You can calculate volts from joules and coulombs, but you can't convert joules to volts since volt and joule units represent different quantities.
The voltage V in volts (V) is equal to the energy E in joules (J), divided by the charge Q in coulombs (C):
V(V) = E(J) / Q(C)
volt = joule / coulomb
V = J / C
What is the voltage supply of an electrical circuit with energy consumption of 60 joules and charge flow of 4 coulombs?
V = 60J / 4C = 15V