From drdino.com in an article written by Dr. Kent Hovind:
Does the Bible contain a mathematical error?
I Kings 7:23–26 and II Chronicles 4:2–5 describe a huge brass bowl built by King Solomon. If the diameter of this bowl was 10 cubits, then the circumference should have been 31.415926...cubits, not just 30 cubits! Any math student will tell you that the circumference of a circle is found by taking the diameter times Pi (3..141592653589793...). This apparent mathematical error caused me, as a new Christian, to doubt the accuracy of the Bible.
The answer is so simple! The diameter of 10 cubits is from outer rim to outer rim, the way anyone would measure a circular object. The circumference of 30 cubits, however, was of the inner circle, after subtracting the thickness of the brass (two handbreadths—one for each side) from which the bowl was made. This would be the number needed to calculate the volume of water.