Both penalties for being late stop at 25 percent. That is the good news, and it is also the trap, because the penalty is the only part of the bill that stops anywhere.
Interest does not stop. A forfeited refund is total. And a return that is never filed never starts the clock that limits how long the IRS has to assess the tax, which is the cost almost nobody counts.
The part with a ceiling, and why it is the small part
Failure to file runs at 5 percent of the tax due per month to a maximum of 25 percent, and failure to pay at 0.5 percent per month to its own maximum of 25 percent. The mechanics, and the ten to one ratio that makes filing on time worth far more than paying on time, are in what an extension buys you.
What matters here is the shape rather than the rate. After five months the failure to file penalty has finished growing. Someone five months late and someone five years late owe the same failure to file penalty. If penalties were the whole story, the marginal cost of staying away would fall to nothing, and a great deal of advice is written as though that were true.
It is not true, because nothing else on the list has a ceiling.
Interest has no ceiling, and it compounds daily
Interest on an underpayment is the federal short term rate plus 3 percentage points, reset every quarter, and it is compounded daily.
For the fourth quarter of 2026 that rate is 7 percent. It ran at 7 percent in the first quarter, 6 in the second and 7 in the third, which is the useful thing to notice: it moves, so a balance carried across years is not carried at one rate, and nobody can quote a single figure for what a stale balance has cost.
Daily compounding on a balance that has already been maximized at 25 percent of penalty is the engine of the eye watering totals people report years later. The penalty made the balance bigger once. The interest has been working on the larger balance every day since, including on the penalty itself.
Not filing does not start the assessment clock
This is the cost that is almost never explained, and it is the one that changes what people decide to do.
Two separate limitation periods run in a tax matter. The IRS generally has three years from the later of the date a return was received and the date it was due to assess tax. Once tax is assessed it generally has ten years from the date of assessment to collect it.
The first clock depends on the taxpayer. Where no return is filed, tax may be assessed at any time. The Advocate’s guidance puts it plainly: the three years begins only when the taxpayer files. The second clock depends on an assessment, not on a return. If the IRS assesses a year on its own figures, through the substitute return described below, the ten years to collect run from that assessment like any other.
So the intuition that quietly drives non filing, that if enough time passes the problem ages out, is backwards. Filing is what starts the assessment period. Not filing keeps a year open, and it can be assessed in year twelve as easily as in year two. A year the IRS has already assessed without a return does have a collection period running, but on a figure that is usually higher than the real return would have produced.
Two related points, both worth knowing before assuming a filed year is closed. Omitting more than 25 percent of gross income extends the assessment period to six years. A false or fraudulent return filed with intent to evade has no limit at all, permanently.
The refund is not delayed, it is forfeited
A refund of withholding or estimated tax has to be claimed within a fixed window. For a year with no return, the late return is itself the claim, and it recovers only tax paid in the 3 years before it is filed, plus any extension of time to file that applied to the year. Withholding and estimated tax count as paid on the original due date, so in practice the window is about 3 years from that date, or 3 years and 6 months for a year that was on extension. After that the money stays with the Treasury, and no reasonable cause or hardship recovers it.
This is the single most common real cost of a missed deadline, because the population that fails to file is heavily weighted toward people whose withholding already covered the tax. They are not avoiding a bill. They are abandoning their own money on a three year timer, usually while believing they are in trouble. The three year window is also why a non filer’s first question should be which years are still inside it, before any question about penalties.
A refund case also has no failure to file penalty and no failure to pay penalty, because both are percentages of tax due. That asymmetry is set out in the October 15 deadline for an extended individual return.
The costs that never appear on a notice
Some of what a missed filing costs is never billed by the IRS at all.
Retirement and disability credits. Self employment income on an unfiled return is never reported to the Social Security Administration, so no credits accrue for it. A self employed person who skips several years has reduced a benefit calculated decades later, and filing late fixes this only up to the SSA’s own limits on revising a record.
Anything that needs a filed return. Mortgages, refinancing, business lending and federal student aid all ask for filed returns. Missing years surface at the point where somebody else’s deadline is running, which is the most expensive moment to discover them.
The return the IRS writes instead. Where a return is not filed, the IRS may prepare a substitute for return, which allows none of the deductions the taxpayer was entitled to. It arrives as a Notice of Deficiency, CP3219N, carrying 90 days, or 150 days if it is addressed to someone outside the United States, to petition the Tax Court or file a real return in its place. The date printed on the notice is the one to work from. The tax on a substitute return is routinely far higher than the tax on the real one, and that date is a genuine cliff rather than a suggestion.
Collection. Unpaid assessed tax leads to levies on wages and bank accounts and to a federal tax lien, which arises automatically once the first demand for payment goes unpaid. Repeated non filing can lead to criminal prosecution, which is rare, and is not the usual outcome, and is the reason the word usual should be doing work in any sentence about it.
Who this actually happens to
The picture most people carry of a non filer is somebody avoiding a bill. It is frequently the opposite.
A large share of unfiled years belong to people whose withholding already covered the tax, and who are owed money. One year was missed during something difficult, an illness, a bereavement, a move, a business failing, and then the second year could not be faced because the first one was still sitting there. The fear is out of proportion to the liability, and the delay the fear causes is the thing that converts a refund into a forfeited one.
That is worth saying plainly because it inverts the order of operations. Someone in that position is not deciding whether to pay. They are deciding whether to collect, on a three year timer, and every month of avoidance is spending their own money.
It also sets the first piece of work on a file with several missing years, before any return is prepared: establish which years are still inside the three year refund window. That is the only part of the position with a hard expiry that nothing reopens. Penalties can be argued later, balances can be paid over time, and assessment and collection periods are long. The refund window is the one that simply closes.
What follows from all of this
File the return. That single act converts an open ended exposure into a defined one, starts the assessment period, preserves any refund still inside its window, and reduces the penalty question to something with a known maximum.
Then deal with the money separately, and in that order. There is relief available for penalties in the right circumstances, and it is assessed on the facts of why the return was late rather than on whether the tax has been paid, so it is not a reason to delay filing while trying to assemble the balance.
If several years are missing, the order to file them in and the years still worth filing at all is a judgment rather than a rule, and the pricing page sets out how back years are quoted, which is separately from a current year return.