I found this formula for rating the difficulty of hikes one day while surfing through Mike´s Hikes and thought it would make a useful calculator.   Of course these ratings are only an indicator of hike difficulty - every hike has its variables beyond elevation gain and distance including weather, experience/fitness, unique trail conditions, starting/ending elevation, etc..  

Rating (Novice) = ( 0.002 x elevation gain [in feet] ) + round trip distance [in miles]
Rating (Expert) = ( 0.0005 x elevation gain [in feet] ) + round trip distance / 2 [in miles]

Note: The equation does not represent the true difficulties for very short and steep hikes.
According to Mikes Hikes, allow 1 hour for each 1,000 feet of elevation gain, and another hour for every two miles of distance.   I would have to agree with Mike when he says "I´ve always found myself hiking faster than this, though."