The bitcoin purchasing power index (bitcoinppi) tells you how many Big Mac hamburgers you can buy with one bitcoin. This is helpful when you want to know the everyday value of bitcoin.
So far the value of bitcoin has only been quoted in fiat currencies like the US dollar, European euro or others. This measure is strained by increases in the overall level of consumer prices (aka inflation).
It might happen for instance that the value of bitcoin in US dollars goes up from 300 USD to 305 USD but the products that one bitcoin buys you (purchasing power) remains the same. Inflation is influenced by monetary policy and should not get in the way of measuring the value of a central bank independent currency.
The bitcoinppi measures the value of bitcoin by its purchasing power of a worldwide available and uniform item - the Big Mac hamburger. This makes the bitcoin purchasing power index agnostic to monetary policy. The bitcoinppi hence lets you express bitcoin's value in a central bank independent way. Much similar to how bitcoin operates technologically as a currency and payment network.
In order to buy a Big Mac with one bitcoin you will need to convert it to a local currency like the pound, peso etc. The local bitcoinppi does exactly that. It tells you how many Big Macs you can buy with one bitcoin in a specific country.
This is because bitcoin exchange rates and Big Mac prices differ from one country to another. When you want to buy Big Macs with a bitcoin in the UK, a different exchange rate applies and the Big Mac price is not the same as in Mexico for example.
The calculation of the local bitcoinppi only uses real local Big Mac prices as recorded by the Big Max index and only local exchange rates as they were available if you wanted to buy or sell bitcoins in that specific country. The bitcoinppi does not use cross-rates to derive exchange rates from cross-currency pairs because these rates are in most cases not accessible to consumers.
The global bitcoinppi is a weighted average of local bitcoin purchasing power indices. It tells how many Big Mac burgers you can buy with one bitcoin on average globally. Each available local bitcoinppi is considered in the global bitcoinppi. The weights are calculated based on a country's population and the GDP per capita in purchasing power parity.
Besides just expressing the value in a central bank independent way, there are multiple other use cases for the bitcoinppi. Note that the bitcoinppi API provides all the raw data you need to use this index in every imaginable way. If you use it for something cool, we would love to hear about it in the comments! Here are a few use case ideas
The bitcoinppi volatility measures the average daily return volatility of of the global bitcoinppi over the last 30 days and expresses it as an annualized value. The volatility data is not provided by our API but you can easily calculate it yourself with daily index information. This is the formula we use to calculate the bitcoinppi volatility
STDEV(ln(bitcoinppi_global(d) / bitcoinppi_global(d-1)) * SQRT(365) with d = 1..30
The bitcoinppi is updated every 15 minutes and is calculated as follows
bitcoinppi_local(country_i) = (btc_price(currency_country_i) / big_mac_price(currency_country_i))
bitcoinppi_global = SUM(bitcoinppi_local(country_i) * weight(country_i)) with i = i..I
weight(country_i) = (population(country_i) / SUM(population(country_i..I))) * 1/3 + (GDPperCapPPP(country_i) / SUM(GDPperCapPPP(country_i..I))) * 2/3
There are three main data sources to calculate the bitcoinppi. We need Big Mac prices, bitcoin exchange rates and macroeconomics data for each country that is included in the index.
The Big Mac index as published by The Economist is a great source for local Big Mac prices. The Big Mac index is updated every six months and so are the Big Mac prices that are contained in it.
Note that the bitcoinppi doesn't use the actual Big Mac index itself. Instead we leverage the fact that the Big Mac index tracks a large number of local Big Mac prices in a reliable way on a regular basis. Whenever there are new prices available from the Big Mac Index they will be imported into the bitcoinppi.
The Big Mac is a hamburger that is served in Mc Donald's fast food restaurants in approximately 40 countries around the world.
For some historical bitcoin prices (until October 2015) we used the CoinDesk bitcoin price index BPI. In these cases we had to rely on cross-rates.
Macroeconomic data is sourced from Trading Economics. Historical values for population and GDP per capita are not yet included. We will update the data in the next weeks. Please note that the index will recalculate based on a different set of historical macro data and will therefore likely change.