Percolation testing apparatus comprising an elongated guide rod having one end to be driven into the bottom of a test hole for simple and sole support of the rod along which a gauge rod is slidable by means of guide brackets on the latter and a scale strip is attached to the upper end of said gauge rod for vertical movement relative to a reference marker supported adjustably upon the upper portion of said guide rod. A float is connected to the lower end of the gauge rod for floating movement vertically in the test hole to move the scale strip relative to the reference marker which is stationary on the guide rod.