Four things a debt payoff spreadsheet has to get right

There is no shortage of free debt payoff spreadsheets. Sorting your debts by rate or by balance is the easy part, and every template does it. The arithmetic underneath is where they differ, and the differences are worth real money because they change the payoff date you are planning your life around.

Here are the four things the engine has to do. Each one is checkable against your own numbers in about a minute, so you can test whatever template you are already using rather than taking anyone's word for it, including ours.

1. The rollover, which is the whole engine

When a debt clears, its minimum payment does not go back into your spending. It joins the extra payment and lands on the next target. That is what makes either the snowball or the avalanche accelerate, and it is the single most common thing a simple spreadsheet leaves out.

The test: clear your smallest debt on paper and look at what the sheet does with the payment that debt was absorbing. If the total going toward debt each month stays flat after a payoff, the rollover is missing and every date after that first payoff is too late. In a four-debt load with minimums of $120, $55, $310 and $210, the amount attacking your debt should climb from $300 of extra to $1,000 by the time the last one is standing.

2. Interest applied before the payment, at the right cadence

Interest accrues, then the payment lands. A sheet that subtracts the payment first and charges interest on the reduced balance understates what you owe every single month, and the error compounds.

balance = (balance + balance x APR/12) - minimum - (extra, if this is the target debt)

A monthly model divides the APR by twelve. Credit cards actually accrue daily, at the APR divided by 365, against the average daily balance. Those are not identical, and it is worth knowing which one you are looking at. On a $12,000 balance at 25.99% with $600 a month going to it, the monthly model says $3,898 of total interest and the daily model says $3,878. The payoff month is the same, 27 either way, and the gap is about half a percent.

That is small enough to plan with and too large to call exact. Our calculator uses the monthly convention, and this is the caveat that comes with it. If you need the number to the dollar for a payoff quote, get it from the lender.

3. Every debt keeps receiving its minimum

The target debt gets the extra payment. Every other debt still gets its own minimum, every month, or the plan you are looking at is a plan that quietly assumes you defaulted on the rest.

The test: add up every payment the sheet makes in month one. It should equal the sum of all your minimums plus your extra payment, exactly. If it equals less, some debt is being starved. If it equals more, the sheet is spending money you did not tell it you had.

4. The case where no ordering works

This is the one almost nothing handles, and it is the one that matters most to the person who needs it.

If a minimum payment is smaller than one month of interest on that balance, the balance grows. Not slowly, and not until some crossover point. Forever, regardless of which debt you attack first, because the payment never reaches the principal.

Take $5,000 at 30% APR with a $40 minimum. One month of interest is $125. The payment is $40. After a year of paying every month on time, the balance is not $4,520. It is about $6,173, and it keeps climbing.

A spreadsheet that reports a payoff date for that debt is not being optimistic, it is being wrong, and the person reading it is being told that patience will fix something patience cannot fix. That balance needs a larger payment, a lower rate through a consolidation or a hardship program, or a conversation with the creditor. The free calculator on this site refuses to produce a date in that case and says why, which is the only honest output available.

Test the one you already have

Four checks, ten minutes, on any template:

1. Does the monthly total going toward debt rise after the first payoff? If not, no rollover.
2. Do month-one payments sum to your minimums plus your extra, exactly?
3. Is interest charged before the payment is applied?
4. Enter a debt whose minimum is below one month of its interest. Does the sheet flag it, or does it print a date?

A template that passes all four is doing the work. Most free ones pass two.

Where a spreadsheet beats a calculator, and where it does not

A calculator is better for the decision. Put your numbers in, see the gap between avalanche and snowball, pick one, and get on with your day. Nothing to maintain.

A spreadsheet is better for the eighteen months after the decision. Balances change, a rate expires, a card gets consolidated, and you want the file to still be yours next year with your figures in it rather than typing them again into a form that forgets. That is the honest split, and it is why both exist here.