cuni >uom>double click on LB you will find the conversion rate
below url will help you
Units of Measure in Purchase Orders - Purchasing (MM-PUR) - SAP Library
http://wiki.sdn.sap.com/wiki/display/SI/Unit+of+Measure+related+settings+in+SAP
CUNI UoM conversion update to 1 TON = 2204.6226 | SCN
Regards
Mohammad