Here is a list of the spelling differences I have noticed: US vs UK Spelling Differences
I have included some variant words in the list where they are close to the UK equivalent (like airplane vs aeroplane),
but not included those where the word is obviously different (like zipcode vs postcode or diaper vs nappy).
Note: old lists include out of date differences - like catsup instead of ketchup; but ketchup is now standard in the USA.
British English allows the use of -ise endings as an alternative to -ize: for example-
organise as well as organize
organised as well as organized
organiser as well as organizer
organisers as well as organizers
organises as well as organizes
organising as well as organizing
organisation as well as organization
organisations as well as organizations
As the -ise ending is optional, the US spelling (-ize) is not different from the UK norm (-ize per Collins and Oxford dictionaries).
So these are not included in the list.
However, the US -yze ending is replaced in the UK by -yse (e.g. the US "analyze" becomes the UK "analyse") as shown in the list.
There are plenty of other examples of UK alternatives (like the word gaol) where the US spelling (jail) is recognised
as valid in the UK -indeed, often the norm- so they're not included in this list.
Some of the US differences are fairly predictable, like:
* the reversal of "re" at the end of words (center for centre)
* the omission of the "a" in the diphthong "ae" (esthetic for aesthetic)
* the omission of the "u" in "our" endings (flavor for flavour).
But the use of single and double l is a minefield:
in some words, the US spellings double the UK single l, but in most endings where we have a double l, the US spelling only has a single l.