Meaning of the word "off the top of my head" in English

What does "off the top of my head" mean in English? Explore the meaning, pronunciation, and specific usage of this word with Lingoland.

off the top of my head

US /ɔf ðə tɑp əv maɪ hɛd/
UK /ɒf ðə tɒp əv maɪ hɛd/

Idiom

without careful thought or checking facts; from immediate memory

Example:
I can't remember her phone number off the top of my head.
Off the top of my head, I'd say it's about 20 miles from here.