T5 fluorescent tubes are five-eighths of an inch or 16mm in diameter, They are narrower than a T8 fluorescent tube, however they provide much better efficiency than a T8 fluorescent tube.
T5 tubes are available in 6W, 8W, 13W, 14W, 21W, 24W, 28W, 35W, 39W, 54W and 80W
It is not possible to directly replace an exisitng T8 fluorescent tube with a new T5 fluorescent tube, as the control gear for each is different.