At equilibrium the rate of the forward reaction is equal to the backward reaction rate. A simple reaction, such as ester hydrolysis
has reaction rates given by expressions
According to Guldberg and Waage, equilibrium is attained when the forward and backward reaction rates are equal to each other. In these circumstances, an equilibrium constant  is defined to be equal to the ratio of the forward and backward reaction rate constants  
The concentration of water may be taken to be constant, resulting in the simpler expression 
This particular concentration quotient, , has the dimension of concentration, but the thermodynamic equilibrium constant, , is always dimensionless.