Furthermore, if you know the diameter, also known as bead seat diameter, chances are you can fit any tire of that diameter of almost any width. If your bike came with very wide tires, it may have wide rims. With those wide rims, it may be a bad idea to install super narrow tires, because that would splay the tires out too much. If your bike came with narrow tires, wide tires might not fit in the frame or fork because the maker didn't provide enough room. Measuring first may help in this case.

You probably will have room for wider tires if your bike came with 38mm wide tires, but you may not crave wider tires.