We British understand American English almost perfectly, but Americans tend to struggle with British English more, especially with our slang or informal language. Part of the reason is that we Brits have a lot more exposure to American English than Americans have to British English. Also, I believe British English is far richer and more varied than American English (it's almost as if American English is dumbed down or simplified) so they struggle in that respect too. I have had a lot of American friends when living overseas, and I often had to "translate" for them because their knowledge of other forms of English (even the original form) was not so good. In a situation where Brits are in a group talking in standard British slang, few American observers would have any idea what was going on.

The problem is not the slangs and accent, we have problem with their use of diction. Typically, American speak more slangs than the British, French fries, chips are American English. But, chips is not equal to french fries. Chips and French Fries are two different things, they are completely different. It looks like that American will understand the British, but the British will not understand the American, In Percentage, American may understand British close to 99%.if they use the right diction. Remark: I was wondering, someone here who claimed to be American, she can not tell the different between chips and french fries. I believe, all Americans who order french fries at the restaurent will not expect to get the potato chips.

Yeah, I think the percentage is pretty low. My American friends get confused a lot when I use British idioms. The accent is generally OK.

We understand the accent, but the average American doesn't know very much slang at all. There's the obvious thing like chips = french fries, and what not... And I know a good deal of them due to just researching them out of boredom, but really Americans don't know alot of them. So, percentege-wise, probably something like 10%, 15%, or something like that.

They can understand the accent but there will always be confusion in relation to the slangs we use.... for example: Hugh Laurie interview regarding slang is quite interesting (Hugh Laurie, british born actor, working in a famous American tv series, HOUSE) http://www.youtube.com/watch?v=wYmrg3owTRE