After presenting the significance of the K0 factor the paper discusses some aspects of the formulae for the coefficient of earth pressure at rest, mainly the well-known Jaky's equation. An improvement for the critical Jaky's assumption is introduced and a modified K0 ...φ relationship is deduced. All the formulae originating from Jaky's idea and some other continuum mechanical or speculative equations are tested against 153 measured data. The original Jaky's equation, and the ones from Vierbiczky and Matsuoka-Sakakibara were found to be the best. -from Author