What is Hp/A section factor?

Hp/A is the ratio of the heated perimeter (Hp) to the cross-sectional area (A) of a steel member. It determines how quickly the steel heats up in a fire. Low Hp/A (heavy sections like UC 254×254) heat slowly and need less intumescent. High Hp/A (light sections like CHS 114.3×3.6) heat quickly and need more. The Hp/A value is found in the SCI Yellow Book or calculated from steel section tables.

How thick should intumescent paint be?

The dry film thickness (DFT) depends on the required fire rating and the section factor. Typical values: 30 min = 250-800 microns, 60 min = 500-1500 microns, 90 min = 1000-2500 microns, 120 min = 1500-3500 microns. The exact DFT is specified in the intumescent paint manufacturer's assessment report (e.g., Nullifire, Sherwin-Williams Firetex) matched to the Hp/A value.

What is the complete intumescent system?

A complete system has three layers: (1) Anti-corrosion primer — typically 75 microns of zinc phosphate or epoxy primer. (2) Intumescent paint — applied in multiple coats to achieve the target DFT. (3) Protective topcoat — 25-50 microns to protect the intumescent from moisture and UV damage. All three components must be from the same manufacturer's tested system.

Can intumescent paint be applied on site?

Yes, but off-site application in a controlled environment is preferred for quality. On-site application requires temperatures above 5°C, low humidity, and no rain exposure until cured. Each coat must be measured with a wet film thickness gauge. Multiple coats are needed for high DFTs — typically 500 microns per coat maximum. Drying time between coats is 4-24 hours depending on the product.

How long does intumescent paint last?

With a proper topcoat, intumescent paint lasts 25+ years in interior environments. External or high-humidity locations reduce the lifespan to 10-15 years. Regular inspection is required — check for cracking, peeling, or damage. Damaged areas must be repaired promptly as the fire protection is compromised. BS 8414 and BS 476 provide testing standards for fire performance.

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Fireproof Paint Calculator — Intumescent Coating by Fire Rating

Calculate intumescent paint quantities by fire rating (30/60/90/120 min) and section factor (Hp/A). Includes primer and topcoat for complete system.

Steel Surface Area

Total steel surface to protect

Fire Rating

As specified by fire engineer

Section Factor (Hp/A)

From SCI Yellow Book

Price Per Litre Intumescent (optional)

How We Calculate This

This calculator determines intumescent paint quantities based on the required fire rating and steel section factor. Higher fire ratings and lighter steel sections need thicker coatings.

The formula

Target DFT = Lookup(Fire rating, Hp/A range)

WFT = DFT ÷ Volume solids %

Intumescent (L) = Steel area × WFT (mm) ÷ 1000 × Wastage

Primer (L) = Steel area × 0.08 L/m²

Topcoat (L) = Steel area × 0.10 L/m²

For example, 100 m² steel at 60 min fire rating, medium Hp/A: DFT = 1000 microns. WFT = 1000 ÷ 0.65 = 1538 microns. Intumescent = 100 × 1.538 ÷ 1000 = 153.8 L + 15% waste = ~177 L.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.