Firestop Pillow Calculator

Intumescent pillow systems are the removable, re-enterable firestop option — bags of expanding material stacked into an opening rather than sealant poured or troweled in, which matters for cable trays and conduit banks where wires get added or pulled later. Opening Width, Opening Height, and Void Percentage all combine into one figure, Void Area, which is simply the opening's area times the fraction that is NOT already occupied by cable or pipe; all three inputs move that number linearly. Pillow Size (1-3) is where the real nonlinearity lives: it is a lookup into three fixed pillow geometries — Small at 8 sq in of face area, Medium at 18, and Large at 32 — so at the same 173-square-inch default void, switching from Medium to Small pillows more than doubles the pillow count (10 to 22) while switching to Large cuts it to 6, even though the wall itself hasn't changed.

Layers Needed comes from a separate calculation entirely: every pillow is 9 inches deep regardless of size, so the engine divides Wall thickness by 9 and rounds up — an 8-inch wall needs only 1 layer, but a 10-inch wall needs 2, even though 10 is barely more than 9. Weight and cost both key off the same Pillow Size choice (0.5 to 1.8 lb and $6 to $14 per pillow), so a size change ripples through every dollar figure on the page. This calculator does not check whether your chosen pillow brand's actual UL listing supports the void percentage or wall thickness you've entered.