The Excel ATAN function returns the inverse tangent or arc-tangent of a number. In geometric terms, the function returns the angle of a right-triangle given the ratio of its opposite side over its adjacent side. The ATAN function is the inverse of the TAN function. For example, if the length of a right-triangle's adjacent side is 3 and the length of its opposite side is 3 to find the angle of the triangle the formula is:
ATAN returns the angle in radians. To convert the result from radians to degrees, multiply the result by 180/PI() or use the DEGREES function. For example, to convert the result of ATAN(1) to degrees, you can use either formula below:
The ATAN2 function is useful for getting the angle corresponding to a point in the Cartesian Coordinate System which forms the shape of a right-triangle. For points in the first and fourth quadrants of the coordinate system, ATAN and ATAN2 will return the same angle as expressed in the formula:
For points in the second and third quadrants of the coordinate system, the ATAN function will return the angle relative to the negative x-axis direction. The ATAN2 function, by comparison, returns the angle relative to the positive x-axis which is the standard for measuring angles.
The graph of ATAN, shown above, visualizes the output of the function. Output of the function is limited to the range from -π/2 to π/2.