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, created in 1986, takes its name from the Big Mac, a hamburger sold at McDonald's restaurants.

The Big Mac index was introduced in The Economist in September 1986 by Pam Woodall as a semi-humorous illustration of PPP and has been published by that paper annually since then. The index also gave rise to the word burgernomics.

One suggested method of predicting exchange rate movements is that the rate between two currencies should naturally adjust so that a sample basket of goods and services should cost the same in both currencies. In the Big Mac Index, the basket in question is a single Big Mac burger as sold by the McDonald's fast food restaurant chain. The Big Mac was chosen because it is available to a common specification in many countries around the world as local McDonald's franchisees at least in theory have significant responsibility for negotiating input prices. For these reasons, the index enables a comparison between many countries' currencies.

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.

https://en.wikipedia.org/wiki/Big_Mac_Index