Standard notation has "f" (note lower case) representing "focal length". That is why f/stops have the `front' slash.

The entire formula is: f/(stop number) = equals diameter of aperture opening.

Bell and Howell (Remember them? - They offered a "lifetime guarantee" on their cameras - the "lifetime" was the period of the company's survival) once had a campaign to substitute "T" numbers for "f/stops": the "T" stop (for transmission of light) was based on actual measurements. Unfortunately, the numbers seemed to indicate "slower" lenses in a time when fast lenses were considered superior -