We estimate the minimal mass of selfgravitating polytropic disks using the famous Sobolev inequality .
This bound resembles the well known mass formula for Lane–Emden stars .
For ideal gas with the polytropic index n = 3 the minimal mass is not smaller than the Jeans mass .
The accuracy of the estimate is verified in a number of numerical examples .
The bound works well for heavy selfgravitating disks and is less useful for light disks .