The difference between the buying exchange rate and selling exchange rate of foreign exchange is called "bid-ask spread", which is generally 1-5%. The calculation method is: (selling price-buying price)/selling price × 100%. It constitutes the source of profits for banks to operate foreign exchange trading business. The smaller the bid-ask spread, the more competitive the operation of foreign exchange banks, or the more developed the foreign exchange market. For example, USD and GBP trade frequently and in large volume, which can form economies of scale. Therefore, the bid-ask difference between the two currencies in new york and London foreign exchange markets is only 0.5‰.

Foreign exchange transactions are usually quoted in dollars. If we know the exchange rates of all currencies against the US dollar, then the exchange rates between any two non-US dollar currencies can be easily calculated. This is the so-called "cross exchange rate" or "arbitrage exchange rate". When calculating the cross exchange rate, it is very important to use the buying price and selling price correctly.

In direct quotation, how to correctly use the buying price and selling price to calculate the two prices of cross exchange rate.

If one of the two sets of currency quotations adopts indirect pricing method, the buying price and selling price of the cross exchange rate of two non-US dollar currencies can be obtained by direct multiplication, for example:

(purchase price) ￥/= (purchase price) $ /× (purchase price) ￥/$

(selling price) ￥/= (selling price) $ /× (selling price) ￥/$

In addition to the buying and selling exchange rates, it is the arithmetic average of the buying and selling exchange rates, that is, the intermediate exchange rate = (buying exchange rate+selling exchange rate) /2. It is the exchange rate used to make the report concise and easy to express. In addition, the middle price is often used to simplify the accounting in the internal accounting process of enterprises. But the intermediate exchange rate is by no means the actual transaction price in foreign exchange trading business.