There are
1,657 mi
from Thornton, CO to Gaithersburg, MD
That's the driving distance. It would take 1 day 5 hours to go from Thornton, Colorado to Gaithersburg, Maryland.
The flight distance (direct flight from Thornton, CO to Gaithersburg, MD) is 1,475.32 mi.
1,657 mi = 1.61 kms