I bought two of these shirts one in the royal blue and one in the black.  I am writing one review for both shirts.

So I have a 15yo who, due to circumstances beyond his control (i.e. an extremely sleepy metabolism even with medication), wears larger than he should adult clothing.  it has been extremely difficult to find street clothes and swim clothes which fit properly.  B/c he's still short statured and he wears a mens medium or large everything - including this swim shirt - is too long and too snug.  I ordered the large.

That being said, if you are a normal guy with normal size for your age, height, etc. this shirt would be fine and should fit as expected.  It is of good quality, a good price, and I have ordered the kid size shirts as well.  I recommend this if you are looking for a rash guard which isn't form fitting.  I would say that it more than likely runs true to size.

We just have unusual circumstances.