DC Comics states on its website that the current Flash, Barry Allen, can generate a running pace close to light speed. According to NASA, light speed is approximately 186,000 miles per second, which means that the Flash could run the circumference of the equator around seven times per second.
In the comics, Allen's speed is so disruptive that he's generated sonic booms with a mere snap of his fingers and phased through solid matter by bypassing object particles. He's also able to outpace gravity by running up the sides of building surfaces. As the Flash ages, however, his maximum speed will decrease: characters classified as "speedsters" are only as fast as their bodies permit.