A standard football field is 100 yards long. Here's how to convert that to kilometers:

* 1 yard = 0.9144 meters

* 1 kilometer = 1000 meters

Calculation:

1. Yards to meters: 100 yards * 0.9144 meters/yard = 91.44 meters

2. Meters to kilometers: 91.44 meters / 1000 meters/kilometer = 0.09144 kilometers

Therefore, a football field is 0.09144 kilometers long.