Double the money supply, and prices roughly double too, eventually

The quantity theory of money holds that in the long run, growth in the money supply mostly shows up as higher prices, not more real output. Irving Fisher formalized this as an equation of exchange, MV = PQ, linking money, its velocity, prices, and output. Milton Friedman later revived the idea, calling inflation 'always and everywhere a monetary phenomenon.'

— Irving Fisher, The Purchasing Power of Money — 1911