Words Ending in -us: Cacti or Cactuses? The Real Answer
Quick Answer
Words from Latin ending in -us often change to -i: one cactus, two cacti. But English now accepts the simple -uses form for most of them too — and for a few words, the Latin form is plain wrong.
See the Pattern First
Look at how these behave. Notice that some have two correct answers.
- One cactus, two cacti — or cactuses. Both are correct.
- One nucleus, two nuclei. Here the Latin form is the normal one.
- One virus, two viruses. Never “virii” — that word does not exist.
- One octopus, two octopuses. “Octopi” is a famous mistake.
So this is not one rule — it is three groups. The table below sorts them for you, which is exactly why these words are confusing when you meet them one at a time.
The Full List — Sorted by What Is Actually Safe
The right-hand column is the form you can always use without being wrong.
| One | Many | One | Many |
|---|---|---|---|
| cactus | cacti / cactuses | focus | foci / focuses |
| genus | genera | hippopotamus | hippopotamuses |
| locus | loci | nucleus | nuclei |
| octopus | octopuses | platypus | platypuses |
| radius | radii | stimulus | stimuli |
| syllabus | syllabi / syllabuses | virus | viruses |
| fungus | fungi | status | statuses |
| rhinoceros | rhinoceroses |
These come up often, so they have their own full lesson with more practice: the plural of fungus and the plural of status and the plural of rhinoceros.
Why “Octopi” Is Wrong (And Why People Say It)
The -us → -i rule only works for words that came from Latin. Octopus is Greek, not Latin. Its Greek plural would be “octopodes”, so “octopi” applies a Latin rule to a Greek word. English simply uses octopuses.
The same trap catches virus. In Latin it was a mass noun with no plural at all, so there is no ancient form to borrow. “Viruses” is the only correct answer, whatever the internet says.
When unsure, add -es. It is never wrong.
The Safe Strategy
You do not have to memorise which words take -i. Use this instead:
- In everyday speech and writing, -uses is accepted for almost all of them.
- In science and maths, the Latin forms are standard: nuclei, radii, stimuli, fungi, loci. Use those in a lab report.
- Three words never take -i: virus, octopus, status.
That is the whole rule in three lines — far easier than learning fifteen separate words.
Mistakes Learners Make (You Are Not Alone)
These are the ones examiners and editors circle:
- ✗ two octopi → ✓ two octopuses (octopus is Greek, not Latin)
- ✗ many virii → ✓ many viruses (“virii” is not an English word)
- ✗ several stati → ✓ several statuses (status keeps the simple ending)
“Octopi” is so common that it appears in newspapers — but it is still not standard. Use “octopuses” and you are always safe.
Your Turn — Quick Quiz
Five questions. Take your time — the table above is still there if you need it.
1. 1. The aquarium has three ____.
2. 2. The computer was infected by two different ____.
3. 3. In a science report, the plural of “nucleus” is:
4. 4. Which of these is ALWAYS safe in everyday English?
5. 5. The circle has two ____ drawn inside it.
Keep Going
You now know something most native speakers get wrong, and you have a safe strategy for any -us word you meet.
Latin left English another group too. What happens when a word ends in -um, like curriculum or memorandum? There the rule is much cleaner — and there are no arguments.
Next lesson: Words ending in -um and -on: curriculum, phenomenon
Sources
I’m Khamis Maiouf. English is not my first language – I learned it as an adult in the UK, passed Functional Skills Level 2 in English, qualified in Hairdressing at Level 3, and I’m now studying Business at university. For over five years I’ve taught ESOL classes in my community and tutored students through GCSE English. I write every lesson on this site myself, for the learner I used to be.
