Assumed the frequency range of interest is up to and including shortwave then the description below is the approach. A loop antenne can be used as passive antenna with or without tuning, or as active antenna with an amplifier. Most probably your question relates to passive wideband reception antennas. Then you can read further. Active antenna can be deduced from the info.
Conversion from H-field A/m (magnetic component) into induced voltage. Common radio use is expression of field strength in the E-field component V/m. Relation: E/H equals 376.73... Ohm. Depending on the environment (in-house or otherwise shielded, or in a wood) this relation deviates: in many practical cases the E-field is more attenuated than the H-field.
The induced voltage from a EM-field is now, metric non-US values:
EMF = 2 * PI * A * N / Lambda for 1 V/m where A is the area of the loop in square meters, N the number of turns and Lambda the wavelength in meters.
The source impedance can be measured and will be in the order of circumference (meters!) multiplied with 1 microHenry (per meter) and multiplied with N^2 in case of more than one winding. Do not forget the load impedance. A circuit simulator can help you to do the math.
Important to know: the short-circuit current of such a loop is a frequency-independent measure of the field strength.
Wideband active loop antenna for use with receivers needs conversion factor (effective height since dimension follows from "volts per volt per meter") of factor 0.4 in load when receiver noise figure is 10 dB, to be sure that:
---> received noise floor on a quiet location converted in an output voltage for the antenna that corresponds to the input noise of the receiver.
Depending on the environmental noise and on the need for better large signal behavior the conversion factor can be (must be) adjusted for local situation.