Both spellings are correct, and both mean exactly the same thing: to make something as effective or efficient as possible. Optimize is the standard spelling in American and Canadian English. Optimise is the standard spelling in British, Australian, and most Commonwealth English. The difference isn’t about meaning — it’s entirely regional, with one real exception worth knowing before you pick a side.
Are Optimize and Optimise The Same Word?
Yes, completely. Both share the same root, the same grammar, and the exact same meaning. Neither spelling changes how the word functions in a sentence. This is a pure regional spelling choice, not a meaning distinction — unlike true homophone pairs where picking the wrong word changes what you’re saying.
Where The Split Actually Comes From
Most writers assume “-ize” is the modern American form and “-ise” is the traditional British one. That’s backwards. The “-ize” spelling has been used in the UK since the 15th century, predating “-ise” by more than a hundred years, and it’s the older, historically British form — not an American import. The idea that “-ize” is strictly American and “-ise” is strictly British is largely a modern, internet-era assumption rather than a historical fact. Wikipediagrammarist
The “-ise” spelling entered later, largely through French influence, since French had settled on “-iser” for equivalent verbs. Over the 19th and 20th centuries, everyday British usage gradually shifted toward “-ise,” while American English standardized firmly on “-ize.” But the older form never disappeared in Britain — it simply became the more conservative, formal choice.
The Oxford Exception
Here’s the detail that surprises most people: the Oxford English Dictionary and Oxford University Press still prefer “-ize” for words like this one, even in British writing. This preference is favored on etymological grounds, since “-ize” corresponds more closely to the Greek root “-izo,” the actual historical origin of the suffix in most of these verbs. Wikipedia
This isn’t a fringe preference. This standard, known as Oxford spelling, combines British conventions with the “-ize” ending and is used by major international organizations, including the United Nations and the World Health Organization, as well as academic journals like Nature. So seeing “optimize” in a British academic paper, a UN document, or a scientific journal isn’t a mistake — it’s a deliberate, well-established style choice, distinct from the “-ise” you’ll see in everyday UK newspapers, marketing copy, and most British web content. stileeducation
The Words That Are Always “-ise” — No Exceptions
This is the part most guides skip, and it’s the actual trap writers fall into. A separate group of common verbs is spelled “-ise” in every variety of English, all the time, with zero regional variation: advertise, advise, apprise, chastise, circumcise, comprise, compromise, despise, devise, disguise, exercise, improvise, supervise, surmise, surprise, and televise. In these words, “-ise” isn’t the same suffix at all — it’s simply part of the English or French root, so “advertize” or “surprize” are flat-out wrong in any variety of English, American included. Only the genuine “-ize/-ise” verb family — optimize, organize, realize, and similar words — actually varies by region. Wikipedia
Which Spelling Should You Use?
| Situation | Best Choice | Why |
|---|---|---|
| Writing for a US or Canadian audience | Optimize | Standard spelling in both varieties |
| Writing for UK, Australian, or general Commonwealth readers | Optimise | Standard everyday spelling in those regions |
| Following Oxford, UN, or a scientific-journal style guide | Optimize | Reflects the older, etymologically preferred Oxford spelling |
| Following AP style or general US publishing guidelines | Optimize | Matches standard American convention |
| No specific style guide given | Either, based on audience | Just stay consistent throughout the piece |
Real institutions don’t even agree with each other: UN editorial guidance has specified “-ize” spelling as its house style, while the European Commission’s Directorate-General for Translation specifies “-ise” for its own documents. If you’re writing for a specific organization, its house style always overrides the general regional habit.
Common Mistakes
- Assuming “-ise” is simply wrong because a US dictionary lists “-ize” first. Optimise is fully correct and standard in British and Commonwealth English.
- Assuming “-ize” is always “the American one” with no British use. Oxford-style British and international writing regularly uses “-ize,” and it’s actually the older form historically.
- Applying the “-ize/-ise” regional rule to words like advertise or surprise. These words are always spelled “-ise” everywhere — there’s no “-ize” version, in any region.
- Mixing both spellings within the same document. Whichever you choose, use it consistently for the verb and its related noun form (optimization/optimisation) throughout.
Everyday Examples
- We need to optimize our website for speed. (US)
- We need to optimise our website for speed. (UK)
- The team optimized the delivery route to save fuel. (US)
- The team optimised the delivery route to save fuel. (UK)
- The UN report used optimize throughout, following its house editorial style. (Oxford/international style)
Frequently Asked Questions
Is optimize or optimise correct?
Both are correct. Optimize is standard in American and Canadian English; optimise is standard in British, Australian, and most Commonwealth English. Neither spelling is a meaning error.
Is optimize American and optimise British?
That’s the common assumption, but it’s not the full picture. “-ize” is actually the older spelling historically used in Britain, and it remains the preferred form in Oxford spelling, used by organizations like the UN and WHO. Everyday British usage favors “-ise,” but “-ize” isn’t strictly American.
Why does Oxford use optimize instead of optimise?
Oxford spelling favors “-ize” because it more closely reflects the word’s Greek root, “-izo,” which is the actual historical origin of the suffix in most verbs of this type.
Are there words that are always spelled with -ise, no matter the region?
Yes. Words like advertise, advise, surprise, comprise, exercise, and compromise are always spelled “-ise” in every variety of English. In these words, “-ise” isn’t the regional suffix at all — it’s simply part of the root, so there’s no “-ize” version.
What spelling should I use for Canadian English?
Optimize. Canadian English generally follows American conventions for the “-ize/-ise” verb family, even though it follows British spelling in other areas, like “colour” or “centre.”
Does it matter which spelling I choose if I’m not writing for a specific region?
Not for correctness, but it matters for consistency. Pick one spelling — and stick with it and its related noun form (optimization or optimisation) — throughout the same document, so the choice reads as intentional rather than like a typo.
Conclusion
Optimize and optimise mean exactly the same thing — the only real choice is regional spelling. Match the version to your audience, stay consistent within a document, and you’ll never trip over this one again.
