The concept of the Quantity Theory of Money (QTM) began in the 16^th century. As gold and silver inflows from the Americas into Europe were being minted into coins, there was a resulting rise in inflation. This led economist Henry Thornton in 1802 to assume that more money equals more inflation and that an increase in money supply does not necessarily mean an increase in economic output. Here we look at the assumptions and calculations underlying the QTM, as well as its relationship to monetarism and ways the theory has been challenged. 

QTM in a Nutshell
The Quantity Theory of Money states that there is a direct relationship between the quantity of money in an economy and the level of prices of goods and services sold. According to QTM, if the amount of money in an economy doubles, price levels also double, causing inflation (the percentage rate at which the level of prices is rising in an economy). The consumer therefore pays twice as much for the same amount of the good or service. 

Another way to understand this theory is to recognize that money is like any other commodity: increases in its supply decrease marginal value (the buying capacity of one unit of currency). So an increase in money supply causes prices to rise (inflation) as they compensate for the decrease in money’s marginal value.  

The Theory’s Calculations
In its simplest form, the theory is expressed as:
 
MV = PT (the Fisher Equation)

Each variable denotes the following:
M = Money Supply
V = Velocity of Circulation (the number of times money changes hands)
P = Average Price Level
T = Volume of Transactions of Goods and Services  

The original theory was considered orthodox among 17^th century classical economists and was overhauled by 20^th-century economists Irving Fisher, who formulated the above equation, and Milton Friedman.

It is built on the principle of "equation of exchange": 

Amount of Money x Velocity of Circulation = Total Spending

Thus if an economy has US$3, and those $3 were spent five times in a month, total spending for the month would be $15.    

QTM Assumptions
QTM adds assumptions to the logic of the equation of exchange. In its most basic form, the theory assumes that V (velocity of circulation) and T (volume of transactions) are constant in the short term. These assumptions, however, have been criticized, particularly the assumption that V is constant. The arguments point out that the velocity of circulation depends on consumer and business spending impulses, which cannot be constant.  

The theory also assumes that the quantity of money, which is determined by outside forces, is the main influence of economic activity in a society. A change in money supply results in changes in price levels and/or a change in supply of goods and services. It is primarily these changes in money stock that cause a change in spending. And the velocity of circulation depends not on the amount of money available or on the current price level but on changes in price levels.  

Finally, the number of transactions (T) is determined by labor, capital, natural resources (i.e. the factors of production), knowledge and organization. The theory assumes an economy in equilibrium and at full employment.  

Essentially, the theory’s assumptions imply that the value of money is determined by the amount of money available in an economy. An increase in money supply results in a decrease in the value of money because an increase in money supply causes a rise in inflation. As inflation rises, the purchasing power, or the value of money, decreases. It therefore will cost more to buy the same quantity of goods or services.