The Excel ACOS function returns the arccosine (inverse cosine) of a number — the angle whose cosine is that value. The result is in radians, from 0 to π.
Syntax
| Argument | Description | |
|---|---|---|
number | Required | The cosine value, which must be between -1 and 1. ACOS returns the angle in radians, from 0 to π. |
How to use it
ACOS is the inverse of COS: it takes a cosine value and returns the angle. The angle is in radians and always lands between 0 and π (0 to 180 degrees):
The input must be between -1 and 1 inclusive; anything outside that range returns #NUM!. Note =ACOS(1) is 0 and =DEGREES(ACOS(-1)) is 180.
ASIN and ACOS are complementary: for any valid value, ASIN(x) + ACOS(x) equals π/2 radians (90 degrees).
Try it: interactive demo
Pick a ACOS example to see the formula and its result.
Practice workbook
Frequently asked questions
Why does ACOS(0) return 1.5708 and not 90?
=DEGREES(ACOS(0)) to get 90.What input range does ACOS accept?
-1 and 1 inclusive, the full range of cosine. Outside that range it returns #NUM!.What range of angles does ACOS return?
0 and π radians (0 to 180 degrees), unlike ASIN which spans −90 to 90.How are ASIN and ACOS related?
ASIN(x) + ACOS(x) = PI()/2 for any valid x, i.e. the two angles always add to 90 degrees.Master functions like this in one day
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