Fire / W-D Calculator

Calculates the W/D ratio (weight per unit length, divided by heated perimeter) for a symmetric I/H section - the figure your fire protection supplier needs to select a coating or board thickness from their own UL-listed product data.

What this figure is used for:W/D is the section factor US fire protection manufacturers' UL Fire Resistance Directory listings are indexed against. It is not itself derived from a single AISC 360 clause.
Scope of this tool - please read before relying on this result:
  • This tool calculates W/D only - it does not calculate a required fire protection thickness. That comes from the specific product's own UL-tested and certified data, not a formula.
  • "Contour" method assumes protection follows the actual steel profile (sprays, intumescent paint). "Box" method assumes a simple rectangular envelope (board protection) - this always gives a higher (less conservative) W/D than the true contour value.
  • Assumes a symmetric I/H section with sharp (unfilleted) corners.
  • "3-sided" exposure assumes the top flange is in full, continuous contact with a concrete slab along its whole width. If there's any gap, use 4-sided exposure instead.
Exposure condition
Protection method
This calculator provides preliminary, indicative structural calculations only, intended to assist with initial sizing and understanding. It is not a substitute for assessment by a qualified structural engineer or fire engineer, and cad-steel.com accepts no responsibility for its use in any final design.