What is correct spelling Willful or wilful?

Answer 1

It's Willful

We-e-ell, yes, BUT...!

In Britain, 'wilful' is the generally accepted spelling, whereas 'willful' is more common in the US. However, the double 'l' should never follow the 'u' except in the adverb, 'wilfully'.

Answer 2

Both "willful" and "wilful" are correct spellings of the same word, but the preferred spelling in American English is "willful," while "wilful" is more common in British English.