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Spray Paint Calculator — Paint Volume for Spraying
Calculate how much paint you need when spraying, accounting for overspray, thinning ratio, and sprayer type.
Combined surface area
2 coats standard
Paint left in hoses, gun, and filters
Typical airless: 0.5-1.5 L/min
Override sprayer default (leave blank to use preset)
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How We Calculate This
This calculator determines how much paint you need when using a spray gun, which always requires more paint than brushing or rolling due to overspray and atomisation losses.
The formula
Brush equivalent = (Area × Coats) ÷ Coverage rate
Spray paint = Brush equivalent × Overspray factor
Total with thinner = Spray paint + (Spray paint × Thinning %)
For example, 50 m² with 2 coats at 10 m²/L = 10 L brush equivalent. With an airless sprayer (1.4×) = 14 L paint needed. With 10% thinning, total mixed volume = 15.4 L.
Overspray factors
The overspray factor is the inverse of transfer efficiency (TE): paint required = paint actually deposited ÷ TE, so a 1.4× factor corresponds to ~71% TE. These presets assume good technique, correct tip size and masking on flat surfaces:
- HVLP: 1.3× — ~77% transfer efficiency (HVLP guns must achieve ≥65% TE to qualify as HVLP)
- Airless: 1.4× — ~71% transfer efficiency
- Air-Assisted Airless: 1.35× — ~74% transfer efficiency
Real-world airless TE is often lower (~50-65%) on complex, exterior or windy work, which can push paint use toward 1.5-2× the brush quantity. For those conditions, use the Custom Overspray % advanced option (e.g. 60-80%) rather than the airless preset.
Frequently Asked Questions
For flat interior walls with masking and good technique, spraying typically uses 30-50% more paint than brushing or rolling. HVLP sprayers are the most efficient at around 30% extra (1.3× factor), while airless sprayers use about 40% more (1.4× factor) under those conditions. This extra is overspray — tiny paint particles that miss the surface. On complex shapes, exterior or windy work the loss is higher: real-world airless transfer efficiency can be ~50%, meaning up to roughly twice the paint. Treat the presets as best-case and use the Custom Overspray % option for adverse conditions.
Most paints need 5-10% thinning with water (for emulsion) or white spirit (for gloss/oil-based) when spraying. Check the manufacturer's data sheet for exact ratios. Some modern paints like Dulux Trade are 'spray-ready' and need no thinning. Over-thinning causes runs and poor coverage.
HVLP (High Volume Low Pressure) sprayers are best for fine finish work — less overspray but slower. Airless sprayers pump paint at high pressure for fast coverage of large areas (walls, ceilings, exteriors) but produce more overspray. Air-assisted airless combines both benefits. For interior walls, airless is fastest; for trim work, HVLP gives the best finish.
An experienced decorator can spray a typical room (walls and ceiling) in 20-40 minutes per coat, compared to 2-3 hours with a roller. However, masking and preparation for spraying takes 1-2 hours. Spraying is most efficient for large, open areas or multiple rooms. For a single small room, rolling may be quicker overall when you factor in setup time.
For emulsion on walls, use a 517 or 519 tip (~10 inch fan, 0.017-0.019 inch orifice). For trim/gloss, use a 310 or 312 tip (6 inch fan, smaller orifice). For primers and undercoats, a 517 works well. The first number doubled gives the fan width in inches (measured at 12 inches from the surface), so a 5xx tip gives a 10 inch fan; the last two digits are the orifice size in thousandths of an inch. If you want a wider 12-14 inch fan, step up to a 617 or 717 tip.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.