SFRM Thickness Calculator: IBC Beam Substitution
Updated 2026-09
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The equation
h₂ = [ (W₁/D₁ + 0.60) ÷ (W₂/D₂ + 0.60) ] × h₁
— International Building Code §722.5.2.2.1, Equation 7-17
| Symbol | Meaning | Units |
|---|---|---|
| h₁ | Thickness of sprayed fire-resistive material in the tested assembly | inches |
| h₂ | Thickness required on your beam | inches |
| W₁ | Weight of the beam in the tested assembly | lb per linear foot |
| D₁ | Heated perimeter of the beam in the tested assembly | inches |
| W₂ | Weight of your beam | lb per linear foot |
| D₂ | Heated perimeter of your beam | inches |
Subscript 1 is always the assembly that was actually tested; subscript 2 is the member you are building. The heated perimeter is the length of the steel profile exposed to fire — for a beam under a slab, three sides rather than four.
Why the limits matter more than the number
Most calculators will return a figure for any input. This one does not, because the code provision is bounded and a number produced outside those bounds is not a code-compliant thickness:
- W₂/D₂ ≥ 0.37. Below that the relationship is not written to hold. A very light member needs a tested assembly of its own, not an extrapolation.
- h₂ ≥ 3/8 in. If the arithmetic returns less, the floor applies — not the calculated value.
- Rating ≥ 1 hour. The substitution procedure does not cover shorter periods.
- Restrained beams: compact sections only, classified under AISC 360.
And above all: the UL design for your specific product governs. The equation adjusts a tested result to a different shape; it does not create a tested result.
What it means for material and cost
W/D is a measure of how much steel stands behind each inch of exposed surface. A light beam heats faster, so it needs more material for the same rating — on the same square foot.
| Your beam's W/D | Required thickness | Board feet per 1,000 sq ft | vs tested beam |
|---|---|---|---|
| 0.50 | 1.31 in | 1,309 | 1.31× |
| 0.84 (tested) | 1.00 in | 1,000 | 1.00× |
| 1.20 | 0.80 in | 800 | 0.80× |
| 2.00 | 0.55 in | 554 | 0.55× |
Arithmetic from the equation with an illustrative tested beam at W/D 0.84 and 1 in of SFRM. One board foot is one square foot at one inch.
Read the first and third rows together: the lighter beam needs about 1.6× the material of the heavier one for the same rating. That is why two quotes for “1,000 square feet at two hours” can differ sharply — they may be pricing different steel. What that does to the price is covered in our fireproofing cost guide.
Why intumescent is not calculated this way
Intumescent coatings protect by expanding into a char, not by insulating mass, and the relationship between section, rating and thickness is specific to each product's chemistry. There is no public code equation for it: the dry film thickness is read from the manufacturer's UL loading table for that product, that section and that rating.
The practical consequence is that a thickness figure for one intumescent product tells you nothing about another, and nothing at all about SFRM. Published DFTs for a single 120-minute rating span roughly 30 to 309 mils depending on the section — see intumescent coverage and section factor for what that does to material consumption, and the intumescent fireproofing cost guide for pricing.
Sources: International Building Code §722.5.2.2.1 and Equation 7-17, including the stated limits on W/D, minimum thickness, rating and restrained-beam applicability. Checked September 2026.