Using Google Analytics

Google Analytics is a great free service for measuring traffic on up to five web sites. It can help you learn things like how many users are visiting your blog — and how often, which of your posts are the most popular, which search terms visitors are finding you through, and where in the world your visitors are coming from.
Some blog themes have an option in the Customize appearance page in the Appearance area for you to simply paste in a Google Analytics ID. If your theme supports that, pasting in the Google Analytics ID and saving are all you need to do to install Google Analytics. If your theme doesn't support that, follow the directions below.

Google's metrics are not real-time, and usually lag about 24 hours.

Step 1.  You'll need a Google/Gmail account. If you don't have one yet, it only takes a minute to sign up.
Step 2.  Head to http://www.google.com/analytics/ and log in with your Google account.
Step 3.  Click "Add Website Profile".
Step 4.  Select "Add a Profile for a new domain".
Step 5. In the URL field, enter your Tumblr blog's URL. This will be something like "david.tumblr.com". If you're using a custom domain, enter that instead.
Step 6.  Click "Finish".
Step 7.  Highlight and copy the code that appears on the next page.
Step 8.  Open your Tumblr Dashboard in a new window, click the name of your blog at the top of the Dashboard, and click Customize appearance.
Step 9.  Paste the Google Analytics code into the "Description" field and click "Save Changes".
Step 10.  Go back to the Google Analytics window and click "Finish".
All done!
You should now see your blog added to your Google Analytics "Website Profiles". It can take Google a little while to start displaying your traffic report. Until Google's system has started processing your data, its "Status" label should read "Tracking code not verified".

Now you can log into Google Analytics and click "View Reports" for detailed traffic metrics about your blog.
Note: If you are using a theme that doesn't include the Description, you will have to put the Google Analytics code into the Custom HTML for your theme. Click the Theme tab instead of the Info tab and then click the Use Custom HTML link at the bottom of the tab. The code can go at the end of the Body area of your Custom HTML.