How Much PageRank Passes Through a 301 Redirect?
We know that PageRank degrades through links (thought to be around 85% through each iterative set of links, give or take) - but how much does it degrade through 301s?
To clear up confusion, Matt Cutts says it's about the same. Meaning: if you 301 redirect one page to another, you'll pass about 85% (or so) of the PageRank to the new page. But is this the end of the story?
Folks often wonder why they 301 redirect a high PageRank page, but the new page doesn't seem to benefit. The answer, I believe, is there's a huge difference between redirecting a page and linking to a page.
For example, if you 301 to an off-topic page - one that differs in subject and relevance to the original - then the 301 is likely to have little value. Several Google patents show how Google might devalue old links pointing to a page if the new links are suddenly much different in anchor text.
So if you redirect a page about "Ford Trucks" to a page about "Red Pandas" - it's unlikely the page about Red Pandas will see much of a boost.
This is the biggest mistake I see people making with 301s. If you want your 301 to pass as much value as possible, make sure to redirect to a page that offers close to similar value as the original as possible. This means finding appropriate category or product pages or blog post that offer the most relevant value to the reader, and not simply 301'ing everything to the homepage.