Use fantastic most often to mean very good, excellent, or sometimes strange / imaginary, depending on context. Cambridge lists both the common “extremely good” meaning and a second sense related to things that are strange or imaginary.

Use fantastical when something feels more storybook-like, strange and wonderful, or so extreme that it hardly seems real. Cambridge defines fantastical as “strange and wonderful, like something out of a story,” and also as something so strange or extreme that it may not seem true or reasonable.

So the practical rule is:

✅ The food was fantastic.

Here, fantastic means very good. That is one of its most common modern meanings in Cambridge’s learner dictionaries.

✅ The film is full of fantastical creatures and impossible landscapes.

Here, fantastical works because the meaning is imaginative, unreal, and almost fairy-tale-like. Cambridge defines it that way directly.

✅ He came up with a fantastic story about hidden treasure.

Merriam-Webster notes that fantastic can also mean based on fantasy or not real, so this use is valid too.

✅ The hotel was fantastic, but the castle in the mountains looked almost fantastical.

This shows the easiest distinction:

The most common mistake is treating fantastic and fantastical as if they always mean the same thing.

They overlap a little, but not completely. Fantastic is much more common in everyday English and very often just means excellent. Fantastical is less common and usually leans harder into the meaning of imaginary, strange, or like something from a fantasy world.

Another common mistake is using fantastical when you only mean great:

❌ The concert was fantastical.
More natural: ✅ The concert was fantastic.

If you mean “amazing” or “very good,” fantastic is usually the safer choice.

Use this memory rule:

A simple shortcut:

If you would clap for it, use fantastic.
If you would illustrate it with a dragon, use fantastical.

That is not a dictionary definition, but it matches the real usage pretty well. Cambridge’s definitions support exactly that split between excellent and story-like / unreal.