Sometimes when you’re setting up the layout for a theme, your WordPress will ask you for the ID of a post, page or category. Easy.
ID for Posts and Pages
Open up All Posts (or All Pages). Find the post (or page) that you want the ID for and then hover your mouse or finger over the post (or page) name.
You’ll see, at the bottom of your browser window, the URL which includes the post (or page) ID
I’m going to hover over the Post ‘Reykjavik Nights’ and the URL will show.
See that URL at the bottom of the browser?
So, the Post ID is 84
ID for categories
Same thing for categories. Hover over the category you want the ID for and look down at the URL in your browser
So the category ID for ‘Nordic Noir’ is 20