Any liquid covers 1604 square feet per WET mil per gallon... WET... So to figure out how much paint you need for 'dry mil thickness' you would:
1604*% Solids= # of square feet per DRY mil per gallon...
So if your paint is 50% solids...
1604*0.50= 802 ft2 per DRY mil per gallon
I like numbers, and I find this interesting.
I tested this on a few SW products. ProMar 200 interior latex is speced at 42% solids, 1.6 mil DFT, and coverage of 350 to 400 sf. Using your formula (1604*.42)/1.6 I came up with 421 sf. Exterior SuperPaint gave me (1604*.35)/1.44, which resulted in coverage of 389 sf.
I would think that if I worked backwards I would get to the 1604 number. Using the mid range on coverage, I usedthis formula for PM: (375/.5)*1.6 and would come up with 1429. With SuperPaint I come up with 1542.
Just so you don't think I'm being a jerk, I really do find this interesting. But the numbers aren't working for me, so I'm wondering what I'm missing.
It could be that the formula is a general guideline, but the difference between PM and SP is about 8%, which seems significant to me. And neither is real close to the 1604 number you cited.
Perhaps of greater interest is why the solids would be higher for an interior paint. Is it in the expectation that only 1 coat would be applied, while with an exterior paint the expectation is 2 coats?
I don't see an immediate reason to know this, but I have long believed that the more I know about the products we apply, the better I can serve my customers. It also helps me understand why one product is better than another in a particular situation. So I am all eyes.