This paper proposes a way to assess total electric field strength by using mono-axial probe (instead of isotropic, tri-axial probe). When mono-axial probe is used, additional conversion factor should be applied. Consequently, the usage of mono-axial probe causes additional uncertainty in measurement results that should be taken into account. Measurement results for seven different environments show that the additional multiplicative conversion factor value of 1.95 should be applied and additional uncertainty in measurement results of 33.07 % should be taken into account.