spending in comparable terms was 2003. In terms of the prevailing exchange rate, SIPRI, RAND, the CIA and the DIA estimated the budget to be between US$30–65 billion. In terms of purchasing power parity, or the relative purchasing strength of the expenditure, the SIPRI estimate was as high as US$140 billion. The Chinese government's published budget at that time was less than US$25 billion. A RAND Corporation study for year 2003 estimated China's defense spending to be higher than the official number but lower than United States Department of Defense calculations. The defense spending of China was estimated, in the mid-range estimate, to be 38 billion dollars or 2.3% of China's GDP in 2003. The official figure was 22.4 billion dollars. Nevertheless, Chinese military spending doubled between 1997 and 2003, nearly reaching the level of the United Kingdom and Japan, and it continued to grow over 10% annually during 2003–2005. In 2010, the US Department of Defense's annual report to Congress on China's military strength estimated the actual 2009 Chinese military spending at US$150 billion. Stockholm International Peace Research Institute (SIPRI) estimates that the military spending of the People's Republic of China for 2009 was US$100 billion, higher than the official budget, but lower than the US DoD estimate. The International Institute for Strategic Studies in a 2011 report argued that if spending trends continue China will achieve military equality with the United States in 15–20 years. Jane's Defence Forecasts in 2012 estimated that China's defense budget would increase from $119.80 billion to $238.20 billion between 2011 and 2015. This would make it larger than the defense budgets of all other major Asian nations combined. This is still smaller than the estimated United States defense budget of $525.40 billion for 2013. However, United States defense spending is slightly declining. In 2017, the magazine Popular Mechanics estimated that China's annual military spending is greater than $200 billion, around 2% of the GDP. In 2019, Peter Robertson, a professor from the University of Western Australia, argued that using conventional currency conversion as opposed to more accurate "purchasing power parity" (PPP) exchange rates dramatically understated China's military capabilities and that China's real military spending was equivalent to US spending of $455 billion, calculated from a PPP perspective. In 2022, the Stockholm International Peace Research Institute estimated the China's military spending is $230 billion dollars which accouted for 1.7% of the country total GDP.