Depends on the pitch.
3/8 refers to the diameter. The pitch refers to both the number of threads per inch but also the shape of the tooth. There are hundreds of different thread forms including UNC, UNF, Whitworth, BSP (British Standard Pipe), BSB and lots of others from other countries.
The same is true with metric. A metric thread may be M8 which is an 8mm thread. Each diameter has a standard pitch, but also pitches either side referred as either metric fine or metric course threads. M8 x 1.25 is standard, M8 x1.0 is fine and M8 x 1.5 is course. You need a matching nut otherwise it won't screw in.
Its getting harder and harder to get imperial sizes in small quantities at economical prices. Easy if your prepared to buy 1000 but 1's and 10's are harder, especially in decent quality materials.