Joint compound is the material where published estimates disagree most, by a factor of five. Knowing which figure to trust saves either a wasted pallet of mud or a trip to the store on finishing day.
Whose number to use
USG, which makes the product, publishes roughly 0.9 gallons per 100 square feet of board for a Level 4 finish. Experienced finishers on trade forums consistently report about one five-gallon bucket per ten sheets — roughly 1.5 gallons per 100 square feet.
The truth sits between them and depends on how straight your framing is, whether sheets went up vertically or horizontally, and how heavy-handed the taper is. This calculator uses 1.2 gallons per 100 square feet.
Finish levels
Level 4 is the normal residential standard: tape coat, fill coat, finish coat on all seams and screw heads. Most walls that get paint are Level 4.
Level 5 adds a thin skim over the entire surface and roughly doubles the compound. It is worth it where glossy paint or strong raking light would show the difference between mudded seams and bare paper.
Level 3 is less compound, and only appropriate where heavy texture is going over the top.
All-purpose, topping, or setting
All-purpose works for every coat and is what most people should buy. Topping compound sands more easily but has less bond strength, so it is finish coats only.
Setting compound — the powder you mix, sold as 20, 45 or 90 minute — cures chemically rather than drying. It is the way to get two coats on in a day, and the only sensible choice for filling deep gaps, but it does not sand easily once set.
Why the third coat uses so little
The three coats are not equal. The first fills the taper and embeds the tape. The second is wider and fills the remaining hollow. The third is a skim, feathered wide, and it removes almost as much as it adds.
Most of the compound in a job goes on in the first two passes. If the third coat is using as much as the second, it is going on too thick, and the sanding afterwards will be much harder work than it needs to be.
Compound is cheap and sanding is not. Thin coats, more of them, less dust.
Sanding, and the alternative
Sanding joint compound produces an extraordinary amount of very fine dust that gets everywhere in a house and is genuinely unpleasant to breathe.
Wet sanding with a damp sponge produces none of it. It is slower, it will not take down a badly overfilled joint, and on a well-applied third coat it works perfectly well.
A pole sander with a vacuum attachment is the middle ground on a large job. Whichever way, a light touch and a good work light held flat against the wall to show the shadows are what get a flat result.