The US federal debt has increased by about $7 trillion under the Obama administration, as of mid-2015. It stood at roughly 11 trillion at the beginning of his first term and, as of mid-2015, stands at roughly 18 trillion.
The increase is both the largest — in nominal dollars, not adjusted for inflation — of any single administration and larger than the sum total of all preceding administrations combined. The second-highest increase was under his immediate predecessor, George W. Bush, with just under $6 trillion. By the end of Obama's second term in 2016, the debt is expected to stand at about $20 trillion.