In United States, the units of miles per hour are used to express the speed of an object. Converting time in seconds to miles per hour is only possible in the context of a speed calculation -- in particular, when the distance associated with the time is given. A typical example is to calculate an average speed of athletes who run a distance -- 100 yards, for example -- for the time measured in seconds.

Divide seconds by 3,600 to convert the time to hours. For example, a time of 45 seconds corresponds to 45 / 3,600 = 0.0125 hours.

Convert the distance to miles. For example, if a runner covers 100 yards, multiply it by 0.000568 to calculate miles; 100 x 0.000568 = 0.0568 miles.

Divide the distance in miles by the time in hours to calculate the speed. In this example, 0.0568 miles / 0.0125 hours = 4.544 miles per hour.