Nofollow is a hint, no longer a hard signal to Google. Google now determines how to distribute link value based on their own algorithms, rather than solely based on tags added by site owners.

Just curious, What type of other's content you want to enhance your site with, without crediting the original site for their effort to create it? If it is content on-topic with your own content on your site, it might be beneficial to link back to them. Outgoing link value is just as much a signal for ranking as is incoming link value.

Thank you for a swift reply.

Both sites belong to me so I am not depriving anybody of credit. I just want to keep as much juice on my first site.

I know nofollow turned into a hint last year but I am grateful for any method to try and prevent any link juice passing from my page containing the iframe to the other site.

Not sure if I would ever use an iframe anymore unless when embedding something like a Youtube video.

Why use it? I am not even sure how Google treats iframes when linked to another site. I would think it would be a negative in terms of ranking for the site using the iframe as technically they are using content from another site. If you need to display the content why not just rewrite it? Plus for mobile phones it can't look that good.

I am using iframes for just part of the page content. They look fine on mobiles.

I believe Google sees the content of the iframe, recognises it belongs to another site and credits that site for the content and also may credit some link juice (the final part is what I am trying to stop as I dont want my site leaking juice to the site it is framing).

I guess this is a commonm problem for sites which display adverts in iframes.

Possibly a simple link back would feed the link juice back but I would prefer it never to be leaked in the first place. Links are easier tp plug with a nofollow hint tag but frames?

I just want to keep as much juice on my first site.

This is not how link juice works. You loose nothing by linking out. Link juice is calculated in steps, the link juice for your page is calculated, then the links are counted and then link juice attributed to the linked pages are calculated. If there are no outbound links the calculation reaches as dead end and fails, which is not acceptable. So to avoid this situation some arbitrary value is used to represent the probability that user leaves the page without a link, or "teleports".

Now to be clear the situation is different if you have a link to site A and to site B and you want to pass all the link juice to site A and none to site B, if you "no-follow" site B then all the link juice passes to A. This assumes of course, that Google doesn't ignore the "no-follow" as @lammert suggested above. But blocking link juice from going to both A and B doesn't increase link juice on your website.

I would warn you that you should be careful with allowing content to be iFramed, be sure that you set your x-frame headers such as to allow only the site that you control to show the iFrame, otherwise you may find your content used by spammers. see link below for details:
[developer.mozilla.org...]

@NickMNS thank you for your assistance.

To clarify my page which includes the iframe on Site A does have a link to another site call it Site B which can be followed..

The framed content is on Site C.

So I would like the link juice to flow from Site A to Site B and not Site C.

From other articles I have read iframes do appear to leak link juice as if they are a link?

I still don't see the value behind using iframes this way, just potential problems. If you are worried about duplicate content rewrite it or reference the content and use use a cross domain canonical link telling Google the original source is on on your other site.

I don't think there is much you can achieve with a "no-follow".

You can add more links to site B. Adding one more will send 2/3 of the link juice to site B, this assumes that Google doesn't consolidate all the links to the same locations.

Assuming that the content of site B is relevant to site C, then add a link on site C to site B.

In the end all this "page rank sculpting" is questionable in terms of effectiveness. Will it actually make a significant difference? Will Google view this as a link scheme and just ignore it all? Or worse, will Google penalize you for it? All for what, to give you 25% more link juice from a single link? It is futile, I would focus more on what makes sense for your users.

Useful feedback thank you all very much.

You loose nothing by linking out.

This is all quite old-school (and probably no longer relevant), but how I remember it is that you would lose PageRank even to nofollow links, it just wouldn't flow through to the linked URL. In other words, it would be lost.

So what happens when you have a page with “ten PageRank points” and ten outgoing links, and five of those links are nofollowed? Let’s leave aside the decay factor to focus on the core part of the question. Originally, the five links without nofollow would have flowed two points of PageRank each (in essence, the nofollowed links didn’t count toward the denominator when dividing PageRank by the outdegree of the page). More than a year ago, Google changed how the PageRank flows so that the five links without nofollow would flow one point of PageRank each.

[mattcutts.com...]

(Note the date of that post, as well as the disclaimer at the top.)

The rel attribute is not valid for the iframe element so adding nofollow would be pointless.

Short answer: you can't, and you shouldn't worry about it.

So I would like the link juice to flow from Site A to Site B and not Site C.

Beware blocked content- it cannot pass any rank.

Surely if you use a redirect but then stop that redirect from being crawled then you've not side-stepped the issue of a dead end as far as PageRank is concerned?

As far as I'm aware Robzilla's post above is correct in that there is nothing you can do to save the PageRank. It's not saved when you use nofollow. It's not saved when you use a redirect and then use robots to stop bots crawling the redirect.

In a nutshell, you can stop it arriving at the destination but you can't stop it being sent.

I don't know how much content you're talking about but I think I'd just rewrite it for the second site if it were me.

Getting to the nitty gritty of what I think this is about: Google has said that it's looking for original, useful content that's been cited by freely-given editiorial links. As I've observed over the years, Google has been relentless in following this up, in a manner which scales.

So, there may be a few scraps of some ranking factors that temporarily exist out there for the compulsively eager, but really, they are inevitably going to be overshadowed by other factors. Indulging in the petty technicalities of the sort the OP is trying to pursue here to save a few drops of PageRank is, IMO, a supreme waste of time and effort.

It's often said that efforts are better spent creating new and original content... and that's true if your talents lie in that direction. That direction involves some real empathy with your audience... some genuine caring for your subject matter... and research, effort, trial and error, and experience in determining what's really needed. That's what Google is mostly seeking to reward.

The potentials for misuse of iframed content are not a secret to Google. A long time back, Matt Cutts said something to the effect that Google is concerned about having its ranking criteria be consistent with the user experience, and that, at some point, Google might decide to clean up the inconconsistencies, but at the time (years back, now) Google felt, essentially, that cleaning up those inconsistencies possible with iframes wasn't worth the effort.

My guess is, by now, that Google has either sorted this out or buried it. I hate to see our members here wasting all that ingenuity to trick Google, when it's almost less effort to create content that's useful and consistent with what Google wants... and for that reason, I'm taking some time and effort now to point this out.

Where you put your efforts is up to you. As the saying goes, YMMV.

(PS: Edited above to clarify.)