It all depends on the input data available to your calculation.
If you know that two plates of metal are parallel and have a voltage difference, and also know the distance between them. The electric field between them is simply the voltage divided by their distance.
If the source of the magnetic field is a current at distant locations, then you can calculate its magnetic potential as the current divided times the distance squared. You can then calculate the field as the derivative of time.
Additional factors can be used in calculations depending on which units are being used.