Stucco Calculator

How the stucco calculator works

Stucco in North America, render in the UK and Australia, parging on a Canadian foundation wall: the material differs a little but the arithmetic is the same. A wall of a given area, coated to a given total thickness, needs a certain wet volume of mix, and each bag makes a fixed amount of it. The calculator multiplies area by thickness to get that volume, adds your wastage, and divides by what one bag yields. The answer is rounded up to whole bags.

The one figure people get wrong is the thickness. Enter the total of every coat you will apply with this product, not the thickness of one coat. A three-coat stucco job over lath is 7/8 in in all; a two-coat sand and cement render is usually 15 to 20 mm; a plaster skim is 2 to 3 mm.

Bag yield and coverage

With US units selected the calculator asks for the yield of one bag in cubic feet, because that is how American data sheets state it. QUIKRETE's base coat stucco sheet gives 0.75 ft³ per 80 lb bag, and lists the same bag covering 20 to 24 ft² at 3/8 in, 15 to 18 ft² at 1/2 in and 10 to 12 ft² at 3/4 in. The calculator reproduces the top of each range: 0.75 ft³ spread 3/8 in thick is 0.75 ÷ (0.375 ÷ 12) = 24 ft². The sheet puts the range down to thickness and waste, which is what the wastage input is for.

With metric selected it asks for coverage in m²·mm per bag: the area one bag covers at 1 mm. It is the same quantity as the bag's wet yield in litres, since a square metre one millimetre thick is exactly one litre. Data sheets state it three ways. Some give kg per m² per mm, so divide the bag weight by that figure: a 25 kg bag at 1.5 kg/m²/mm is 16.7 m²·mm. Some give the yield in litres, which you can enter as it is. Some give an area per bag, and you multiply it by the thickness it applies to. Boral's Uni-Render sheet quotes 2.5 to 6 m² per 20 kg bag, depending on thickness and the surface, in layers of 3 to 4 mm. Take the figure for your thickness and surface: 3 m² at 4 mm is 12 m²·mm.

Worked examples

A 430 ft² gable end in three-coat stucco at 7/8 in: 430 × 0.875 ÷ 12 = 31.4 ft³ of wet mix. At 0.75 ft³ a bag that is 41.8 bags, and with 10% wastage 46 bags, which at $11 a bag is $506. In practice the finish coat is a different product, so run the tool twice. The two base coats at 3/4 in need 26.9 ft³ and 40 bags with wastage. The 1/8 in finish needs 4.5 ft³ and 7 bags, using the finish coat's own yield, which for QUIKRETE is also 0.75 ft³. That is 47 bags in total, one more than the single run, because each product rounds up on its own.

A 40 m² rendered wall at 15 mm with a bag rated 15 m²·mm: 40 × 15 = 600 m²·mm, or 600 litres of wet render, so 40 bags before wastage and 44 with 10% added. At £9 a bag that is £396. A 60 m² wall at 20 mm with a 25 kg bag at 1.5 kg/m²/mm (16.7 m²·mm) is 1,200 ÷ 16.7 = 71.9 bags, 80 with wastage. A 12.5 m² ceiling skimmed at 3 mm with a finishing plaster covering 20 m²·mm is 1.9 bags, so two bags with 5% wastage.

If you mix sand and cement yourself, the Wet Volume row is the figure to batch from, and the bag count does not apply.

Thickness and coats

Section R703.7 of the International Residential Code, Chapter 7 requires cement plaster to be applied in at least three coats over metal or wire lath and at least two coats over masonry or concrete, installed to ASTM C926. It also sets the curing gaps: 48 hours before the second coat and seven days before the finish. The three-coat system is a 3/8 in scratch coat, a 3/8 in brown coat and a 1/8 in finish, 7/8 in in all. One-coat stucco systems run 3/8 to 1/2 in and are installed to their own evaluation reports.

In the UK, external rendering is designed and applied to BS EN 13914-1, and the BSI page for the standard gives its scope. Traditional sand and cement render goes on in two coats of 8 to 12 mm, the first scratched to key the second, and bagged one-coat renders are usually applied at 10 to 20 mm in one or two passes. Australian bagged renders are thinner still, 3 to 4 mm a layer to a 10 mm maximum for a product like Uni-Render, so a 15 mm total there means a different product or a sand and cement undercoat. Whatever the market, the printed coverage assumes an even thickness. Old brickwork with raked joints, dashed finishes and hand mixing all eat material, which is why the default wastage is 10%.

What the count does not include

Lath, fixings, corner beads, stop beads, movement joints, mesh and the water-resistive barrier behind the stucco are separate orders. Nor does the calculator deduct openings: take doors and windows off the wall area yourself, but leave small ones in, because the returns around them use as much material as they save. Colour, texture and acrylic finish coats have their own coverage figures and are best run as a second calculation with the finish coat's yield.

Using this with other BuildMetricLab tools

The brick and block calculators size the wall that carries the render, the drywall calculator does the boards on the inside face, and the plastering time calculator turns the area into hours. All BuildMetricLab tools run in your browser with no sign-up, and the formula is on the page so the maths can be checked.