Traditionally exhaust catalysts have been dimensioned and adjusted to suit vehicles on the basis of the characteristics and parameters of the ceramic catalyst. The metal catalyst differs, however, so much from the ceramic one that the dimensioning principles of the ceramic catalyst are not altogether appropriate for it. The metal substrate gives the catalyst designer a great deal of latitude in dimensioning, so that the optimum function-durability-price relationship can be achieved. In the metal catalyst the cell density can easily be changed. The geometric surface area, pressure drop and conversion efficiency will all change at the same time. This paper examines the effect of cell densities and catalyst volumes on the functioning of the catalyst. The catalyst performance depends strongly on the temperature. The effect of thermal insulation on a metal catalyst is also examined.