A new scale-up concept for microwave assisted flow processing is presented where modular scale-up is achieved by implementing microwave cavities in series. The scale-up concept is demonstrated for case studies of a packed-bed reactor and a wall-coated tubular reactor. With known kinetics and reaction temperature, a packed-bed reactor gave a conversion of 99% with the highest production rate of 170 kgprod/kgcat·h for esterification of acetic acid and ethanol catalyzed by ion-exchange resin in 18 cavities. A similar approach for a multicomponent reaction of benzaldehyde, piperidine, and phenylacetylene catalyzed by a thin Cu film in a wall-coated tubular reactor gave 99% conversion with the highest production rate of 7740 kgprod/kgcat·h in 28 cavities. In both cases, the pseudo first order reaction rate with respect to the limiting reactant yielded a typical rise in conversion and production rate. In a packed-bed reactor-heat exchanger operated at a temperature between 343 and 348 K, the conversion in the esterification reaction increased from 22% to 88% when the number of cavities was increased from one to eight. The experimental conversions matched the predictions of a packed bed reactor model within 5%. The production capacity in flow reactors, restricted to smaller sizes due to a limited microwave penetration depth and dominated mainly by the reaction kinetics, was increased by modular scale-up with implementation of the microwave multicavity assembly.