F. Supp. 2d or F.Supp.2d? Spacing reporter abbreviations (and F.3d vs F. Supp. 3d)

Short answer. In the national form it is F. Supp. 2d, with a space between each piece, but F.3d, closed up. The rule behind both: single capital letters close up with each other, a series number such as 2d or 3d is treated like a single capital, and any abbreviation longer than one letter (Supp., Ct., App.) gets a space on each side. F.Supp.2d is a closed-up style you will meet in some state courts' own opinions. The traditional national form keeps the spaces (on word limits, see below).

The rule [Convention]
Single capitals close up with each other, and a series number closes up with them: F.3d, F.4th, and D.D.C. for the District of Columbia's district court.
A longer abbreviation is set off by spaces: F. Supp. 3d, S. Ct., 2d Cir.
Write series and circuits as 2d and 3d, never 2nd or 3rd.
A free public-domain citation manual describes the same rule (adjacent single capitals close up; a capital does not close up with an abbreviation that is not a single capital) and lists F. Supp. 2d, F.3d and F.4th as its ordinal examples.

Two real citations (each checked against its source)
A district court decision in the Federal Supplement:
Mata v. Avianca, Inc., 678 F. Supp. 3d 443 (S.D.N.Y. 2023).
A court of appeals decision in the Federal Reporter:
Park v. Kim, 91 F.4th 610 (2d Cir. 2024) (per curiam).

F.3d vs F. Supp. 3d
They are two different reporters, not two editions of one. Today the Federal Reporter (F.3d, F.4th) is where federal courts of appeals decisions are cited, and the Federal Supplement (F. Supp. 2d, F. Supp. 3d) is where federal district court decisions are cited. Older district court decisions, from before 1932, appear in F. and F.2d, because the Federal Supplement did not exist yet. A new series only restarts the volume numbers, so the "3d" in F.3d and the "3d" in F. Supp. 3d each mean "third series" of a different set of books.
[Convention] The reporter tells the reader the level of court; the parenthetical says which court. Keep the district or the circuit in it: (S.D.N.Y. 2023), (2d Cir. 2024).

Why the Supreme Court looks different. On page 405 of its 2024 Loper Bright opinion, the Court's footnote cites a D.C. Circuit decision in the Federal Reporter's fourth series as "F. 4th," with a full space, and names the court "CADC." The Court follows its own style there. In a brief following the national form, the D.C. Circuit's own decision in Loper Bright, with the Supreme Court's later action on it, is:
Loper Bright Enters. v. Raimondo, 45 F.4th 359 (D.C. Cir. 2022), vacated, 603 U.S. 369 (2024).

Common mistakes
"F.Supp.2d" or "F. Supp.2d" in a brief that follows the national form.
"F. 3d" or "F. 4th", copied from a Supreme Court opinion.
"F. Supp. 2nd" or "F.3rd".
An F.3d citation with a district court in the parenthetical, or an F. Supp. citation with a circuit. One of the two is wrong.

Closing up spaces to meet a word limit
[Jurisdiction-specific] The Colorado Court of Appeals addressed this in a Policy on Citations dated Nov. 13, 2025. It tells parties not to strip out or underline spaces, or tinker with citation spacing in other ways, to get under the word limit, and a brief that ignores it can be struck.
[Unresolved] Elsewhere, manuals differ. The public-domain manual's commentary reads the Bluebook's 21st edition as letting writers close up F. Supp. 2d to save space, while noting that the result may look wrong to many legal readers. Check your court's rules before you try it.

Check a draft. Our checker flags a reporter abbreviation that is spaced differently from its standard form, and a series written 2nd or 3rd: https://citationcode.com/checker (opens in a new tab)

Read more
Real district court citations: https://citationcode.com/examples/how-to-cite-a-federal-district-court-opinion (opens in a new tab)
The Abbreviations chapter (its opening is free to read): https://citationcode.com/book/abbreviations (opens in a new tab)