20 - 40 %
||Stock JB, Rauch B, Roseman S. Periplasmic space in Salmonella typhimurium and Escherichia coli. J Biol Chem. 1977 Nov 10 252(21):7850-61 abstract & p.7860 left column 2nd paragraphPubMed ID334768
||P.7851 left column 3rd paragraph: "The cytoplasmic, periplasmic, and total cell volumes were all derived from the distributions of solutes in cell suspensions, The rationale is illustrated in Fig. 1. Inulin, because it is large, cannot penetrate the outer membrane and is located only in the extracellular fluid. Sucrose, without the aid of a permease, cannot cross the plasma membrane and is located only in the periplasm and extracellular fluid. Water moves freely throughout the cell it is located in the cytoplasm, periplasm, and extracellular fluid."
||Abstract: "The volume of the periplasmic space in Escherichia coli and Salmonella typhimurium cells was measured. This space, in cells grown and collected under conditions routinely used in work with these bacteria, was shown to comprise from 20 to 40% of the total cell volume." P.7860 left column 2nd paragraph: "When the enteric bacteria are grown under a variety of conditions and are suspended in their growth medium or its equivalent, such as Medium 63, the periplasmic space contains from 20 to 40% of the cell water. Although total cell volumes may change as a function of growth conditions, or stage of growth, or other variables, the ratio of periplasmic to total cell water remained relatively constant." For periplasmic fractional volume of 8-16% in E. coli estimated from cryo-electron micrographs See BNID 100010. For periplasmic volume of 30% see PMID 824274 p.334 left column top paragraph