Calendar year vs. fiscal year
| Calendar year | Fiscal year | |
|---|---|---|
| Definition | 12 consecutive months, January 1 – December 31 | 12 consecutive months ending on the last day of any month except December |
| Who typically uses it | Nearly all individuals; most sole proprietors, partnerships, and S corporations | Certain C corporations and businesses with a natural non-calendar operating cycle |
| When it ends | Always December 31 | Varies by the business’s chosen fiscal year-end |
Source: Internal Revenue Service
Who must use a calendar tax year
Most taxpayers don’t get to choose. They’re required to use a calendar year. That includes:
- Individual taxpayers filing Form 1040
- Sole proprietors and single-member LLCs (unless they elect corporate tax treatment)
- Most partnerships
- S corporations
These entities are required to use a calendar year unless they keep formal books and records on a fiscal-year basis and meet specific IRS conditions for adopting a fiscal year instead.
Who can elect a fiscal tax year
C corporations have the most flexibility here. They can choose any 12-month period as their tax year, including a fiscal year that ends when a company’s own business cycle naturally winds down (a retailer ending its year after the holiday season, for example, rather than mid-quarter on December 31).
Some businesses also use a 52/53-week tax year, which always ends on the same day of the week (say, the last Saturday of a given month) rather than a fixed calendar date. This keeps each fiscal year almost exactly 52 weeks and simplifies year-over-year comparisons for businesses that operate on a weekly retail or accounting cycle.
Once a tax year is adopted, changing it isn’t automatic. It requires IRS approval via Form 1128, Application to Adopt, Change, or Retain a Tax Year.
Tax year vs. filing year: avoiding the common mix-up
One of the most common points of confusion: the return you file in April 2026 covers the 2025 tax year, not 2026. The “tax year” refers to the period the income was earned in; the “filing year” or filing season is simply when you prepare and submit the return covering that period.
This distinction matters practically, not just semantically. Using the wrong year’s tax brackets, standard deduction amount, or contribution limits because you confused the tax year with the filing year is a common and avoidable mistake, especially since those figures change annually.
What a short tax year is
A short tax year is a tax year of less than 12 months. It typically comes up in a few specific situations:
- A new business is formed partway through what would otherwise be its first full tax year.
- A business changes its entity type in a way that ends one tax year and starts another.
- A business switches from a calendar year to a fiscal year (or vice versa), creating a shorter transition period to bridge the two.
Adopting or changing to a short tax year requires filing Form 1128 and a separate short-period tax return covering just that partial year.
Why your tax year matters
Your tax year determines more than just bookkeeping mechanics. It impacts:
- Your filing deadline
- Which tax brackets apply to your income
- Standard deduction amounts
- Credit and deduction rules and limits
A business with a fiscal year ending in June, for example, files by a different deadline than a calendar-year filer and applies the tax rates in effect during its 12-month period.
Check the tax filing deadline for your situation, and consider filing a tax extension if you need more time once you know which tax year you’re working with.
