Stucco is estimated by volume, not by area, because the thickness changes by a factor of three depending on which system you are installing. A one-coat system over foam and a traditional three-coat system over lath are the same wall and nearly triple the material.
Three-coat versus one-coat
Traditional three-coat stucco goes over metal lath and totals about seven eighths of an inch: a three eighths inch scratch coat, a three eighths inch brown coat, and an eighth inch finish. It is the durable system and it is what most older stucco houses have.
One-coat systems go over rigid foam board and run three eighths to half an inch total. They install faster and cost less, and they depend entirely on the foam and the weather barrier behind them.
Enter the total thickness for whichever system you are using. The quick buttons cover three eighths, seven eighths and the heavy end.
Coats go on in stages, and so does the buying
You do not apply seven eighths of an inch at once. The scratch coat goes on and is scored, then cures. The brown coat goes on and is floated flat, then cures. The finish coat is last.
Each stage needs its own curing time — often a week between coats, and the scratch and brown coats have to be kept damp while they cure or they will crack.
That means you can buy in stages too, which is worth doing because mix that sits through a wet month is mix you throw away. Work out the total here, then buy per coat.
What is behind it matters more than the mix
Stucco is not waterproof and was never meant to be. It absorbs water and releases it. What keeps the wall dry is the weather-resistant barrier behind it and the drainage plane it creates.
Two layers of building paper is the traditional approach, and the reason for two is that wet stucco bonds to the first layer and destroys it as a barrier — the second layer is the one still doing the job in year ten.
Weep screed at the base is not optional either. It is the metal trim that lets absorbed water out at the bottom of the wall. Stucco run down to grade with no weep screed traps water against the framing.
Openings and why they are subtracted here
Doors and windows come off the area automatically, using three by seven feet for a door and three by four for a window. Change the counts to match your wall.
Openings in stucco need their own detail work — casing bead around the perimeter and flashing above — and that trim is a separate line item from the mix.
Corners are the other trim item. Corner aid goes on every external corner and it is what keeps the edge straight and stops it chipping.
Cracking is normal, within limits
Fine hairline cracking in a stucco wall is expected. Cement shrinks as it cures and stucco is a thin layer of cement on a moving building.
What is not normal is cracking wide enough to admit water, cracking in a diagonal from a window corner, or cracking that follows a lath seam. Those point at movement, missing control joints, or lath that was not fastened properly.
Control joints break the wall into panels so that shrinkage has somewhere to go. Large unbroken expanses of stucco crack in the middle instead.