The Big Mac Index is an informal economic indicator created by The Economist magazine in 1986. It is a lighthearted yet widely recognized method of comparing the purchasing power parity (PPP) between different countries' currencies by using the price of a Big Mac as a benchmark. The index is based on the theory of the law of one price, which states that in an efficient market, identical goods should have the same price in different countries after adjusting for exchange rates.

The idea behind the Big Mac Index is simple: McDonald's Big Mac burger is a standardized product with consistent ingredients and preparation methods across the globe. Therefore, any differences in the burger's price between countries should primarily be due to factors such as exchange rate fluctuations and disparities in local economies and labor costs.

To calculate the Big Mac Index, The Economist collects the prices of a Big Mac in various countries and then converts those prices into a common currency, usually the US dollar. By comparing these converted prices, they can determine if a particular currency is undervalued or overvalued relative to the US dollar. If the actual price of the burger is lower in a foreign country after conversion, that country's currency is considered undervalued. Conversely, if the price is higher, the currency is considered overvalued.

The Big Mac Index is not meant to be a precise economic tool but rather an illustrative and accessible way to understand exchange rate dynamics and relative currency values. It can highlight cases where a currency may be significantly over or undervalued, which can have implications for trade balances and international competitiveness.

Despite its informality, the Big Mac Index has garnered significant attention over the years and is often cited in discussions about global economics and currency markets. It serves as a conversation starter and a way to engage the public in understanding complex economic concepts.

While the Big Mac Index has its limitations and critics, it remains a popular and widely recognized tool for comparing the relative value of currencies in an engaging and straightforward manner. Additionally, it has inspired other similar indices that use different goods or services to compare currency values, further contributing to public awareness and understanding of international economics.