Which is the correct spelling: "Granddad" or "Grand-dad" or "Grandad"?
According to Google NGram, in British English the spelling "grandad" is more popular than "granddad", however American English, the spelling "granddad" is more popular than "grandad".
American English:
British English:
Both exist, apparently.
The NOAD, says
granddad |ˈgranˌdad| (also grandad) noun informal - one's grandfather.
grandad |ˈgranˌdad|, Noun - variant spelling of granddad .
The OALD says the same thing, and is more specific, saying that "Granddad" is North-American English. If someone from U.S./UK can confirm, it would add more info.