The purpose of this study is to propose a simple and convenient equation to describe a relation between overturning ratios of rectangular rigid bodies and input motions. Some equations have already been proposed to represent criteria for overturning of rectangular rigid bodies. However, they just indicate minimum magnitude of input motion by which rigid body might overturn and they do not give us any information about overturning ratios.Firstly, in this study, many dynamic response analyses of rectangular rigid bodies were conducted using several rectangular models with different widths and heights and several input motions with different magnitudes and different frequency characteristics. The results have made it clear how overturning ratios of rectangular rigid bodies are influenced by sizes and aspect ratios of rigid bodies and magnitudes and predominant frequencies of input motions.Secondly, a new equation was formulated based on these studies to express overturning ratios of rectangular rigid bodies. This equation is expressed by a logarithmic normal distribution of maximum acceleration or maximum velocity of an input motion. The mean value of the distribution is expressed by aspect ratio and size of a rectangular rigid body.Furthermore, the validity of the equation was confirmed by comparison with relations between overturning ratios of tombstones (most tombstones are rectangular in Japan) caused by the 1993 Kushiro-oki earthquake or the 1995 Hyogo-ken Nambu earthquake and estimated maximum velocity of ground motion at each graveyard.The proposed equation is applicable to all things that can be supposed to be rigid bodies. It is expected to apply to various problems including estimation of overturning damage of furniture in a building due to an earthquake.