Motor speed is proportional to......?

[infamous_Q]

im a little confused here. im trying to find the relation between speed of a dc motor and it's voltage, and ne thing else that matters. in some old encyclopedia i found that n (rpm) = E / K*(magnetic flux per pole)

but once again, K is a conversion constant which i dont know. are there any other relations out there that someone could point me in the direction of?

thanx very much!