Depth of field is a result of the size of the aperture.

For example:
A 35mm lens at f/1.4 has a 25mm aperture.
A 100mm lens at f/4 has a 25mm aperture.
A 400mm lens at f/16 has a 25mm aperture.
They will all have the same depth of field at those f/stops.

That is the math behind it. Once you know the size aperture (depth of field) you like to shoot at you can start comparing lenses.