There are two questions in every refund claim. Was it filed on time? And how much can it recover? The second question is answered by the lookback rule in section 6511(b)(2), and it surprises people who did everything right on the first.

The lookback is a cap. It limits the refund to tax paid within a window of time before the claim was filed. A claim can be timely and still be capped at zero if the payments happened outside the window. This guide explains how the cap works, why deemed payment dates matter and where the traps are.

The three caps

Section 6511(b)(2) sets three limits, depending on when the claim was filed.

  • Claim filed within the 3-year period. Under section 6511(b)(2)(A), if the claim was filed during the 3-year period in section 6511(a), the refund cannot exceed the portion of the tax paid within the period immediately preceding the claim equal to 3 years plus the period of any extension of time for filing the return.
  • Claim not filed within the 3-year period. Under section 6511(b)(2)(B), the refund cannot exceed the portion of the tax paid during the 2 years immediately preceding the claim.
  • No claim filed. Under section 6511(b)(2)(C), if the IRS allows a refund without a claim, the cap is what would have applied had a claim been filed on the date the refund is allowed.

The 3-year period mentioned in the first two caps is the three-years-from-filing period in section 6511(a). So the question of which cap applies turns on whether the claim was filed within three years of the return. The refund claim deadline guide covers that period.

Why deemed payment dates decide most cases

The lookback measures when tax was paid. For most individuals, the tax was paid through wage withholding or estimated payments during the year. Section 6513(b) says withholding is deemed paid on the fifteenth day of the fourth month after the close of the taxable year, and estimated tax is deemed paid on the last day prescribed for filing the return, without regard to extensions. Section 6513(a) treats any other payment made before the due date as made on the due date.

So a calendar-year individual's withholding and estimated payments are all treated as paid on the April due date. That single date is what the lookback window must reach. The guide on deemed payment dates explains those rules in detail.

Extensions add to the window

Section 6511(b)(2)(A) adds the period of any extension of time for filing to the three-year lookback. That addition exists so that a taxpayer who files on an extension and then claims a refund within three years of filing can still reach payments deemed made on the original due date.

IRM 25.6.1 explains how the IRS applies it. When a claim is filed within three years of the original return, the lookback is three years plus the full length of any valid filing extension. For claims filed after December 26, 2025, the IRM says disaster postponement periods are treated like extensions and added to the lookback, without counting overlapping days twice. For claims filed on or before December 26, 2025, the IRM says disaster postponements did not change the lookback.

The extension has to be valid. An extension request that was never filed or was invalid does not add anything.

Where timely claims recover nothing

The classic trap is a late original return. Suppose an individual's withholding for a year is deemed paid on the April due date, and no return is filed for years. When the return is finally filed, the return itself is the claim under Treasury Regulation 301.6402-3(a)(5). The claim is timely because it is filed within three years of the return, which is the same day. The lookback then asks whether the tax was paid within three years before the claim. If the April due date was more than three years earlier, the withholding falls outside the window and the cap is zero.

That is how people with old unfiled years lose refunds they were clearly owed. The guide to late returns that claim refunds walks through it.

A second trap involves later payments. If you paid an audit assessment, the two-year rule in section 6511(a) can keep the claim period open for two years after that payment. But if the claim is filed more than three years after the return, section 6511(b)(2)(B) limits the refund to amounts paid in the two years before the claim. Earlier payments on the same year fall outside the cap.

The cap is a dollar limit, not a list of payments

IRM 25.6.1 includes a useful warning for IRS employees: the lookback is just a formula that sets a maximum refund amount; it is not a rule about which specific payments are refunded. The IRM says the IRS generally follows last-in, first-out rules by transaction date when issuing refunds, so a refund may sometimes include payments dated before the lookback period begins.

For planning, the takeaway is to think in amounts. Add up the tax paid within the window. That total is the most you can get back, before interest.

The postmark rule helps at the edge

Claims filed by mail near the edge of a window can be saved by the timely mailing rule. Treasury Regulation 301.7502-1(b)(2) says section 7502 applies when a claim is delivered after the last day of the lookback period in section 6511(b)(2)(A), as long as its requirements are met. The regulation's example involves a claim postmarked on the last day and received three days later; the claim is deemed filed on the postmark date and the refund is allowed.

Section 301.7502-1(f) adds a rule for late original returns that serve as claims: if the postmark falls within three years, plus extensions, of the date the tax is deemed paid, the late return and claim are treated as filed on the postmark date. See the guide to the timely mailing rule.

Exceptions to the cap

Some special refund periods also lift the cap for the items they cover. Section 6511(d) provides that for bad debts and worthless securities, net operating loss and credit carrybacks and foreign tax credits, the refund may exceed the amount paid within the normal lookback to the extent the overpayment is attributable to those items. Those rules are covered in the guide to special refund periods.

A consent extending the assessment period also changes the math. Section 6511(c)(2) sets a special cap for claims filed after a consent: tax paid after the consent was executed, plus the amount that would have been allowable if a claim had been filed on the date the consent was executed.

A quick self-check

  • When was the original return filed, and is the claim within three years of that date?
  • Was there a valid filing extension, and how long was it?
  • On what dates was the tax paid or deemed paid?
  • Does the total paid inside the window cover the refund you expect?

The bottom line

If the self-check shows a gap, act on the timing before the substance. A claim filed now can be supported later; a payment that slips out of the window cannot be brought back.

A refund claim has to be timely, and the refund is capped at tax paid within the lookback window: three years plus extensions for claims filed within three years of the return, two years otherwise. Deemed payment dates in IRC 6513 decide whether withholding and estimated payments fall inside the window. Late original returns are where most refunds are lost. File early, count from the deemed dates and use the postmark rule if you are close.