Stucco Calculator

Bag count comes from the wall area, the coat thickness, and each product's coverage. The calculator multiplies your net wall area by the applied thickness of each coat, then divides by the plaster a bag yields at its rated thickness, using bags = area x thickness x waste / (coverage x reference thickness). A typical 80 lb base coat bag covers about 25 sq ft at 3/8 in, and an 80 lb finish bag covers about 70 sq ft at 1/8 in. Enter your own coverage numbers from the bag label for the closest estimate.

Traditional stucco is a three-coat system defined by ASTM C926. The scratch coat goes on first at about 3/8 in and is raked so the next coat can grip it, the brown coat adds another 3/8 in to true up the wall, and the finish coat adds about 1/8 in of color and texture. That totals roughly 7/8 in over metal lath. Over solid masonry you often skip the scratch coat, so set its thickness to 0 for a two-coat job.

You need metal lath wherever stucco goes over wood framing, sheathing, or any surface the plaster cannot bond to directly. A standard expanded-metal lath sheet is 27 in by 96 in, or 18 sq ft, but side and end laps consume about 15% of each sheet, so plan on roughly 15 sq ft of real coverage. The calculator counts sheets only when you pick a framed substrate and skips lath for masonry or concrete. Add building paper or a weather-resistive barrier behind the lath as your local code requires.

Yes. A common base coat is 1 part plastic or Portland cement to 3 parts sand by volume, which the calculator shows as an alternative to bagged base coat. As a rule of thumb that is about one 94 lb bag of cement plus 3 cu ft of sand for every 3 cu ft of base plaster. The finish coat is almost always a bagged product, so the site-mix figures replace only the base coat bags. Site mixing is usually cheaper at volume but takes more labor and quality control.

Enter the count plus the width and height of your doors and windows, and the calculator subtracts their combined area from the gross wall area before sizing materials. For example, one 3 by 6.75 ft door and four 3 by 4 ft windows remove about 68 sq ft. If the openings add up to the whole wall or more, the result clears because there is nothing left to coat. Leave the counts at 0 for a solid wall.

Waste covers material lost to mixing, spillage, uneven substrates, and trimming around openings. For stucco, 5 to 15% is typical, and the calculator defaults to 10%. Use the high end for rough or irregular walls with many openings, and the low end for large, flat runs. The waste percentage scales the bag, lath, sand, and cement totals together.