How much cord blood is needed for a transplant?

The crucial thing is not the volume of the cord blood collection, but the number of stem cells it contains. Transplant doctors develop recommendations based on the Total Nucleated Cell count, or TNC, because it is the easiest measure to reproduce between different labs.    For treating cancer, the transplant dose should be at least 25 million TNC per kilogram of patient body weight (1 kilogram equals 2.2 pounds).  The average cord blood collection holds 8.6 million TNC per mL.  Thus, the optimal transplant dose requires harvesting:
1.3 mL of cord blood for every pound of patient weight, -or-
2.9 mL of cord blood for every kg of patient weight

However, as more transplant centers are adopting the practice of giving adult  patients "double cord blood transplants" with two cord blood units, it is less critical for both units to have adequate cell dose.

References:
Reed, W et al., Blood 2003;101(1):351
Barker, JN et al., Blood 2005;105:1343-1347
Eapen, M et al. Lancet 2007;369:1947-54
Rocha & Gluckman Brit. J. Haematology 2008;147:262-274
Delaney, C et al., Brit. J. Haematology 2009;147:207-216
Michallet et al. 2010 Blood 2010;116:Abstract#361