President Barack Obama and his wife Michelle paid more than $93,000 in taxes on a gross income of $477,383 last year, with an effective tax rate of 19.6 percent, according to tax returns published by the White House on Friday. The Obamas’ gross income is down from $481,098 in 2013 and represents the couple’s lowest earnings since 2004, prior to Obama’s election to the U.S. Senate, Bloomberg reported.

The bulk of the Obamas’ income comes from the president’s $400,000 salary, followed by earnings from sales of Obama’s best-selling books, which generated $88,181 in income. The president reported receiving $51,167 from Random House, the publisher of his memoir "Dreams from My Father," and $43,722 from Dystel & Goderich Literary Management. Last year, the couple reported making $116,000 from book sales. Other adjustments brought their gross income down to $477,383.  

The Obamas donated nearly 15 percent of their income to charity, with the largest sum going to the Fisher House Foundation, which supports the families of service members and veterans. More than $22,012 was donated to the organization, one of 33 charities supported by the couple, including the American Red Cross and Mosaic Youth Theatre of Detroit.

While the president’s income may have declined slightly, his net worth has been estimated at around $7 million, according to financial disclosure forms released last year. This sum might make the Obamas far wealthier than the average American but makes Obama far from the wealthiest U.S. president.

All of the last four presidents have been millionaires, a study by 24/7 Wall Street found, with Bill Clinton among the top 10 richest U.S. presidents ever. Though Clinton may have been the poorest president elected in the 20th century-- something he made a point of in his campaigns -- since his departure from office in 2001, the former president and his wife Hillary Clinton have enjoyed considerable financial success, amassing millions from book deals and speaking engagements. This fortune places Clinton on the top 10 list, with George Washington topping the list with a net worth of $525 million in today's dollars.