The original form of the Kelvin equation, published in 1871, is: 
where:
* = vapor pressure at a curved interface of radius 
* = vapor pressure at flat interface () = 
* = surface tension
* = density of vapor
* = density of liquid
* ,  = radii of curvature along the principal sections of the curved interface.
This may be written in the following form, known as the Ostwald–Freundlich equation:
where  is the actual vapour pressure,
 is the saturated vapour pressure when the surface is flat,
 is the liquid/vapor surface tension,  is the molar volume of the liquid,  is the universal gas constant,  is the radius of the droplet, and  is temperature.
Equilibrium vapor pressure depends on droplet size.
* If the curvature is convex,  is positive, then  
* If the curvature is concave,  is negative, then 
As  increases,  decreases towards , and the droplets grow into bulk liquid.
If the vapour is cooled, then  decreases, but so does . This means  increases as the liquid is cooled.  and  may be treated as approximately fixed, which means that the critical radius  must also decrease.
The further a vapour is supercooled, the smaller the critical radius becomes. Ultimately it can become as small as a few molecules, and the liquid undergoes homogeneous nucleation and growth.
The change in vapor pressure can be attributed to changes in the Laplace pressure. When the Laplace pressure rises in a droplet, the droplet tends to evaporate more easily.
When applying the Kelvin equation, two cases must be distinguished: A drop of liquid in its own vapor will result in a convex liquid surface, and a bubble of vapor in a liquid will result in a concave liquid surface.