1 Jackson's texts states 70% of body fluid is intracellular. 2  Extracellular fluid (1/3 of body water). 3  Per Guyton's illustration, for a 40 litre body, about 15 litres is extracellular, which amounts to 37.5% Again, this is close to the 1/3 rule of thumb cited here. 4  Plasma (1/5 of extracellular fluid).