Kilobytes to bytes?

[gugliotti]

I am confused, one kilobyte is equal to 1024 bytes or 1000 bytes?

I see different opinions about this definition, for example

https://www.gbmb.org/kb-to-bytes

http://www.global-converter.com/main-data-quantity-converter/

I understand there are different definitions, my question is which one of the two is commonly used.