The circular tooth width (thickness) is the arc length of a tooth, generally measured at the pitch diameter.
Think of the gear profile as a simple circle with diameter equal to the pitch diameter. If your gear has 30 teeth the thickness will be the circle's circumference (pi * pitch diameter) divided by the number of teeth  to get the arc length of a single tooth/space sector. Then divide that by 2, since the tooth thickness is equal to the tooth space.
This reduces to pi/(2*diametral pitch) regardless of number of teeth. The corresponding tooth space is the same value. Usually the teeth are thinned a little to create backlash to prevent binding.