To estimate OIP3 of low power RF amplifiers from DC power operating conditions is of a good help at the beginning of the design work. We introduce a simple formula intended for estimation of OIP3 of Bipolar Junctions Transistors. Harmonic balance simulations and measurements were carried out to validate usability of this formula. The results show that there is more relation between OIP3 and the collector current than between OIP3 and the collector emitter-voltage. The proposed formula is applicable for collector currents in the range of 2 mA to 8 mA and for collector-emitter voltages in the range of 2 V to 3 V.