Plaster calculator for walls and floors

How the plaster calculator works

Bags = area × thickness × yield, plus extra for waste, divided by the bag weight and rounded up. The yield comes from the data sheet, and US sheets give it per bag: Quikrete says an 80 lb bag of base coat stucco makes about 0.75 cu ft of mortar, Custom says a 50 lb bag of LevelQuik RS covers 46 to 48 sq ft at 1/8 in.

1. Area. Length × height for walls, length × width for floors. Several areas add up.
2. Volume. Area × thickness. 160 sq ft at 3/4 in is 160 × 0.0625 = 10 cu ft.
3. Powder. Volume × pounds of powder per cubic foot of layer. 80 lb for 0.75 cu ft is 106.67 lb/cu ft; 50 lb for 48 sq ft at 1/8 in (0.5 cu ft) is 100 lb/cu ft.
4. Waste. The calculator adds 10%. Quikrete's own table runs from 24 down to 20 sq ft per bag at 3/8 in, so in the worst case you need up to 20% more than the full yield. 10% sits in the middle; raise it on rough walls.
5. Bags. Powder with the waste ÷ bag weight, rounded up.

In metric countries the data sheets give the yield in kg/m² per mm, and the advanced options show it as kg per m³ of layer: 1.7 kg/m² per mm is 1,700 kg/m³.

Worked example: a 20 × 8 ft stucco wall

These are the values the calculator opens with in feet: base coat stucco 3/4 in thick, the scratch coat and the brown coat of a three-coat job, in 80 lb bags.

• Area: 20 × 8 = 160 sq ft
• Volume: 160 × 3/4 in = 10 cu ft
• Powder: 10 × 106.67 = 1,066.7 lb, 1,173.3 lb with 10% extra
• Bags: 1,173.3 ÷ 80 = 14.7, so 15 bags of 80 lb
• Coats: 2 coats of 3/8 in
• Water: about 5 1/2 quarts per bag

One bag at 3/4 in covers 12 sq ft, the top of Quikrete's 10 to 12 sq ft range.

For a floor, a 12 × 10 ft room with 1/4 in of self-leveling underlayment needs 2.5 cu ft, 250 lb of powder, 275 lb with the extra: 6 bags of 50 lb.

In the UK, with metres: a 4 × 2.7 m wall with 15 mm of lime-cement base coat plaster at 13.3 kg/m² per cm is 10.8 m², 215.5 kg of powder, 237 kg with 10% extra, so 10 bags of 25 kg and 5.75 L of water per bag.

How many bags per 100 and 300 sq ft

10% extra for waste, yields from the Quikrete, USG and Custom data sheets.

Product and thickness	One bag covers	100 sq ft	300 sq ft
Stucco 3/8 in, 80 lb	24 sq ft	5 bags	14 bags
Stucco 3/4 in, 80 lb	12 sq ft	10 bags	28 bags
Veneer plaster 1/16 in, 50 lb	150 sq ft	1 bag	3 bags
Veneer plaster 3/32 in, 50 lb	100 sq ft	2 bags	4 bags
Self-leveler 1/8 in, 50 lb	48 sq ft	3 bags	7 bags
Self-leveler 1/4 in, 50 lb	24 sq ft	5 bags	14 bags
Self-leveler 1/2 in, 50 lb	12 sq ft	10 bags	28 bags

Coverage halves each time the thickness doubles, which is why a floor that needs 1/2 in in its low spots eats bags fast.

Bags, water and coats on the data sheet

Base coat stucco comes in 80 lb bags; veneer plaster and self-leveling underlayment in 50 lb bags. The water goes by the bag: about 5 1/2 quarts for Quikrete's 80 lb base coat, 12 to 15 quarts for 50 lb of USG Diamond Veneer Finish, 5 1/4 to 5 3/4 quarts for 50 lb of LevelQuik RS. The calculator turns this into a percentage of the powder weight so that it still works if you change the bag size.

Each product has its own thickness range. Quikrete puts scratch and brown coats on at a minimum of 3/8 in each, so 3/4 in of base coat means two coats. USG applies Diamond Veneer Finish at 1/16 to 3/32 in, over its IMPERIAL gypsum base or smooth interior concrete. LevelQuik RS goes from featheredge to 1 1/2 in in a single pour. When your thickness goes past the limit of one coat, the calculator counts the coats.

Many US skim coats on drywall are done with ready-mixed joint compound from a bucket. That is sold by the gallon, not by the bag: if you use it, type the yield from its label in the advanced options.

Common mistakes

• Using the coverage at the wrong thickness. A self-leveler bag that covers 48 sq ft at 1/8 in covers 12 sq ft at 1/2 in.
• Measuring only the thin spots. On an uneven floor the low spots take most of the material: measure several points with a straightedge and use the average depth.
• Picking the product by name, not by thickness. Veneer plaster above 3/32 in or stucco base coat below 3/8 in is outside its data sheet; past those limits the calculator points you to the other product.
• Forgetting the water. Fifteen bags of base coat stucco take about 20 gallons of clean water.
• Buying to the exact bag. Running out halfway through a wall stops the job while the coat is setting: keep the extra and buy it all at once.