Intumescent Paint Calculator — Fire Protection for Steel & Timber

This calculator estimates the volume of intumescent fire protection paint needed for steel or timber surfaces based on the fire rating required, surface area, and the steel section's Hp/A factor.

Steel calculations

Paint volume = Surface area ÷ Coverage rate

For a thin-film intumescent the coverage rate is fixed by the paint's volume solids and the required dry film thickness (DFT), not by the number of coats: Coverage (m²/litre) = Volume solids % × 10 ÷ DFT (microns). One litre of 100% solids covers 1,000 m² at 1 micron. At 85% volume solids (Nullifire SC902) and 0.72mm DFT this is ~1.18 m²/litre; at 1mm it is 0.85 m²/litre. The coat count is an application-method choice only — it splits the wet film per coat but does not change the total volume.

The required DFT depends on the fire rating and the Hp/A value of the steel section. Higher Hp/A values (thinner sections) heat faster and need a thicker coating for the same fire rating. The Hp/A adjustment here is indicative — the binding DFT must be read from the chosen product's loading table (it differs for 3-sided beams, 4-sided beams and columns).

Typical DFT (Nullifire SC902, Hp/A 150)

• 30 min: ~0.46mm DFT, coverage ~1.8 m²/litre
• 60 min: ~0.72mm DFT, coverage ~1.2 m²/litre
• 90 min: ~1.51mm DFT, coverage ~0.56 m²/litre
• 120 min: ~2.37mm DFT, coverage ~0.36 m²/litre

DFT figures are the SC901 & SC902 loading-table values for a 3-sided I-section beam at Hp/A 150; coverage is derived from the 85% volume solids. Actual DFT spans roughly 0.46–2.4mm at a given rating depending on section factor, orientation (3- vs 4-sided) and critical temperature, so always refer to the specific product's assessment and data sheet, and allow extra for loss in mixing and application.

Timber calculations

Timber intumescent paint is typically applied at 400-500 microns WFT (wet film thickness) per coat. Manufacturer-quoted spreading rates sit around 5–8 m²/litre per coat (for example Envirograf HW Clear at 8 m²/litre per coat for 30/60-minute protection), so this calculator uses a 6 m²/litre default for the finished system — adjust it in Advanced options to match your product. One or two coats are standard depending on the manufacturer's specification and the fire rating required.

UK standards

Intumescent coatings for structural fire protection are assessed to BS 476-21, BS EN 13381-8 (steel) and BS EN 13381-7 (timber). Fire rating requirements are specified in Approved Document B of the Building Regulations.