femto-
"femto" is a 5-letter English headword indexed on PlainSpell.
The verdict
“femto-” is uncommon English (outside the top frequency list), classed as a prefix. The kind of spelling people second-guess.
- Unranked
- below top-frequency English
- 6
- letters
According to Wiktionary data (CC BY-SA, analyzed September 3, 2026) - In the International System of Units and other metric systems of units, multiplying the unit to which it is attached by 10⁻¹⁵ (short scale quadrillionth or long scale billiardth).
Corpus desk
Index EN-femto · femto- · English
femto- · unranked · 0 variants · 0 confusables
- FREQ-UNRANKED Unranked
- LEN-LONG 6 letters
- VOW-2 2 vowels
- CONFUS-0 No pairs
- VAR-0 0 variants
- BOOK-THIN "F" thin
- PHOTO-UNK Peer pending
Read with this headword: Spelling guide for this pattern
Desk codes from Wiktionary headword shape, FrequencyWords rank tier, confusable-pair count, generated misspelling depth, letter-cohort book size, and photo-finish peer. Not a difficulty grade.
| Property | Value |
|---|---|
| Headword | femto- |
| Language | English |
| Part of speech | Prefix |
| Letters | 6 |
| Misspellings tracked | 0 |
| Confusable pairs | 0 |
| Source | Wiktionary (kaikki.org) |
Long-tail rank of “femto-” in English
Rare enough to double-check
femto- is uncommon English outside the top frequency list, classed as aprefix. Rarity is why readers land here mid-draft. Gloss: "In the International System of Units and other metric systems of units, multiplying the unit to which it is attached by 10⁻¹⁵ (short scale quadrillionth or long scale billiardth).".
The misspelling generator found no plausible variants for femto-, typically a sign the spelling maps closely to how the word sounds. No close-neighbour confusable shows up for this headword in our dataset, since its spelling is unusual enough that it doesn't cluster with a lookalike.
Etymologically, the entry records: From Danish femten (“fifteen”), with reference to the (negative) exponent in the relevant power of ten; the same principle also explains atto-. The correct English form is femto-, spelled F-E-M-T-O--.
Definition
- 1In the International System of Units and other metric systems of units, multiplying the unit to which it is attached by 10⁻¹⁵ (short scale quadrillionth or long scale billiardth).
Etymology
From Danish femten (“fifteen”), with reference to the (negative) exponent in the relevant power of ten; the same principle also explains atto-.
This word in other languages
Definitions, pronunciation, and etymology for this entry are drawn from Wiktionary via the kaikki.org structured extract (CC BY-SA). See the methodology for how each field is sourced and updated.