First, remember that in the forex markets investors trade one currency for another. Therefore, currencies are quoted in terms of their price in another currency.
In order to express this information easily, currencies are always quoted in pairs (e.g. USD/CAD). The first currency is called the base currency and the second currency is called the counter or quote currency (base/quote). For example, if it took C$1.20 to buy US$1, the expression USD/CAD would equal 1.2/1 or 1.2. The USD would be the base currency and the CAD would be the quote or counter currency.
Now that we know how currencies are quoted in the marketplace, let's look at how we can calculate their spread. Forex quotes are always provided with bid and ask prices,
similar to what you see in the equity markets. The bid represents the price at which the forex market maker is willing to buy the base currency (USD in our example) in exchange for the counter currency (CAD). Conversely, the ask price is the price at which the forex market maker is willing to sell the base currency in exchange for the counter currency. Forex prices are always quoted using five numbers; so, for this example, let's say we had a USD/CAD bid price of 120.00 and an ask of 120.05. Thus, the spread would be equal to 0.05, or $0.0005.
To learn about the basics of the forex market, check out A Primer On The Forex Market and Getting Started In Forex .