Supply/Demand is a concept adopted into forex, inspired by futures/stocks, which is centralized. It is closely related to order books/order flow which takes a different path.
Anyways, Forex is not centralized (thus the term Over-The-Counter)
Most people mistake the 'Volume' option on their platform (in forex) to depict the actual volume.. truth is that it is only the broker (or their liquidity network's volume in some cases).
Supply/Demand has lot of different meanings to different people... one commonly used method is to find out key areas (or zones) of price interaction.. By price interaction, it could be reversals, consolidation, re-visiting the same zone (these phenomenon's are often used by pattern traders. Ex: Double tap, head and shoulders, triangles, doji etc).
Trading off SD requires time, effort and experience. Its one of the best ways to get started with pure price action trading (there are some indicators to help guide you, but don't rely on them too much, use the indi's as a way to help you find the SD zones yourself).
If you search for Supply demand trading or supply demand strategy forex you'll find a ton of information to help you get started.
To answer your question: How to calculate supply and demand, you first should know what it means, how to identify it on the charts. SD trading has its own lingo.. Drop-base-Drop, Rally-base-Drop and so on.
@Buitre.. the eHow article inclines towards Volume analysis.
Supply/Demand is more to do with buyers and sellers. For example, volume can be high but prices might drop.. In other words, more people will step in to sell the currency. So if the previous day shows high buying volume (green bar) then next day price can drop just as much and the volume bar will still be green (if not greener).