Both “includible” and “includable” are correct English, and either spelling will be understood. For tax and legal writing, use “includible”: that is the spelling the Internal Revenue Code uses in every section where the word appears, and it is the form the U.S. Government Publishing Office Style Manual directs federal writers to follow.
Both Spellings Are Grammatically Correct
English has two suffixes that turn verbs into “capable of being” adjectives: “-able” and “-ible.” The “-able” suffix is the productive one, so new coinages almost always use it (searchable, clickable, downloadable). The “-ible” suffix is older and survives mainly in inherited words like admissible, deductible, and defensible.
“Include” historically takes “-ible,” but because “-able” is the modern default, “includable” has become a standard alternative. Merriam-Webster actually lists “includable” as its primary headword and treats “includible” as a variant. Neither spelling is wrong, and swapping one for the other never changes the meaning of a sentence.
The Internal Revenue Code Uses “Includible”
In U.S. tax law, the choice is settled. The Internal Revenue Code uses “includible” every time the concept appears, and the term carries real weight in several core provisions:
- Section 1504 defines which entities may file a consolidated return by establishing the term “includible corporation,” spelled with “-ible” throughout.1Office of the Law Revision Counsel. 26 USC 1504 – Definitions
- Section 457 uses “includible compensation” to describe the earnings base against which contribution limits are measured for state and local government deferred compensation plans.2Office of the Law Revision Counsel. 26 U.S. Code 457 – Deferred Compensation Plans of State and Local Governments
- Section 83, governing restricted stock and equity grants, describes when an amount would be “includible in the gross income” of the employee receiving the stock.3Office of the Law Revision Counsel. 26 U.S. Code 83 – Property Transferred in Connection With Performance of Services
No section of the Internal Revenue Code uses the “-able” spelling. Congress has spelled it “includible” for decades, and every amendment has followed suit.
Federal Publications Follow the Same Rule
The U.S. Government Publishing Office Style Manual, which governs spelling and usage across federal publications, directs writers to use “includible” over “includable.”4GovInfo. U.S. Government Publishing Office Style Manual So the “-ible” spelling is standard not only in the tax code but across federal regulatory drafting generally.
Where “Includable” Has Support
Outside federal government writing, opinion splits. Merriam-Webster gives “includable” top billing. The University of Illinois Tax School style guide goes further, flatly instructing its writers to use “includable, not includible.” A practitioner who follows the dictionary or a respected tax education program could reasonably land on the “-able” form.
The disagreement is narrower than it looks. Dictionary editors track general usage across all English writing, where the intuitive “-able” pattern dominates. The GPO and Congress are making a different choice for a specialized vocabulary. Both positions are internally consistent; the question is which audience you are writing for.
Does the Spelling Choice Create Legal Risk?
No. Using “includable” on a tax return, in a contract, or in an SEC filing will not change the legal effect of the document. Courts and the IRS interpret documents by their substance, not by whether you matched Congress’s preferred vowel. A variant spelling between two recognized English words is the kind of minor slip that gets corrected on its face rather than litigated.
What to Use
If you are drafting or reviewing anything that references the Internal Revenue Code, use “includible.” You will match the statute, the GPO Style Manual, and the expectations of tax professionals who read these provisions daily. In a consolidated return workpaper discussing “includible corporations” under Section 1504, spelling it “includable” will not invalidate the filing, but it signals that the writer took the concept from a secondary source rather than the code itself.
For writing that does not touch federal tax or regulatory language, use whichever spelling your organization’s style guide prescribes. With no house preference, “includable” follows the more common English pattern and is the dictionary’s first choice. Pick one, hold to it within a single document, and move on.