Reference angle is the smallest angle that the terminal side of a given angle makes with the x-axis. It is positive and always lies between 0 and 90 degrees (Acute angle). Reference angle can be in any of the four quadrants. When determining the reference angle, vertically drop (raise) the line to the horizontal axis (x-axis). Angles in first quadrant are their own reference angles. The angle is measured by the amount of rotation from the initial side to the terminal side.
Reference Angle

Let $\theta$ be the given reference angle. Given below is the table to find reference angle in different quadrants:

Quadrant I 
Quadrant II  180$^{\circ}$ - $\theta$
Quadrant III  $\theta$ - 180$^{\circ}$
Quadrant IV  360$^{\circ}$ - $\theta$