Big Mac index

 From the Economist:  

..a Big Mac costs $5.36 in America, by today’s exchange rate it should cost £4.34 in Britain. In fact, the meal costs £3.79, or $4.68. That suggests that the pound is undervalued against the dollar by about 13%. ... Our model suggests that the pound ought to rise by about 3% by 2028...

Of the three other currencies we analysed, China’s and Japan’s should also rise over the next five years, by an estimated 7% and 9%, respectively. The euro, the only currency of the four that was overvalued, is forecast to decline by 1.4%