How to estimate fireproofing thickness from W/D
For the same fire-resistance rating, a heavier steel shape needs less fireproofing than a light one because more steel mass per inch of heated perimeter heats up more slowly. That relationship is captured by the W/D ratio, the weight in pounds per foot divided by the heated perimeter in inches. Enter the shape dimensions and this tool computes area, weight, heated perimeter and W/D, using a 3-sided perimeter for beams with the top flange against a slab and full contour for columns.
Thickness is scaled with the thickness adjustment relationship used in the UL Fire Resistance Directory, h = h_ref x 1.6 / (W/D + 0.6), which is valid for W/D of 0.37 and above. The reference anchors assume a representative SFRM requiring 1/2 in at 1 hour, 1 in at 2 hours and 1-5/8 in at 3 hours when W/D equals 1.0, so inputs below 0.37 are blocked rather than extrapolated.
The governing thickness is always the value listed in the specific UL design number for the product, assembly and restraint condition, tested to ASTM E119. Gypsum board enclosures and intumescent coatings are listed as typical layer counts and dry film ranges because their thickness comes from the listing or the manufacturer loading chart. Treat this output as a budget and coordination figure, then confirm it with the fireproofing submittal and the structural engineer of record.
Frequently asked questions
A beam with its top flange in contact with the slab is protected on three sides, so the heated perimeter is smaller and W/D is larger. A column of the same shape is protected on all four sides, lowering W/D and usually calling for more thickness.
Not arbitrarily. The rating belongs to a tested assembly at a listed thickness, so thinner application voids the listing. To go thinner you need a different product or UL design that is listed at that thickness.