According to a new report from the Center on Nonprofits and Philanthropy at the Urban Institute, the nonprofit sector contributed $887.3 billion to the U.S. economy, or 5.4 percent of gross domestic product, in 2012.

The report, The Nonprofit Sector in Brief 2014: Public Charities, Giving and Volunteering (17 pages, PDF), found that the number of registered nonprofit organizations increased some 8.6 percent between 2002 and 2012, from 1.32 million to 1.44 million, while total revenue for the 35 percent of nonprofits that filed a Form 990 grew 36.2 percent (adjusted for inflation), to $2.16 trillion, and assets increased 21.5 percent, to $4.84 trillion. More than half the registered nonprofits in the U.S. in 2012 were 501(c)(3) public charities, and they accounted for more than three-quarters of the sector's revenue ($1.65 trillion) and expenses ($1.56 trillion) and more than three-fifths of its assets ($2.99 trillion).

The study also found that 50 percent of public charity revenue came from fees from private entities for goods and services, with smaller shares from public sources (23.1 percent), private contributions (12.9 percent), government grants (9.2 percent), and investment income (3.6 percent).

In addition, the report found that although 25.4 percent of adults in the United States volunteered in 2013, collectively contributing an estimated 8.1 billion hours, or $163 billion worth of work, the volunteer rate was the lowest since the annual survey was launched in 2002. The median number of hours per volunteer also fell, from fifty-two hours to fifty.