Hyphens in Numbers: When Twenty-One Needs a Dash
Last updated 24 August 2026
The rule is narrower than most people expect, and it has no exceptions worth worrying about: hyphenate the compound numbers from twenty-one to ninety-nine, and nothing else. Forty-two takes a hyphen. Four hundred does not. One hundred forty-two takes exactly one, between the forty and the two.
Search data for this site shows people typing “forty two spelling” and “forty six spelling” without the hyphen, and “thirty-one number” with it, which is a fair summary of the confusion: the hyphen looks optional and is not.
Where the hyphen goes
A written number is built from groups of three digits. Inside each group, the only join that takes a hyphen is the one between the tens word and the units word:
| Number | Written | Hyphens |
|---|---|---|
| 42 | forty-two | one |
| 40 | forty | none — nothing follows the tens word |
| 100 | one hundred | none — hundreds are joined by a space |
| 142 | one hundred forty-two | one |
| 1,142 | one thousand one hundred forty-two | one |
| 999,999 | nine hundred ninety-nine thousand nine hundred ninety-nine | two, one per group |
The last row is the useful one. Long numbers do not accumulate hyphens as they grow; they accumulate groups, and each group contributes a hyphen only if its last two digits fall between 21 and 99.
Where it never goes
- Not between a digit word and “hundred”. It is four hundred, never four-hundred.
- Not before a scale word. Two thousand, six million — spaces, not hyphens.
- Not in the teens. Thirteen through nineteen are single words already, so there is nothing to join. Fifteen is one word; “ten-five” is not a number.
- Not in round tens. Twenty, thirty, forty and their siblings stand alone.
Ordinals follow the cardinal
An ordinal changes only the last element of the number, and the hyphen stays exactly where the cardinal put it:
- twenty-one → twenty-first
- forty-two → forty-second
- ninety-nine → ninety-ninth
- one hundred twenty-one → one hundred twenty-first
The mistake to avoid is treating the hyphenated compound as a single word and suffixing the whole thing: forty-twoth is not a form. Only “two” becomes “second”.
Why the rule exists at all
English inherited two ways of building numbers below a hundred: single words for the units, the teens and the round tens, and compounds for everything else. The hyphen marks the compound as one number word rather than two separate ones, which matters when a number sits next to other words. Twenty one dollars could be misread as twenty items and one dollar; twenty-one dollars cannot.
The same logic is why the hyphen stops at ninety-nine. Above that, the structure is no longer a compound of two words but a sequence of groups, and English marks that with spaces.
Fractions and adjectives
Spelled-out fractions take a hyphen between numerator and denominator — two-thirds, three-quarters — which is a separate rule that happens to look similar. And when a number is part of a compound adjective before a noun, the whole phrase is usually hyphenated: a twenty-one-year-old applicant, a forty-two-page report. That hyphenation belongs to the adjective, not to the number.
Check any specific number
Every number page on this site prints the hyphenated form, so you can confirm one rather than reason about it — see 42 in words, 121 in words, or type your own into the converter. The broader question of when to spell a number out at all, rather than use digits, is covered in the guide to spelling numbers in words.