We present a new formulation of McKenzie's simple uniform stretching model that is based on two reference levels, one near 3.6 km and the other near 7.8 km below sea level. In the absence of lithosphere, the asthenosphere would reach these levels if no oceanic crust were formed. The first level is for hot asthenosphere, the other is for asthenosphere cooled to thermal equilibrium. The instantaneous motion as well as the total vertical motion, produced by uniform stretching of the lithosphere, is expressed simply as a function of the elevation difference between the starting level and respectively the 3.6 and 7.8 km reference levels. In addition, we show that the behavior of the lithosphere under extensional strain is different above and below the 2.5 km-deep asthenosphere geoid. Below this level, oceanic accretion starts rapidly; above it, extensive thinning of the lithosphere produces subsidence until the asthenosphere geoid level is reached, enabling the asthenospheric material to break through to the surface. At low strain rate, pieces of the lower lithosphere may detach and sink in the asthenosphere. This process results in uplift and is taken into account in the formulation proposed.