You CANNOT use micro sieverts per hour (uSv/hr), MREM/hr, mR/hr, or any other such energy-based unit with a Geiger counter without first knowing the exact energy of the object you are testing. With this knowledge, you can calculate your equivalent dose from exposure, or even calibrate your Geiger counter to do it for you.
NOTE: The math I use was created by me, but is widely used in science, and not new. DO NOT USE the techniques or math I show you for anything you really need to be 100% sure of. This is for fun and there are many additional variables I did not calculate.
USE AT OWN RISK
This is only an approximate means.
The real method for finding the energy of a given isotope is to use a scintillator to measure the energy emitted by the sample. If you REALLY must use a Geiger counter, you might try something like this:
Sv/hr = E(joules) * A(Bq) * 3600s * Wr * H * (100-(r/(ρ cm^3/CSDA cm^2)))/kg Hr
E = energy per Bq in joules
A = activity in Bq
3600s = 60seconds x 60min
Wr = weighting factor of radiation. Gamma and Beta=1, Alpha=20
H = weighting factor of body part. Use 1 for entire body
/kg Hr = the entire effective energy is divided per kilogram, and per hour.
The last part calculates the change in energy of particle with respect to position within a medium. ρ is the density of air. dE/dx, basically.
Watch the video to see in detail.