Stucco Calculator
This stucco calculator gives bags per coat, the lath and paper underneath, and the sand and cement for a site mix, for three-coat, two-coat and one-coat systems.
Bags By Coat
Site Mix, Sand and Cement
Mixing on site instead of buying pre-blended bags. Usually cheaper on a large job and always more work.
Stucco Cost
How to use the Stucco Calculator
Enter the wall area and pick the system. That choice decides everything else: a three-coat job over lath is seven eighths of an inch of material in three passes, a two-coat job over masonry is half that, and a one-coat system is a proprietary product at three eighths.
The result gives bags per coat, not as one total, because the coats are different thicknesses and are usually bought at different times.
Site Mix works out the sand and cement instead, for jobs big enough that pre-blended bags stop making sense.
Stucco formula
net area = length × height − openingsbags per coat = net area ÷ coverage per bag at that thicknesslath sheets = net area ÷ 32 sq ftsite mix volume = area × thickness
Coverage falls with thickness, which is why the brown coat takes about twice the bags the finish does.
| Coat | Thickness | Coverage per 80 lb bag | Purpose |
|---|---|---|---|
| Scratch | ⅜ in | ~27 sq ft | Keys into the lath, scored to grip the next coat |
| Brown | ⅜ in | ~27 sq ft | Builds thickness and makes the wall flat |
| Finish | ⅛ in | ~80 sq ft | Colour and texture only |
| One coat | ⅜ in | ~27 sq ft | Proprietary fibre-reinforced base |
A standard three-coat build is ⅜ + ⅜ + ⅛ = ⅞ inch over metal lath.
Worked example
A 40 × 9 ft wall with 60 sq ft of openings, three coat, 10% waste:
- Gross: 40 × 9 = 360 sq ft, less 60 = 300 sq ft
- Plus 10%: 330 sq ft
- Scratch: 330 ÷ 27 = 13 bags
- Brown: 330 ÷ 27 = 13 bags
- Finish: 330 ÷ 80 = 5 bags
- Lath: 330 ÷ 32 = 11 sheets
Thirty-one bags in total, and twenty-six of them are for the two coats nobody sees. That is the shape of every stucco job.
What the result means
The finish coat is the cheap one. It is an eighth of an inch of colour. The scratch and brown coats are three eighths each and take four fifths of the material, budget accordingly.
Lath and paper come first and are not optional over framing. Two layers of weather-resistive barrier and self-furring metal lath go on before any stucco, and they are what the scratch coat keys into. Over solid masonry you can skip them and drop to two coats.
Stucco is a system, not a coating. The barrier drains water, the lath holds the stucco, the coats build thickness and the finish sheds. Leave one out and the wall leaks, usually two years later, out of sight.
Common mistakes
- Budgeting the total thickness as one coat. Three coats at ⅞ inch is not one pass, it is three, with cure time between them.
- Skimping the brown coat. It is what makes the wall flat. A thin brown coat shows in every raking light for the life of the building.
- One layer of paper. Most codes want two over wood sheathing. The inner layer is the drainage plane.
- Not moist-curing. Stucco needs water to cure, not to dry. In hot weather it wants misting for the first couple of days or it cracks.
- Rendering straight onto smooth concrete. Nothing keys to it. It needs a bonding agent or lath.
Frequently asked questions
How many bags of stucco do I need?
For a three-coat job, about one 80 pound bag per 27 square feet for each of the scratch and brown coats, and one per 80 square feet for the finish. A 300 square foot wall with ten percent waste takes about 13 bags of scratch, 13 of brown and 5 of finish.
How thick should stucco be?
Seven eighths of an inch total for traditional three-coat over metal lath, three eighths scratch, three eighths brown and an eighth of finish. Two-coat work over solid masonry is about half an inch, and proprietary one-coat systems are three eighths plus finish.
How much sand and cement for stucco?
A 3:1 to 5:1 sand to cement ratio by volume, depending on the coat. For 300 square feet at seven eighths of an inch that is about 0.85 cubic yards of mortar, which at 4:1 is roughly 0.8 cubic yards of sand and about 12 bags of Portland cement.
How much lath do I need for stucco?
One 27 by 96 inch sheet covers about 18 square feet after laps, though sheets are often quoted at their full 32 square feet. Divide your wall area by the coverage your supplier states and allow for the overlap at every edge.
Can I stucco over concrete block?
Yes, and you can usually skip the lath and paper and go straight to a two-coat application, because block gives the stucco something to key into. Smooth poured concrete is different and needs either a bonding agent or lath.
How long between stucco coats?
Typically 48 hours between the scratch and brown coats and at least seven days before the finish, with moist curing throughout. Hot dry weather needs misting. Rushing the cure is the most common cause of cracking.
Sources and assumptions
Coat thicknesses follow ASTM C926 for portland cement plaster: three-eighths scratch, three-eighths brown and one-eighth finish over metal lath, totalling seven eighths of an inch.
Coverage per bag is a typical figure for 80 lb pre-blended stucco base at the stated thickness and varies by product. Check the bag.
Lath sheets are counted at 32 sq ft nominal; effective coverage after laps is lower and depends on the lap your installer uses.
Weather-resistive barrier requirements, lath fastening, control joints and curing are governed by code and manufacturer instruction and are outside this calculation.
Coat thicknesses follow ASTM C926, the specification for portland cement plaster.