What is the syntax for ACOS in Excel?

The syntax for ACOS is: =ACOS(number)

What are common errors with ACOS?

#NUM! if number outside -1 to 1

What is ACOS (arccosine)?

ACOS is the inverse of COS - it returns the angle whose cosine is the given number. If COS(60°)=0.5, then ACOS(0.5)=60° (in radians: π/3 ≈ 1.047). Input must be between -1 and 1.

How do I convert ACOS result to degrees?

ACOS returns radians. Convert to degrees: =DEGREES(ACOS(0.5)) returns 60. Or multiply by 180/PI(): =ACOS(0.5)*180/PI(). Always remember Excel trig functions use radians by default.

How do I calculate the angle between two vectors?

Use the dot product formula: angle = ACOS((A·B)/(|A|×|B|)). In Excel: =ACOS(SUMPRODUCT(A,B)/(SQRT(SUMSQ(A))*SQRT(SUMSQ(B)))). Convert to degrees with DEGREES() for a readable result.

Math & Trig

ACOS Function in Excel

Returns the arccosine of a number in radians.

Syntax

=ACOS(number)

Arguments

number (required): Cosine value (-1 to 1)

Examples

=ACOS(0.5) - Arccosine of 0.5 - Result: 1.047 (π/3)

ACOS in reporting workflows

Use absolute references when copying formulas across rows.
For conditional totals see [SUMIF](/functions/sumif/) and [SUMIFS](/functions/sumifs/).
Filtered lists may need [SUBTOTAL](/functions/subtotal/) instead of SUM on visible cells only.

Common errors

#NUM! if number outside -1 to 1

Use cases

Trigonometry
Angle calculations
Engineering

Frequently asked questions

What is ACOS (arccosine)? ACOS is the inverse of COS - it returns the angle whose cosine is the given number. If COS(60°)=0.5, then ACOS(0.5)=60° (in radians: π/3 ≈ 1.047). Input must be between -1 and 1.
How do I convert ACOS result to degrees? ACOS returns radians. Convert to degrees: =DEGREES(ACOS(0.5)) returns 60. Or multiply by 180/PI(): =ACOS(0.5)*180/PI(). Always remember Excel trig functions use radians by default.
How do I calculate the angle between two vectors? Use the dot product formula: angle = ACOS((A·B)/(|A|×|B|)). In Excel: =ACOS(SUMPRODUCT(A,B)/(SQRT(SUMSQ(A))*SQRT(SUMSQ(B)))). Convert to degrees with DEGREES() for a readable result.

Editorial review

Reviewed by Excel.Directory Editorial Team. Updated May 2026.