Technical reference

Intumescent Coverage and Section Factor

Updated 2026-09

Key Facts

Theoretical coverage is fixed arithmetic: one US gallon spread one mil thick covers 1,604 sq ft at 100% volume solids. Divide by the required DFT and multiply by the solids fraction.
Intumescent products run 65–80% volume solids, so at 70% a gallon covers about 37 sq ft at 30 mils but only 3.6 sq ft at 309 mils — a ten-fold difference on the same square foot.
The driver is the section factor: a member with a lot of exposed perimeter behind little steel heats faster and needs more coating. Published DFTs for one 120-minute rating run from about 30 mils to 161 mils on a W10×39 beam and 309 mils on a 10×10×¼ HSS column.
There is no universal thickness-per-rating table. The DFT is read from the manufacturer's UL loading table for that specific product, section and rating — a figure from one product says nothing about another.

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The constant

Theoretical coverage (sq ft per gallon) = 1,604 ÷ DFT(mils) × volume solids

The 1,604 is geometry, not a product property: one US gallon is 231 cubic inches, and spread to a thickness of one mil (0.001 in) it covers 231,000 square inches — 1,604 sq ft. Everything else in the calculation is the product's volume solids and the thickness you need.

At 70% solids — mid-range for intumescent:

Required DFT	Coverage	Gallons per 100 sq ft	Material vs 30 mils
30 mils	37.4 sq ft/gal	2.7 gal	1.0×
45 mils	25.0 sq ft/gal	4.0 gal	1.5×
80 mils	14.0 sq ft/gal	7.1 gal	2.7×
161 mils — W10×39, 120 min	7.0 sq ft/gal	14.3 gal	5.4×
309 mils — 10×10×¼ HSS, 120 min	3.6 sq ft/gal	27.5 gal	10.3×

Theoretical coverage ignores overspray, surface profile and waste, so real consumption is always higher. The DFT figures are published worked examples, not a specification.

Section factor: why the DFT differs so much

All five rows above can describe the same fire rating. What changes is the steel.

The governing property is the section factor — exposed perimeter relative to cross-sectional area, written Hp/A in European practice and as its inverse W/D (weight per linear foot over heated perimeter) in US codes. A heavy section has a lot of steel absorbing heat behind each inch of exposed surface, so it warms slowly and needs less coating. A light section is the opposite.

This is why the W10×39 beam at 161 mils and the 10×10×¼ HSS column at 309 mils carry the same 120-minute rating with roughly double the thickness: the tube has far more perimeter for the steel it contains. On the same square foot, that is double the material.

For SFRM the equivalent adjustment is codified and can be calculated — see the SFRM thickness calculator. For intumescent it is not.

There is no universal DFT-per-rating table

This bears stating plainly, because the tables that circulate online imply otherwise. Required dry film thickness is product-specific, section-specific and rating-specific, and it is read from the manufacturer's UL loading table for that combination. The numbers on this page are published worked examples used to show the spread — they are not a specification, and a figure from one product cannot be carried across to another.

What this explains about published prices

Cost guides quote intumescent fireproofing at $2–$4, $4–$12, $10–$30 and $15–$40 per square foot, usually without reconciling the spread. The arithmetic above is most of the reconciliation: if material consumption alone varies ten-fold across sections at one rating, a single per-square-foot figure cannot describe the work. Add finish requirements on exposed steel and access, and the range is not sloppiness — it is the honest shape of the answer.

The intumescent fireproofing cost guide breaks the bands down by rating and scope, and what intumescent coating is covers how char expansion works, thin versus thick film, and the hydrocarbon premium.

Sources: coverage constant from gallon geometry (231 in³); volume solids range and the published DFT worked examples as cited on our intumescent cost guide. Checked September 2026.