⚠️ Note that this post hasn't been updated for at least a year and the information may be outdated, proceed with caution! (Last updated: July 20, 2022)
Option 1: Find it from Settings
You can find your HubSpot blog ID by going to your Settings (the gears icon in the top right corner), navigating to Website > Blog on the sidebar and choosing your blog (if you have multiple):
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If you look at the page URL in the address bar at the top of your browser, you'll see that it looks something like this:
https://app.hubspot.com/settings/123456/website/blogs/123456789/default
The blog ID is the number that you'll find after /blogs/
(highlighted above)
Option 2: Find it from Blog Analytics
When you navigate to Marketing > Website > Blog and select your blog (if you have multiple), you can click on "View Blog Analytics" at the bottom of the page.
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When you're on the blog analytics page, look at the page URL in the address bar at the top of your browser, it'll be something really long like:
https://app.hubspot.com/analytics/123456/pages?chartType=AREA&metric=rawViews&pageType=blog-posts&sortOrder=ASC&sortProperty=PUBLISHED_TIMESTAMP&stacked=true&backLink=https%253A%252F%252Fapp.hubspot.com%252Fwebsite%252F12345%252Fblog%252Fposts&blog=123456789
The blog ID is the number at the very end, after &blog=
(highlighted above)