The Flash is the Fastest Man Alive in the DC Universe. Quicksilver is one the most prominent speedsters in the Marvel Universe. Of the two of them, which is the faster? How does Quicksilver's speed compare to that of The Flash's? With some exploration into comic books past and a little bit of math, a decently definitive answer can be found.
It should be noted that the source of The Flash and Quicksilver's super speed come from two very different places. Pietro was born with his powers, due to his unlocked X-gene as a mutant. He's always had his powers as a result of genetics. Comparatively, The Flash got his powers when he was stuck by lightning, granting him access to the Speed Force, an energy field which gives him his power that is responsible for moving the very fabric of time and space forward.
According to official stats found in the comics, and compiled courtesy of books like The Marvel Encyclopedia from DK Publishing, it's believed that Quicksilver was able to run up to 175 miles per hour. However, after taking Isotope E from the being known as the High Evolutionary, Quicksilver's speed dramatically increased to hypersonic speed, in other word speeds greater than the speed of sound, such as Mach 5 or over. His max speed is recorded to have be have been near Mach 10 at one point, though it's believed by some in the Marvel Universe that he hasn't yet reached the limits of how fast he can go.
While hypersonic speeds are nothing to sneeze at, the Flash's speed from the Speed Force absolutely trumps Quicksilver's... by a lot. Amazingly, The Flash is way faster than anything Quicksilver has ever displayed in the comics thus far. Flash has moved so fast before that he can phase through solid objects, and can also create enough friction and momentum where he's able to throw lightning bolts at his foes. However, his most impressive display of speed occurred within the pages of 1997's JLA #89.
At the time, Wally West was the current Flash, though he still had the same access to the Speed Force that Barry Allen did. When a nuclear blast was about to decimate Chonjin, North Korea, The Flash managed to save all 532,000 people just before the blast struck, all within .0001 of a microsecond. To the outside observer, it would have looked as though the entire population just appeared out of nowhere 35 miles away from the blast zone.
0001. microseconds is insanely fast. To put that into perspective with some math, if the the total distance back and forth Flash would travel to bring 1 or 2 people back would be 70 miles, this means he travelled 70 miles roughly 354,667 times which equals 24,826,690 miles (with 1.5 as an average per trip). The issue says that it took him a microsecond (a millionth of a second) to do this, so that means he was traveling 24,826,690/ 0.00000000001 which comes out to 2,482,669,000,000,000,000 miles per second. Dividing that by the speed of light (186,000) brings it to 13,000,000,000,000. The Flash's top speed was 13 trillion times the speed of light.
Unfortunately, at those levels of speed, Quicksilver doesn't hold a candle to the Flash. However, this was back in the 90's, and The Flash has been rebooted more than a couple times since then along with the rest of the DC Universe. Perhaps there's more of an upper limit now to how fast he can go that would make Quicksilver closer to matching the Scarlet Speedster. Perhaps not. It's hard to know until one of them goes faster than previously seen before or can't hit a certain speed, thus revealing their limits. In any case, it's interesting to see the top two speedsters compared from opposing universes all the same.