A Reference angle is the smallest angle obtained when measured from the terminal side. If the angle is acute the reference angle is the angle itself.
Reference Angle
Reference Angle Calculator is a amazing online which calculates the reference angle for any given angle if any angle is entered in the block provided. So there is no need to get messed up with these angles just enter the angle and get you reference angle instantly.
Here is given the way to find the reference angle
Step 1 : Read the problem and given angle $\theta$, analyze in quadrant the angle $\theta$ lies.

Step 2 : Based on which quadrant the angle $\theta$ lies, use the following operations
In I Quadrant $\theta$ = $\theta$
   II Quadrant $\theta$ = 180o - $\theta$
   III Quadrant $\theta$ = $\theta$ - 180o
   IV Quadrant $\theta$ = 360o - $\theta$
Put the value of $\theta$ in operations based on which quadrant it lies and get the answer.