How much paint do I need?
One gallon covers about 350 square feet of smooth, primed surface in one coat. Rough, porous or bare surfaces drink it and drop closer to 250. This does the arithmetic both ways.
Result
This is material only — it does not include primer, which should be calculated as its own coat, or the extra you want on the shelf for touch-ups.
Questions
How many square feet does a gallon of paint cover?
About 350 square feet on a smooth, primed surface in one coat. On rough, porous or previously unpainted surfaces, plan for closer to 250.
Should I include primer in the coat count?
Calculate primer separately — it is a different product with different coverage, and mixing it into the coat count understates both.
Does this account for waste?
No. Rounding up to whole gallons absorbs a little, but on a large job add a gallon for touch-ups.
Or stop doing this in a calculator
Aerys prices the whole job from your own catalog and production rates, sends it with an accept link and an invoice already attached, and then tracks what the job actually cost against what you quoted.