China reported its Gross Domestic Product to be 56.88 trillion Yuan in 2013. I am going to assume this is "Nominal GDP", not adjusted for inflation, but I do not know that for certain. I got this from a Chinese newspaper Zinhuanet HERE. On January 1st of 2014 the official exchange rate was 1 Renminbi (Yuan and Renminbi are used interchangeably, sort of) exchanged for $.16529 US cents.
So, putting the GDP in Yuan in dollar terms at the market exchange rate we would take 56.88 Trillion Yuan multiplied by $.16529 and that would equal $9.4 Trillion US dollars.
At the end of 2013 the US Real GDP was $16.768 Trillion dollars (Nominal dollars)
Either way you figure it US GDP is about $7 Trillion more than China's using current (Jan 1, 2014) market exchange rates.
What about many/most economists preferred measure of exchange rates: the Purchasing Power Parity (PPP)?
According to theWorld Bank, the PPP exchange rate (2011 is the latest calculation) is 1 Renminbi exchanges for $.28 US cents.
If we take 56.88 Trillion Yuan and multiply by $.28 US cents, the PPPexchange rate, we get $15.926 Trillion US dollars, about $800 million shy of the US GDP at the end of 2013.
The big picture here suggests the Yuan is UNDERVALUED relative to the US dollar. Instead of the actual market exchange rate where $1.00 US dollar "buys" 6.04 Yuan (or 1 Yuan buys $.16529 US cents) it should buy only 3.57 Yuan ( or 1 Yuan buys $.28 US Cents) based on PPP.
In other words, the dollar should be weaker (depreciate) and the Yuan should be stronger (Appreciate).
But it is not, hence the difference in nominal GDP's based on actual exchange rates as compared to PPP.
I hope that makes some sense. Quite the difficult concept to walk through!