Conversion voltage - power

Hi everyOne ,
i want to know whethere there is any rule to convert Power ( in Watt or dB) to Voltage level (in V ) ? because i had found a lot of explixite conversions in so many documentations ( for example 0.21mW/cm2 = 28 V/m and 34 V/m=0.31mW/cm2), so is there any rule to make such conversions ? thank you for your help ,
and good luck for all ,

You are first asking about converting power to voltage level but quoting field strength and power density units - a rather different problem.

The latter refers to wave propagation laws and free space impedance, the former simply to load resistance.

yes , thank you ,
what is the load resistance to consider , in the two cases

in the second case i mean ;)

In the most simple case P = V * I

there are 3 unknowns, you need to know the values for 2 of them to determine the third

that is simple algebra

The question is implicitly presuming a specific wave propagation scenario, e.g. plane waves. Under circumstances, it can be related to free space impedance of 377 ohms, similar to P=V2/R. That's apparently the case for the numbers you are giving

(28 V/m)2/377 = 2 W/m2 = 0.2 mW/cm2

https://en.wikipedia.org/wiki/Impedance_of_free_space

that is helpful for me thank you FvM , and thanks for all ,
solved .