Foot per second (fps)

The foot per second is a unit of speed equal to one foot travelled in one second. Its symbol is fps, sometimes written ft/s. It is the imperial and US customary counterpart of the metre per second, and it appears wherever a short-duration speed is measured in a country that works in feet.

Ballistics is its main home. Muzzle velocities are quoted in feet per second almost universally in English-language sources, so a rifle cartridge at 3000 feet per second or a subsonic pistol round below 1100 are standard descriptions among shooters. The threshold matters because the speed of sound at sea level is about 1125 feet per second, and a bullet that crosses it produces a sonic crack that suppressors cannot remove.

American engineering uses it for anything falling, flowing or rotating over short intervals. Acceleration due to gravity is 32.174 feet per second squared, a number as familiar in American textbooks as 9.81 metres per second squared is elsewhere. Fluid velocities in pipes, conveyor speeds and the descent rates on aircraft instruments follow the same convention, though aviation more often uses feet per minute for vertical speed.

Sport measurement in the United States uses it for pitch and swing speeds where a short flight makes miles per hour feel coarse, and film and animation borrow the abbreviation for a different quantity entirely, frames per second, which is a frequency rather than a speed.

Falling bodies gave the unit its place in the classroom. In imperial terms gravity accelerates an object at 32.174 feet per second every second, so a stone dropped from rest is moving at about 32 feet per second after one second and 64 after two, and the height fallen is sixteen feet in the first second. Those round-ish numbers were the staple of American and British physics teaching for a century, and they survive in rules of thumb: a fall from a first-floor window reaches roughly 25 feet per second, and the terminal velocity of a human body in air is about 175. Each of them describes a hazard in the unit the trade already uses.

One foot per second equals 0.3048 metres per second exactly, about 1.09728 kilometres per hour, or about 0.681818 miles per hour.

Foot per second conversion table