I want to find out how to convert UK and US dress sizes.  I have found
conflicting information on clothing tags, in reference material and on
the internet (some tags/sites say add 2 to US size, others say add 4. 
See below for links to conflicting sites:
Add 2 - http://www.metric.fsworld.co.uk/siclothing.htm
Add 4 - http://www.laurenlangley.com/size.html)

My question has two parts:
1/ Is there a definitive conversion formula?  If so, which one is it?
2/ Why is there conflicting ideas about dress size equivalencies?  Is
there some kind of logical reason?  If so, what is the reason?

Thanks

Jody

Hi Jody~

There's no doubt about it...the British are less afraid than Americans
of larger clothing sizes!

The reason for the difference in sizing stems from the fact that
American and British dress sizes were developed independently.

Until clothing was readily manufactured in the 1920s or so, there was
no need to standardize clothing sizes; most women's dresses were made
by dressmakers, seamstresses, and tailors, and were sewn to
custom-fit. No sizing was used at all--just the customer's
measurements.

Some clothing has been available ready made since the 19th century,
but not until 920s did it start to become socially acceptable to wear
mass-produced clothing--so it wasn't till then that any attempt at
sizing standards was attempted. Since that time, dress sizes have
changed several times; in the U.S. the numbers almost always go
down--perhaps in an attempt to make women feel skinnier. There's never
been a real attempt to have a unified sizing system between the U.S.
and the U.K.

You really cannot use any "method" for figuring out what a U.S. size
is compared to a U.K. size, so any site that tells you to just add a
few numbers is inaccurate.

The British did not have the U.S. sizes in mind when they created
their own system, and Americans did not have the U.K. system in mind,
either. Each system is based upon what are supposed to be "average"
sizes for women in each country. Any chart that compares U.K. and U.S.
sizes may be slightly inaccurate for this reason; for example, an
American size 8 might have a larger bustline than a dress from the
U.K. that is similar in every other way.

Therefore, the best way to compare sizes is to use a good chart, but
do bear in mind that there are still slight differences in
measurements.

Here is a good chart for U.S., U.K., European, and Japanese sizes:
http://www.henryandjune.com/hj/stsize.asp?sz=MS-STD#intl

The first chart you mention in your question is also a good reference.

Hope this helps!
kriswrite

Great answer - satisfied my need for info and also my uncontrollable curiosity :)

Thank you very much kriswrite - this really clarified things for me