The Flash, when imbued with the speed force, can reach speeds faster than light. He’s been shown to be able to punch hard enough to stun Superman.
However, how strong is he? Well to answer that we’ll delve into a bit of physics.
First off we’ll analyze the most basic thing The Flash does: running. Simple physics demonstrates the enormity of the Flash’s strength. Force = Mass x Velocity. Now follow along, the average leg weighs 20 percent of a person’s body weight. According to DC Comics, the Flash weighs 175lbs, meaning each leg weighs approximately 35lbs. Meaning that at the leisurely pace of Mach 4 the Flash would exert more than 24,000 lbs of force with each step on the floor.
20% of 175= 35.
Force= 35lbs x 3072mph (Mach 1=768. Mach 4= 768 x 4).
Force= 107520 newton’s.
107,520 newtons= 24,170 lbs.
What’s been shown above is only the force exerted by the Flash at the speed of sound. However it’s a whole different ballgame when he reaches the speed of a light. According to Einstein’s Special Theory of Relativity, mass reaches infinity when traveling at the speed of light. So the Flash is perfectly capable of throwing punches with infinite force behind them. Not only that, he’s able to control it due to the speed force (the physics bending aura that protects him when traveling at such high speeds) and can choose to punch with the force of planets or entire solar systems.
As shown below:
So in summary, The Flash is accelerating at speeds difficult to quantify. This gives him the ability to exert massive amounts of force, essentially giving him super strength.