Form 945-X: the timing that turns a late-found withholding error into a zero-penalty correction

The interest-free rule

Form 945-X is an adjusted return under IRC 6205. An underpayment reported on it is an interest-free adjustment if the X-form is filed "by the due date of the return period in which the error was ascertained" and the additional tax is paid with it (IRM 20.1.2.3.9(2)). For Form 945, whose return period is the calendar year, that means: an error found in any month of 2026 is corrected interest-free if the 945-X is filed and paid by February 1, 2027 (January 31, 2027 is a Sunday). An error found on January 15, 2027 belongs to the 2027 period and has until the start of February 2028.

The X-form has no due date of its own. "Since the above returns are not 'required' to be filed by a specific due date (except to qualify for an interest-free adjustment), the penalty for filing late does not apply to these returns" (IRM 20.1.2.3.9(3)). There is never a failure-to-file penalty on a 945-X.

Why a timely X-form is exempt from the deposit rules

The question a payer worries about is the deposit penalty: the withholding should have been deposited monthly or semiweekly in the year it was withheld, and it was not. The regulation answers it. Treas. Reg. 31.6302-1(c)(7) exempts a timely adjusted return from the monthly and semiweekly deposit rules on the increase; the IRM applies it as follows: "If the amount of TC 298/308 interest-free adjustment is paid by the received date of the adjusted return ... the amount timely paid will be deemed to have been timely deposited" (IRM 20.1.4.21.5(2)). The payment "may be made by a check or money order with the adjusted return, by EFT, or by other methods," so paying with the return does not trigger the 10% avoidance penalty that normally attaches to a required deposit paid any other way (IRM 20.1.4.21.5(3)).

The consequence is that a payer who finds in September that March's backup withholding was under-deposited by $8,000 owes, if it files and pays a 945-X before the next February 1, exactly $8,000. No deposit penalty, no late-payment penalty, no interest.

Three scenarios

The middle row is the trap. Filing the X-form on time but sending the payment separately later loses the deposit exemption on the unpaid amount; "If the adjusted return is timely but the tax is not paid with it, the amount of the unpaid tax is subject to a 10 percent FTD penalty." Pay with the return, or by EFTPS the same day.

When the X-form is late

An X-form filed after the due date of the discovery period is processed as an ordinary adjustment (TC 290), not an interest-free one, and "a corrected ROFTL for the new total tax is required to test deposit timeliness on the whole year" (IRM 20.1.4.21.5(6); 20.1.4.21.5.1(3)). The corrected schedule puts the additional liability on the dates it was actually withheld, or should have been, and the deposits made during the year are re-applied against the corrected totals. Any shortfall is late from the original due dates. The Form 945 deposit penalty calculator computes that: enter the corrected liabilities and the deposits actually made.

An increase on an X-form filed late is also where the IRS default application of deposits matters most, since the re-applied deposits go to the most recent periods first and can leave an early-year liability entirely uncovered. The IRC 6656(e) designation is available within 90 days of the resulting penalty notice.

The liability schedule

An amended Form 945-A is "generally not required" with an X-form, with two exceptions: when the original schedule was invalid (blank, negative, or not agreeing with the return), and when a decrease is being reported on a year where a penalty was charged on the higher tax (IRM 20.1.4.21.5.1(1) and (2)). On a decrease where the penalty had been averaged because no good schedule existed, the IRS simply re-averages the lower tax; where the penalty was computed on the higher tax, the net penalty after the decrease is capped at 10% of the net tax plus 5% of any net tax still unpaid, a floor worth checking a CP 210 against (IRM 20.1.4.21.3(13)).

A corrected or first-time schedule received after a penalty was assessed "must be considered and the FTD penalty adjusted" (IRM 20.1.4.18.2), but note the reverse: a revised schedule triggers a full recompute "even if no penalty was originally assessed" (IRM 20.1.4.21.2(2)). Correcting a 945-A can increase the penalty when the corrected dates are earlier than the ones originally reported. Oral requests to change dates or amounts on the schedule are not accepted; a signature is required only when the total tax changes (IRM 20.1.4.21.2(3)).

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