.
Then check your cardholder agreement. The section is usually headed “minimum payment” or “how we calculate your minimum payment”, it runs to a paragraph or two, and it is the only document that settles which formula applies to your account.
What the law requires an issuer to disclose, and what it leaves to them, is the reason this gap exists at all. The Credit CARD Act of 2009 added the minimum payment warning that appears on US statements, showing what paying only the minimum costs in time and money. The disclosure rules sit in Regulation Z, 12 CFR 1026, and the Consumer Financial Protection Bureau publishes plain answers about how card interest works.
What no rule requires is that the issuer tell you which formula produced the figure. That is left to the agreement, which is why two calculators can both be right and still disagree.
Our payoff calculator prints its assumption next to every result, and the minimum payment page computes both formulas side by side rather than picking one for you. Not because that is clever, but because the assumption is the part that actually decides the answer, and hiding it is how two correct calculators end up disagreeing by a decade.
Questions
- If the maths is the same, how can the answers differ?
- Because the maths is the smallest part of the calculation. Compound interest is settled arithmetic and every tool gets it right. What differs is the assumptions each one makes before the arithmetic starts: which minimum payment formula, whether interest is added before or after the payment, whether cents round as they post, and whether a partial final payment is counted as a whole month.
- Which answer should I trust?
- The one whose assumptions match your cardholder agreement. That is a different question from which tool is better written. A calculator assuming a 2% flat minimum is correct for a card that uses one and wrong for a card that does not, and a tool that does not state its assumption gives you no way to tell which case you are in.
- How much can the assumptions actually change the answer?
- On an $8,400 balance at 22.9%, paying only the minimum, the two common minimum formulas differ by years. The payment shape is the single largest source of disagreement between calculators, and it is the one almost none of them disclose.
- Does rounding really matter?
- Over one month, no. Over 250 months it compounds. A tool that carries full floating point precision and only rounds for display will show a schedule whose interest column does not add up to its own total, which is a visible sign that the rounding convention was never decided.