There is no difference in meaning: these are two spellings of one word. Realize is standard in American English and is also used by Oxford University Press in Britain; realise is the dominant British, Irish, Australian and New Zealand spelling. "I didn't realise how late it was" and "I didn't realize how late it was" say exactly the same thing.
The core difference
Spelling, and nothing else. Both forms carry both senses of the word: to become aware of something (She realised she'd left her keys inside) and to make something real or convert it into money (The plan was never fully realised; They realised £40,000 on the sale). The same split applies right through the word family: realised/realized, realising/realizing, realisation/realization.
Which to use
- Writing for the US or Canada: use realize.
- Writing for the UK, Ireland, Australia, New Zealand, South Africa or India: realise is the safe default and is what most readers expect.
- Academic or reference publishing in Britain: check the house style. Oxford University Press and several journals use -ize spellings; the Guardian, the Times and most newspapers use -ise.
Be honest about this one
It is worth saying plainly: -ize is not an Americanism that crept into British English. It is used in Britain's most authoritative dictionaries, and British writers who use it are not making a mistake. Equally, -ise is not sloppy. The only real error is inconsistency within a single document — realise in one paragraph and organize in the next.
The rule you can apply
Pick your convention from your audience, then apply it to the whole -ise/-ize family: organise, recognise, apologise, criticise, emphasise. But do not over-apply it. A set of words ends in -ise in every variety of English, American included, because the -ise is not a suffix there at all:
- advise, devise, revise, supervise, televise, improvise
- surprise, comprise, compromise, promise
- exercise, arise, disguise, despise, advertise
Nobody writes advize or surprize. If you are ever unsure, ask whether removing the ending leaves a word: real + -ise works, so it is in the family; surpr- is not a word, so it is not.
The mistake to watch for
Word processors set to US English will silently flag realise as a misspelling, and autocorrect will change it. Set your document language before you start, especially in a job application or a submission where a stray -ize signals you did not check who you were writing for.
Frequently Asked Questions
Is "realise" wrong in American English?
It will read as a foreign spelling to American readers and most US style guides and spellcheckers will mark it. It is not a different word, but if you are writing for a US audience, use realize.
Which spelling should I use in a UK job application?
Realise. It is the spelling most British readers expect and the one used by nearly all UK newspapers and employers. Realize would not count against you, but realise is the uncontroversial choice.
Is it realisation or realization?
Follow whichever spelling of the verb you have chosen — realise goes with realisation, realize with realization. Mixing them within one piece of writing is the only genuine error.