Bone and connective tissue contain 15% of the total body fluid (that's 9% of the mass of the organism) but they are slow to mobilise and wont participate in infusion physiology to a relevant degree. The rest is the functional ECF.
Functional ECF is about 30% of the total body fluid, or about 20% of total body mass (18% to be precise)
This is a small but important compartment; 2.5% of total body fluid is locked up in these secretions.
The compartment is characterized by being formed from the secretions of epithelial cells, and it exist in epithelial-lined cavities.
anaesthesiamcq.org, as always; my source for most of this information has been the Kerry Brandis Anaesthetic Viva book. Everyone should have one.
Schoeller DA, van Santen E, Peterson DW, Dietz W, Jaspan J, Klein PD: Total body water measurement in humans with 18O and 2H labeled water. Am J Clin Nutr 1980, 33(12):2686-2693.