Plaster bags = area in square yards ÷ coverage per ton × 2,000 lb ÷ 50 lb per bag. For 500 sq ft (55.6 sq yd) of USG Red Top gypsum plaster over gypsum lath, mixed with 2.5 cu ft of sand per 100 lb, the published coverage is 206 sq yd per ton, so the wall takes 0.27 tons, or 12 bags of 50 lb with 10% waste, and 15 cu ft of sand. Switch to a skim coat or to plasterboard sheets above and the same area gives 6 bags of skim or 18 sheets.
How the plaster is calculated
There are three ways to plaster a room and each has its own arithmetic.
Sand plaster (basecoat). A neat gypsum plaster is mixed with sand on site and applied over lath or masonry in two or three coats. USG publishes coverage in square yards per ton for each product and base. The range depends on how much sand goes into the mix, so the calculator asks for 2.0, 2.5 or 3.0 cu ft of sand per 100 lb of plaster and moves linearly between the published low and high figure. Bags come from tons: tons × 2,000 ÷ 50.
Skim and one-coat. Thin coats are sold by the bag with a coverage at a stated thickness: 25 kg covers 10 m² of multi-finish at 2 mm, for example. That is 1.25 kg per m² per mm, so the powder is area × thickness × rate. Set your own thickness and the bag count follows.
Plasterboard. Board is counted by area over sheet area, rounded up, with a waste allowance for offcuts. Plasterboard is the UK, Irish, Australian and New Zealand name for drywall, and the same sheets get a skim coat on top.
Basecoat bags = ⌈(area ÷ 0.8361 ÷ coverage) × (1 + waste) × 2,000 ÷ 50⌉. Skim powder = area × thickness × kg per m² per mm × (1 + waste). Sheets = ⌈area × (1 + waste) ÷ sheet area⌉.
Plaster coverage by product and base
USG coverage for sanded basecoat, square yards per ton (the low figure is for 2.0 cu ft of sand, the high for 3.0):
| Plaster | Gypsum lath | Metal lath | Brick, block, concrete |
|---|---|---|---|
| Red Top, sanded | 180 to 232 | 114 to 148 | 140 to 181 |
| Structo-Base, sanded | 154 to 214 | 99 to 136 | 120 to 167 |
| Structo-Lite, mill-mixed perlite | 140 | 89 | 109 |
Metal lath takes far more plaster, because USG has you push the scratch coat through it to form full keys, so its coverage is about a third to two fifths lower than over gypsum lath. Structo-Lite already contains its aggregate and is mixed with water only, about 3 to 3.75 gallons a 50 lb bag. These figures are for the basecoat only. The thin finish coat over it is separate and not counted here.
Skim and one-coat plaster rates
Rates for a 25 kg bag, taken from British Gypsum Thistle datasheets as listed by TradeCalculator (we could not open the datasheets directly):
| Plaster | Bag covers | At thickness |
|---|---|---|
| Multi-finish skim | 10 m² | 2 mm |
| Bonding coat | 3.5 m² | 8 mm |
| Hardwall | 3 m² | 11 mm |
| Browning | 3.5 m² | 11 mm |
| One-coat | 2.25 m² | 13 mm |
Rough blockwork uses more plaster than smooth board, so add waste; 10% is the default. For the 500 sq ft example, a 2 mm skim needs 116 kg before waste and 128 kg with it: six 25 kg bags.
How many sheets of plasterboard do I need?
Divide the area plus waste by the sheet area. A 4 × 8 ft sheet covers 32 sq ft, and USG's boards are 4 ft wide in 8 to 16 ft lengths. A metric 1200 × 2400 mm sheet at 12.5 mm covers 2.88 m² (Knauf Wallboard). The 500 sq ft example needs 18 of either size once 10% is added. The drywall calculator adds screws, tape and corner bead to the sheet count, and the drywall mud calculator covers the joint compound.
Plaster or drywall, and what comes next
Once the walls are done, work out the paint for them. Ceilings can be plastered or hung as a drop ceiling, and any wall you open up is a chance to add insulation. Start from the room area with the square footage calculator. More wall and ceiling tools are on the walls and ceilings hub.