I am guessing that a 20k square lawn is roughly 12000 square feet. That is about 3 times the normal size of an average lawn at around 4000 square feet or just under 5k. When using a mister, the application is much less than when spraying. However, it really depends on how heavy you appy the product. You could very easily do a 20k lawn with 4 gals if you mist lightly enough. The question is, what are you trying to kill and what product are you using?