How do you say grandparents in Japanese?

There isn't one single word for "grandparents" in Japanese. You need to say "grandfather" and "grandmother" separately, and then depending on the context, you might connect them with a conjunction like "to" (と - and).

* Grandfather: おじいちゃん (ojiichan) - This is the most common and generally polite term.

* Grandmother: おばあちゃん (obaachan) - This is the most common and generally polite term.

To refer to both grandparents together, you would say:

おじいちゃんとおばあちゃん (ojiichan to obaachan) - Grandfather and Grandmother

There are other, more formal or less common terms depending on the relationship and level of formality, but おじいちゃん (ojiichan) and おばあちゃん (obaachan) are the standard and widely understood terms.