Stucco Calculator

How much stucco you actually need

A 20 × 8-ft wall — 160 sq ft — takes 18 bags of stucco for a standard three-coat job: 15 bags of base coat and 3 bags of finish coat, plus 10 sheets of metal lath. That is 80-lb bags at the industry coverage figure of 25 sq ft per bag at 3/8 in, applied over a 3/4 in combined scratch-and-brown thickness, with a 10% waste factor. The finished wall carries about 1,300 lbs of stucco — roughly 8 lbs per square foot — which is why the lath and its fasteners matter as much as the mix does.

Stucco is priced and sold by the bag, but it is designed by thickness. Get the thickness right and the bag count follows automatically. Get it wrong and you either run out halfway up a wall or end up with a coat too thin to resist cracking.

The three systems, and which one you have

Almost every residential stucco job is one of these. The system is dictated by what you are sticking it to, not by preference:

ASTM C926, the standard that governs application, sets 7/8 in as the nominal total over metal lath on frame construction and 1/2 in as the minimum over unit masonry. Those two numbers cover the overwhelming majority of jobs, and they are what the two preset modes above use. The "Custom" mode is there for proprietary one-coat systems, whose manufacturers publish their own thicknesses — follow the bag, not the standard, when you are using one.

The reason frame walls need nearly twice the thickness is that stucco on framing is a self-supporting slab hanging on lath. It has to carry its own weight and resist racking. On solid masonry the stucco is bonded directly to something rigid, so it only has to be thick enough to be durable.

Coverage: how far one bag goes

Bagged stucco base coat covers about 25 sq ft per 80-lb bag at 3/8 in. Coverage scales inversely with thickness, so doubling the thickness halves the area:

The mental shortcut: at 3/8 in, one 80-lb bag does 25 sq ft. Everything else is that number scaled. A 40-lb bag does half; a 94-lb bag of portland used in a site mix is a different calculation entirely (see below).

The math, walked through

For the default 20 ft × 8 ft wall with no openings, three-coat system:

1. Net area: 20 × 8 = 160 sq ft.
2. Base coat thickness (scratch + brown): 3/8 + 3/8 = 3/4 in = 0.0625 ft.
3. Base volume: 160 × 0.0625 = 10 cu ft.
4. Bag yield at 80 lb: 25 sq ft × 0.03125 ft = 0.78 cu ft per bag.
5. Base bags: 10 ÷ 0.78 = 12.8, plus 10% waste = 14.1 → 15 bags.
6. Finish coat at 1/8 in: 160 ÷ 70 = 2.3, plus waste = 2.5 → 3 bags.
7. Lath: 160 ÷ 16 sq ft of effective coverage per sheet → 10 sheets.

Note that base and finish are different products. Base coat is a coarse sanded cement mix; finish coat carries finer sand and, in most colored systems, integral pigment. Buying all base coat and troweling it thin for the finish gives you a rough, blotchy wall that will not hold colour.

Metal lath, and why the sheet count is not area ÷ sheet size

Expanded metal lath comes in 27 in × 97 in sheets — 18.2 sq ft of material. But lath has to lap 1/2 in on the sides and 1 in on the ends, and it must lap over framing members, so effective coverage is about 16 sq ft per sheet. That is the figure the calculator uses. Ordering by the raw sheet area leaves you roughly 12% short.

Two things go under the lath on a frame wall and neither is optional: two layers of Grade D building paper or an equivalent water-resistive barrier (one layer is a code violation in most jurisdictions and a callback waiting to happen), and corrosion-resistant fasteners at 6 in on centre vertically into every stud. The lath goes on cup side up, so the little scoops catch and hold the scratch coat. Installed upside-down, the scratch coat sags off the wall — this is the single most common amateur error, and it is invisible until the mud will not stay put.

Mixing your own versus buying bagged

Site-mixed stucco is cheaper at volume. The traditional proportions by volume, per ASTM C926:

• Scratch and brown coat: 1 part portland cement, 3/4 to 1-1/2 parts lime, 3 to 5 parts damp sand (measured against the combined cementitious volume).
• Finish coat: 1 part portland, 3/4 to 1-1/2 parts lime, 1-1/2 to 3 parts sand — richer and finer than the base.
• Or use plastic cement (portland with plasticiser already in it) at 1 part to 3 parts sand, and skip the lime.

A 94-lb bag of portland plus roughly 0.5 cu yd of sand per four bags yields around 4.5 cu ft of base-coat mix — call it six 80-lb bagged equivalents for about half the material cost. The break-even is somewhere around 40 to 50 bags, or roughly 500 sq ft of three-coat wall. Below that, the mixer rental, the sand delivery, and the risk of batch-to-batch colour variation eat the saving. Above it, site mixing wins clearly — which is why no professional stucco crew buys bagged base coat.

One caution that has nothing to do with cost: colour consistency in a finish coat is unforgiving. If you are using an integrally coloured finish, buy every bag from the same lot number and mix whole bags only. A half-bag batch to finish a wall will read as a visible patch in raking afternoon light, permanently.

Curing matters more than mix design

Stucco gains strength by hydration, not by drying, and a wall that dries too fast cracks. The rules that actually govern outcomes:

• Moist-cure each base coat for 48 hours — a light fog from a garden hose twice a day. Skipping this is the most common cause of map cracking across a whole elevation.
• Wait 7 days between the brown coat and the finish coat in a traditional three-coat job. The brown coat needs to do most of its shrinking before you lock a finish over it.
• Apply between 40 °F and 90 °F, and not onto a wall in direct afternoon sun. Below 40 °F hydration effectively stops; above 90 °F with wind, the surface skins over before the body has set.
• Do not stucco with rain forecast inside 24 hours. Fresh stucco washes, and the streaks are permanent.

Control joints and where cracks come from

Stucco cracks. The question is whether it cracks where you decided or where the wall decided. Code and industry practice put control joints at a maximum of 144 sq ft of unbroken panel on frame walls, with no panel dimension over 18 ft and no length-to-width ratio worse than 2.5:1. Joints also belong at every floor line and wherever the substrate changes.

Beyond joints, the reliable crack sources are: corners of window and door openings (fit diagonal lath strips there), lath that is not tied through to framing, a scratch coat that was not scored, and — most often — inadequate curing. Nothing in the mix compensates for a wall that dried out in 6 hours.

Stucco, plaster, render, and EIFS are not the same thing

The terminology varies by country and it causes real ordering mistakes:

• Stucco (US) — exterior cement-based coating. Called render in the UK and Australia, Putz in German-speaking countries. This calculator sizes it.
• Plaster (US) — the interior equivalent, usually gypsum-based over blueboard or lath. Different material, different coverage, and it must never go on an exterior wall.
• EIFS — Exterior Insulation and Finish System: foam board with a thin polymer-modified base coat, reinforcing mesh, and an acrylic finish. Total thickness is around 1/4 in, it is not cement stucco, and its material take-off is by system component rather than by bag. Do not use these numbers for EIFS.
• Acrylic finish — a synthetic topcoat sometimes used over cement base coats. Sold by the bucket, covers 80–120 sq ft per 5 gallons, and is a different calculation from a cement finish coat.

What stucco costs

Material only, at 2026 US prices:

• Bagged base coat — $14–$20 per 80-lb bag.
• Bagged finish coat — $18–$28 per 80-lb bag; coloured finishes at the top of that.
• Metal lath — $12–$18 per 27 × 97 in sheet.
• Grade D paper — $30–$50 per roll (about 500 sq ft, and you need two layers).
• Site mix — portland at $14–$18 per 94-lb bag, lime at $12–$16 per 50-lb bag, masonry sand at $30–$45 per cubic yard delivered.

For the 160 sq ft default wall, expect roughly $300–$420 in materials for a three-coat job. Installed by a contractor, three-coat stucco runs $8–$14 per square foot in most US markets — labour is the overwhelming majority of that, which is the usual reason DIY stucco is attractive despite being genuinely hard work.

Frequently asked questions