The Southwest Airlines Of The Gym Business

FORTUNE -- Sometimes the fastest path to growth is to keep it simple.

Case in point: Planet Fitness, the fastest-growing "full-size" health club chain in the U.S., which opened its 700th location last week. For just $10 per month, its members get a no-frills experience that CEO Chris Rondeau says makes his company the Southwest Airlines of the gym industry.

That approach means the business is easy to franchise—all you need is a good location and basic equipment. The Newington, N.H.-based operation, in which investment firm TSG Consumer Partners bought a majority stake last year, has nearly tripled its store count since 2008—including 150 new gyms this year. The company projects that it will hit 1,000 total locations in 2015, with an eventual goal of 2,000-plus gyms across the country. The company currently has some 4.5 million members. System-wide sales reached $679.5 million in 2012.