The number of people practicing yoga increased 29 percent in four years, according to the latest Yoga in America study, released by Yoga Journal. The study found that 20.4 million Americans practice yoga, compared to 15.8 million from the journal's 2008 study. In addition, practitioners spend $10.3 billion a year on yoga classes and products, including equipment, clothing, vacations and media. The previous estimate from the 2008 study was $5.7 billion.

Data for this survey were collected by Sports Marketing Surveys USA on behalf of Yoga Journal. The 2012 study indicates that 8.7 percent of U.S. adults, or 20.4 million people, practice yoga. Of current non-practitioners, 44.4 percent of Americans call themselves "aspirational yogis"—people who are interested in trying yoga.

"The number of practitioners and the amount they spend has increased dramatically in the last four years," says Bill Harper, vice president and group publisher of Active Interest Media's Healthy Living Group, the media group that owns Yoga Journal. "Companies that want to advertise to the health and fitness market for women will want to make sure that yoga is a strong segment in their target."

The study also collected data on age, gender, and other demographic and lifestyle factors. Of the yoga practitioners surveyed, 82.2 percent are women. The majority of today's yoga practitioners (62.8 percent) fall within the age range of 18-44. Of practitioners, 38.4 percent have practiced yoga for one year or less, 28.9 percent have practiced for one to three years and 32.7 percent have practiced for three years or longer.

Practitioners who consider themselves novices outnumber those who consider themselves experienced. In the study, 44.8 percent classify themselves beginners (22.9 percent are new to yoga while 21.9 percent are beginning to practice yoga after taking some time off), 39.6 percent consider themselves intermediate and 15.6 percent consider themselves expert/advanced.

The top five reasons for starting yoga were flexibility (78.3 percent), general conditioning (62.2 percent), stress relief (59.6 percent), improve overall health (58.5 percent) and physical fitness (55.1 percent).