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Machine Plaster Calculator — Spray Plaster Coverage & Productivity
Calculate spray plaster bag quantities, water and mix volume, spray time, and daily throughput (m²/day) for machine-applied plaster.
Total area to be sprayed
Standard: 1 sprayer + 1 flattener
Enter your supplier price per bag
12% typical for machine plaster (includes overspray)
PFT G4 typical: 15 L/min
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How We Calculate This
This calculator estimates material quantities and job duration for machine-applied (spray) plaster. Machine plaster has different coverage rates to hand-applied plaster due to the consistent application thickness and overspray factor.
The formula
Bags = (Wall area ÷ Coverage per bag) × Wastage factor
Spray time is calculated from the total wet-mortar volume divided by the pump output rate. Mortar yield is taken per product — Knauf's MP75 data gives a fresh-mortar yield of 100 kg ≈ 125 litres, so a 25kg bag yields roughly 31 litres. At a typical pump output of 15 L/min, one MP75 bag is sprayed in about two minutes. Total job time accounts for all stages: spray, flatten, rule, float, and trowel finish.
Coverage rates (per 25kg bag)
- Knauf MP75: 3.1 m²/bag at the 8mm minimum machine thickness. MP75 uses ≈10 kg/m² at 10mm (≈1.0 kg/m²/mm), so at 8mm the consumption is ≈8 kg/m² and a 25kg bag covers 25 ÷ 8 ≈ 3.1 m². (Knauf's UK/SIA data quotes ~10 kg/m² at 10mm; the NL ATG sheet quotes a lower 8 kg/m², under which 3.1 m²/bag still over-orders — so 3.1 is conservative either way.)
- Knauf Goldband: 2.9 m²/bag at 10mm (≈8 kg/m² per 10mm; 2.9 is a conservative ordering figure vs the ~3.0 m²/bag datasheet rate)
- ThistlePro PureFinish: 10 m²/bag at 2mm (sprayable finish coat — British Gypsum PDS)
- Thistle HardWall: 3 m²/bag at 11mm (spray/hand undercoat — British Gypsum PDS)
Wastage for machine plaster is typically 12% (higher than hand application) due to overspray, material left in hoses, and the initial setup/purge cycle.
Frequently Asked Questions
Machine (spray) plaster is typically 2-3 times faster than hand application. A 2-person team (one spraying, one flattening) can cover 80-120 m² per day with a PFT G4 or G5 pump, compared to 30-40 m² per day by hand. The time saving comes from the spraying speed — the flatten, float, and trowel stages take roughly the same time as hand application.
The most common machine plaster pumps in the UK are the PFT G4 and PFT G5 (also known as the Knauf PFT range). The G4 is a continuous mixer/pump suitable for most interior plastering jobs with a plaster output of roughly 6-20 L/min. The G5 is a larger version for high-volume commercial work. You also need a compressor, spray gun, and supply hoses. Machines can be hired from most plant hire companies for £100-£200/day.
No — machine plasters are specifically formulated for spray application. They have a different particle size, longer working time (90-120 minutes vs 60-90 for hand plaster), and optimised flow properties for the pump. Knauf MP75 is the most popular UK machine plaster. You cannot put standard hand-mix plaster through a spraying machine — it will clog the pump and give a poor finish.
Water demand is set per product, not by a single figure: Knauf MP75 takes about 16.25 litres per 25kg bag (0.65 L/kg), Knauf Goldband about 16.5 litres, British Gypsum Thistle HardWall about 15 litres, and the sprayable ThistlePro PureFinish finish about 11.5 litres per 25kg bag. The machine mixes plaster and water continuously as it pumps, so you need a clean mains supply with adequate pressure (minimum 2 bar) and flow (minimum 15 L/min) connected directly to the machine. A 1,000-litre IBC tank with a pump is a good backup. Always follow the bag for the exact ratio.
Machine plastering requires: (1) significant floor and window protection due to overspray, (2) expensive equipment (hire or purchase), (3) a reliable water and power supply, (4) minimum job size to be economical (typically 100+ m²), and (5) trained operatives. For small rooms or patch repairs, hand application is more practical. The equipment setup and cleaning takes 30-60 minutes each end of the day.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.