The true mathematical formula for plugs per square foot the one that Purdue uses is: Plugr 850 = 8 Plugs per square foot Lawn Solutions WB 21" and Stander = 6.857 per sq foot The tightest rolling tine pattern used in a WB rolling tine aerator. Blue Bird = 5.8299 plugs per square foot. ** disclaimer ** I'm not being compensated by Plugr or Lawn Solutions. However since the Plugrs burn up an alarming amount of tines in the soil here I'd sure appreciate a cut rate on replacement 5/8 tines. Free tines for life would be good. My comments here on Lawnsite have helped sell both Plugr 850's and Lawn Solutions WB Also due to my marketing angle I will not run any aerator with tine spacing wider than 3.5" x 6". The Plugr 400's and 600's pull a 3.63" x 6" pattern. Lawn Solutions WB & stander are 3.5" x 6"