m2=3 ft x 3 ft the short of it is, if the strain says 400g. m2 means under optimal conditions(ie, sufficient wattage, grow skills, you should be able to pull off 400 grams in a 3 ft x 3 ft tent with that strain. Now, the best wattage is .5 watts per square foot. so, for a 3 ft. x 3ft. tent(3x3=9) 9square feet.
9 square feet x .5 watts =450watts
So when people say something about growing a gram per watt being a good yield, now you know its optimal!
So, if you have a 3 x 3ft tent with a 450x light and you yield 400 grams, you are doing pretty good.
PS-not a marketing gimmick true facts!:thumbs: