Fire Resistance Rating Calculator

Achieved Rating is calculated differently by Material Type. For Concrete, it interpolates IBC Table 722.2.1.1's minimum equivalent thicknesses for the selected Concrete Aggregate between the tabulated 1, 1.5, 2, 3 and 4-hour points — 3.5 inches of siliceous concrete for 1 hour, 5.0 inches for 2 hours, and so on — so a 4-inch siliceous wall lands at roughly 1.3 hours, not the 2.0 hours a flat "1 hour per 2 inches" rule of thumb would suggest. For CMU, it uses IBC Table 722.3.2, which is written in EQUIVALENT thickness — the solid thickness the unit's concrete would occupy if recast without core holes — computed here as Thickness times CMU Percent Solid, against a normal-weight (calcareous or siliceous gravel) unit, the most common commercial CMU: a nominal 8-inch hollow unit at about 53% solid has an equivalent thickness near 4 inches and lands around a 1-to-2-hour rating, while the same block solid-grouted at 100% reaches 4 hours. For Gypsum Board, the effective thickness comes from Gypsum Layers instead of the Thickness field — each 5/8-inch Type X layer contributes a full hour PER SIDE in standard GA/UL-rated wood- and steel-stud wall assemblies (UL U305 for one layer each side, UL U301 for two) — so entering a Thickness value while Gypsum Board is selected has no effect, and Gypsum Layers is capped at two because three- and four-hour walls are built from masonry or purpose-designed shaft-wall assemblies rather than by stacking more board. For Protected Steel, the entered thickness is the sprayed fire-resistive material (SFRM) thickness, not the steel section — bare structural steel carries essentially no rating on its own, so treat the per-inch figure here as an order-of-magnitude placeholder and size real fireproofing from the UL listing for your actual member.

For Wood, the per-inch figure is deliberately conservative relative to the AWC NDS Chapter 16 char rate of about 1.5 inches per hour, because a timber member's real rating comes from its residual section's load capacity after charring, not from thickness alone. Switching Material Type moves Achieved Rating far more than adjusting Thickness within a single material does, since the underlying rating mechanisms are completely different — concrete and CMU resist by mass and thickness, gypsum by calcination of its chemically bound water, and timber by charring while the residual section carries load. Required Rating comes from IBC Table 601, which sets minimum fire-resistance ratings for building elements by CONSTRUCTION TYPE (I-A through V-B), not by occupancy group — occupancy classification governs which construction types are allowed for a given area and height under Table 506.2 and what separations are needed under Table 508.4, but it never sets a single required-hours figure for an assembly on its own. Pick the Construction Type your building is classified as and the Building Element you are checking. Treat every figure here as a planning-level estimate: always verify the actual UL-listed assembly or ASTM E119 test data for a specific real-world wall or floor assembly before relying on a rating for code compliance.