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Pressure-loss characteristics of a variety of single- and double-substrate metal-foil and ceramic-substrate converters with tapered and truncated inlet and outlet headers were measured in room-air flow, hot-gas flow, and engine-exhaust tests. Test data in the three different media correlated with the inlet-pipe Reynolds number when expressed as a loss coefficient, i.e., pressure loss normalized by the inlet-pipe dynamic head. Because restriction measurements made in different media correlate well as a Reynolds number-dependent loss coefficient, inexpensive room-air test data can be used to estimate converter pressure losses in the engine environment. The normalized losses in the substrate varied inversely with inlet-pipe Reynolds number, ranging from, e.g., 6 at Re = 30 000 to 2 at Re = 200 000. The remainder of the losses occurred in the inlet and outlet headers and in the section between the substrates.