Why the coat system, not the wall, drives the bag count
Area is the easy half: wall length × height × the number of walls, minus the square footage of doors and windows. The half that decides your order is thickness. Stucco is sold by coverage at a stated depth, and coverage and depth are inversely proportional — double the thickness and you halve the square feet a bag will cover. A traditional three-coat system over lath builds roughly 7/8 in total: a scratch coat of about 3/8 in that is combed horizontally to key the next layer, a brown coat of about 3/8 in that is rodded flat and is the coat that actually makes the wall straight, and a finish coat of about 1/8 in that carries the colour and texture. A one-coat or two-coat system applied over a solid substrate such as concrete or block builds only about 3/8 in. That is why the same wall can take well over twice as many bags in one system as the other, and it is the single biggest reason people under-order.
This calculator takes your coverage figure as a rate at 3/8 in and rescales it for the system you choose, so if your bag quotes coverage at a different depth, convert it first or the answer will be off by that ratio. Two practical adjustments are worth making on top of the raw number. First, do not deduct small openings aggressively — the material saved is largely spent again on the returns and reveals around the frame, so many estimators only deduct openings above roughly 10 sq ft and treat anything smaller as a wash. Second, add a waste allowance of around 10%, higher if the substrate is uneven, since the brown coat has to fill every low spot to leave a flat plane and that fill is real volume that no area calculation can see. Order the finish coat in a single batch as well: colour is mixed per batch, and a wall finished from two different lots can band visibly once it dries.
Coverage varies by product, mix consistency, substrate and applied thickness — always check the coverage and thickness stated on the bag, and follow the manufacturer's system for lath, curing and moisture control.