How many nanoseconds does it take light to travel 2.50 feet in a vacuum

A handy-dandy "rule of thumb" that I and many other engineers

have been using for years is:

1 nanosecond = = = > 1 foot.

Let's figure it out and see whether that's even close:

Speed of light in vacuum = 299,792,458 meters per second.

(299,792,458 m/s) x (1 sec / 10⁹ nanosec) x (3.28084 ft/meter)

= (299,792,458 x 1 x 3.28084 x 10⁻⁹) ft/nanosec

= 0.983571 ft/nanosec.

That's 1.64% less than 1.000000.

The engineer's handy-dandy rule of thumb is valid.

2.5 feet would take

(2.5 x 0.983571 sec) = 2.45893 nanoseconds