Curious about the math in brandons answer here. It's actually  .027 **bushels** per sq ft. which adds up to **1.215 lbs per sq ft** at **45 lbs** of barley to a bushel.

an acre is 208 ft sq.
if we calculate for the least successful amount of 5 bushels per acre.
at the expected weight of 45 lbs of barley per bushel

(5/208)*45 = 1.0817 lbs of barley per sq ft.

this fellow claims he got one 330ml finished beer per sq ft at the gravity he was targeting. https://brewingbeerthehardway.wordpress.com/2015/08/02/one-beer-per-square-foot/

but generally it takes 2 lbs of grain per gallon of wort. so in theory you should be able to get 5 gallons with 10 sqft of planting area. I have that much room in the sun facing windows of my NYC apartment!