The Big Mac Index is published by The Economist as an informal way of measuring the purchasing power parity (PPP) between two currencies and provides a test of the extent to which market exchange rates result in goods costing the same in different countries. It “seeks to make exchange-rate theory a bit more digestible”. The index takes its name from the Big Mac, a hamburger sold at McDonald’s restaurants.

The Big Mac PPP exchange rate between two countries is obtained by dividing the price of a Big Mac in one country (in its currency) by the price of a Big Mac in another country (in its currency). This value is then compared with the actual exchange rate; if it is lower, then the first currency is under-valued (according to PPP theory) compared with the second, and conversely, if it is higher, then the first currency is over-valued.

For example, using figures in July 2008:
1.the price of a Big Mac was $3.57 in the United States (Varies by store)
2.the price of a Big Mac was £2.29 in the United Kingdom (Britain) (Varies by region)
3.the implied purchasing power parity was $1.56 to £1, that is $3.57/£2.29 = 1.56
4.this compares with an actual exchange rate of $2.00 to £1 at the time
5.[(2.00-1.56)/1.56]*100= +28%
6.the pound was thus overvalued against the dollar by 28%

Limitations:

The burger methodology has limitations in its estimates of the PPP. In many countries, eating at international fast-food chain restaurants such as McDonald’s is relatively expensive in comparison to eating at a local restaurant, and the demand for Big Macs is not as large in countries like India as in the United States. Social status of eating at fast food restaurants like McDonald’s in a local market, what proportion of sales might be to expatriates, local taxes, levels of competition, and import duties on selected items may not be representative of the country’s economy as a whole.

In addition, there is no theoretical reason why non-tradable goods and services such as property costs should be equal in different countries: this is the theoretical reason for PPPs being different from market exchange rates over time. The relative cost of high-margin products, such as essential pharmaceutical products, or cellular telephony might compare local capacity and willingness to pay, as much as relative currency values.

Nevertheless, economists widely cite the Big Mac index as a reasonable real-world measurement of purchasing power parity

Source: http://en.wikipedia.org/wiki/Big_Mac_Index