Let me teach u something valuable in a few words ( I am a graduated economist,soon to be master of economy science-35 years old full time trader )
Forex differs from equity markets. Equity and bond markets process trades through central exchange. Centralized trading means all orders move thru a central point thus you can track the orders and calculate the volume size ( equities, bonds, futures, etc...)
On the contrary Forex market there is NO CENTRAL EXCHANGE. Remember that! No order processing. Each bank that trades in the Forex market will know the volume flow through their offices and no-one gathers that data from all the banks or brokerages, so no-one really knows the total balance volume per given time.
Its called FOREX SPOT MARKET, there is no central location. By definition FX market is risk driven,speculative worldwide currency inter-bank or inter-dealer market that uses a floating exchange rate system.
Large banks, hedge funds, government organisations and independent money institutions continually provide exchange rates for each other and for the rest of the markets. The most recent quotation from one of these banks is considered the market's current pricing for that currency whereas you can benefit from ECN no dealing desk quotations.
The only known "volume" in FOREX MARKET is tick volume, or volume used from currency futures.
Now I hope u have learned something