Short Tax Year: What Triggers It, Filing, and Deadlines

A short tax year filing is a return that covers less than twelve months, and getting it right depends on why the period is short. If your entity just formed or is winding down, you generally compute tax the same way you would for a full year. If you changed your accounting period, you have to annualize the income, follow special rules for depreciation and estimated payments, and meet a deadline measured from the close of the short period rather than from year-end.

What Causes a Short Tax Year

A short tax year is any tax period lasting less than twelve months for which a return is required.1Office of the Law Revision Counsel. 26 USC 443 – Returns for a Period of Less Than 12 Months Two things can create one, and the distinction drives everything else about the return.

A voluntary change happens when a business switches its fiscal year-end with IRS approval. A company moving from a December 31 year-end to a September 30 year-end files a return for the stub period from January 1 through September 30. That switch triggers the annualization calculation.

A mandatory short year arises when an entity comes into existence or ceases to exist partway through what would otherwise be its tax year. A corporation formed on August 1 that adopts a calendar year files its first return for August 1 through December 31.2eCFR. 26 CFR 1.443-1 – Returns for Periods of Less Than 12 Months A dissolving corporation files a final return running from January 1 through its dissolution date. For these mandatory short years the tax requirements are generally the same as for a full-year return, and annualization is typically not required.3Internal Revenue Service. Tax Years

Getting IRS Approval for a Voluntary Change

If your short year comes from a voluntary change in accounting period, you need IRS approval before filing. The vehicle is Form 1128, Application to Adopt, Change, or Retain a Tax Year.4Internal Revenue Service. About Form 1128, Application to Adopt, Change or Retain a Tax Year Partnerships, S corporations, personal service corporations, and trusts may all need to file it.

Many entities qualify for automatic approval under Revenue Procedure 2006-46 if they meet the conditions. One restriction: if you’re requesting a natural business year and have already changed your accounting period within the 48 months ending with the last month of the requested tax year, you generally won’t qualify for the automatic track. Entities that fall outside the automatic criteria have to request prior consent from the Commissioner, which involves a user fee and a more rigorous justification.

Form 1128 asks for your current year-end, the requested new year-end, and the business reason for the change. Common accepted reasons include conforming to the accounting period used for financial reporting or matching a controlling shareholder’s tax year. Filing late or incorrectly can result in the IRS rejecting the change, which means the short-period return you filed may not be accepted.

Annualizing Income for a Voluntary Change

When a short year results from a change in accounting period, you must annualize taxable income. Without annualization, a six-month return would push less income through the lower brackets and artificially reduce the effective rate.1Office of the Law Revision Counsel. 26 USC 443 – Returns for a Period of Less Than 12 Months

The calculation has three steps:

  • Annualize the income. Take modified taxable income for the short period, multiply by 12, and divide by the number of months in the short period. A six-month short year with $100,000 in modified taxable income produces annualized income of $200,000.
  • Compute the tax on that annualized amount using the standard rate schedules, as though it were a full year.
  • Prorate the tax back down. Multiply the tax by the number of months in the short period and divide by 12. If the annualized tax is $42,000, the actual tax for the six-month period is $21,000.

The statute says “modified taxable income,” not plain taxable income. For individuals, this means figuring adjusted gross income for the short period, subtracting actual itemized deductions (you must itemize on a short-period return), and adjusting the personal exemption amount proportionally.5Internal Revenue Service. Publication 538 – Accounting Periods and Methods

Entities that exist only for a mandatory short period skip annualization. Their tax is computed the same way as for a full-year return ending on the last day of the short period.3Internal Revenue Service. Tax Years

The Alternative Method That Can Lower the Tax

Standard annualization sometimes produces a tax higher than what the taxpayer would have owed over a full twelve months. Section 443(b)(2) offers an alternative based on actual income over a twelve-month reference period. If you can show what you actually earned during that window, the IRS may reduce the annualized tax to the greater of two figures: a proportional share of the tax on the twelve-month income, or the tax on the short-period modified taxable income alone.1Office of the Law Revision Counsel. 26 USC 443 – Returns for a Period of Less Than 12 Months The reference period is normally the twelve months starting on the first day of the short period.

The catch: you have to file the return using the standard annualization method first and pay that amount. Applying for the reduction is treated as a claim for refund. The application deadline is no later than the due date (including extensions) for the first full tax year that ends on or after the date twelve months from the start of the short period. You’ll prepare a hypothetical return for the full twelve-month period to substantiate the lower figure, and the burden is on you to prove the alternative produces a lower tax.

Depreciation and Section 179 in a Short Year

Depreciation is one of the areas where a short year creates real computational work. Under MACRS, you first calculate full-year depreciation and then prorate it: multiply the full-year figure by the number of months (including partial months) the property was treated as in service during the short year, and divide by twelve.6Internal Revenue Service. Publication 946 (2025), How To Depreciate Property

The applicable convention also shifts:

  • Half-year convention. Property is treated as placed in service at the midpoint of the short tax year, not the midpoint of a calendar year. Find the midpoint by dividing the number of months in the short year by two.
  • Mid-quarter convention. If your short year is three months or less, you must use the mid-quarter convention for all property placed in service during that period. For longer short years, divide the short year into four quarters and find the midpoint of each.
  • Mid-month convention. This works the same way regardless of whether the tax year is short: property is always treated as placed in service at the midpoint of the month.

Section 179 expensing is not prorated for a short tax year. The full deduction limit applies even if the period is only a few months long. The deduction still cannot exceed your taxable income from active trades or businesses for the short period, which is a tighter constraint when the period is compressed.

Estimated Tax Payments

Corporations facing a short tax year don’t always owe estimated tax. No payment is required if the short year covers fewer than four full calendar months, or if the total tax shown on the return is less than $500.7eCFR. 26 CFR 1.6655-5 – Short Taxable Year

For a short year of four or more full months, the standard installment dates apply. If the first installment would fall before the 15th day of the fourth month of the short tax year, it gets pushed to the first regular installment date that falls on or after that 15th day. When a tax year ends early due to an acquisition or accounting period change, the final installment is due on the date the next installment would have been due had the year continued normally. One exception: if that date falls within thirty days of the last day of the short year, the final installment is due on the 15th of the second month after the month the short year ends.7eCFR. 26 CFR 1.6655-5 – Short Taxable Year

A newly formed entity gets some flexibility. A taxpayer with an initial short tax year can make estimated payments as though it were a calendar year taxpayer until it files the return for that initial period. If the entity then chooses a fiscal year when filing, it won’t face an estimated tax penalty for the mismatch during the initial period.

Filing Deadlines by Entity Type

The filing deadline for a short year return is measured from the close of the short period, not from the end of a calendar year:

A short period ending in June gets special treatment for C corporations. For tax years beginning before January 1, 2026, the return is due on the 15th of the third month rather than the fourth. Starting with tax years beginning in 2026, the standard fourth-month deadline applies to all C corporations regardless of when the year ends.8eCFR. 26 CFR 1.6072-2 – Time for Filing Returns of Corporations

Each return must mark the exact start and end dates of the short period at the top of the form. If the short year came from a voluntary accounting period change, attach a statement showing the annualization calculation: short-period taxable income, annualized income, tax on the annualized amount, and the prorated final tax. Taxpayers using an alternative method under Section 443(b)(2) also attach the hypothetical twelve-month computation.

Extensions

You can request an automatic extension for a short tax year return by filing Form 7004.10Internal Revenue Service. About Form 7004, Application for Automatic Extension of Time To File Certain Business Income Tax, Information, and Other Returns The extension is generally six months. Estates and trusts filing Form 1041 receive five and a half months, and C corporations with short tax years ending in June may have a seven-month extension for years beginning before 2026, dropping to six months for years beginning in 2026.11Internal Revenue Service. Instructions for Form 7004 (12/2025)

Form 7004 has a specific checkbox on line 5b where you indicate the reason for the short tax year. If the reason is a change in accounting period, the entity must have already applied for approval on Form 1128 or met the conditions that exempt it from that requirement. If none of the listed reasons apply, check “Other” and attach a statement explaining the circumstances. An extension gives you more time to file, not more time to pay. Estimated tax is still due by the original deadline.

S Corporation Terminations Mid-Year

When an S corporation loses its S election mid-year, whether through revocation, exceeding shareholder limits, or another disqualifying event, the tax code splits the year into two short tax years: an “S short year” ending the day before the termination takes effect and a “C short year” beginning on the effective date.12Office of the Law Revision Counsel. 26 USC 1362 – Election, Revocation, Termination Each short year gets its own return under its own rules.

The default method for splitting income is pro rata allocation. The IRS takes each item of income, loss, deduction, and credit for the full year, assigns an equal portion to each day, and splits the totals based on how many days fall in each short year.12Office of the Law Revision Counsel. 26 USC 1362 – Election, Revocation, Termination This is simple but can produce odd results if a major transaction occurred on one side of the termination date.

The alternative is an election to close the books on the termination date, assigning income and expenses to whichever short year they actually occurred in. Every shareholder who held stock at any point during the S short year, plus every shareholder on the first day of the C short year, must consent to the election.13eCFR. 26 CFR 1.1362-3 – Treatment of S Termination Year Closing the books usually makes more sense when income is concentrated in one period, but unanimous shareholder consent can be the harder problem.