diff --git a/ClosedXML/Excel/CalcEngine/Functions/MathTrig.cs b/ClosedXML/Excel/CalcEngine/Functions/MathTrig.cs
index cc56c25..aa1ff5b 100644
--- a/ClosedXML/Excel/CalcEngine/Functions/MathTrig.cs
+++ b/ClosedXML/Excel/CalcEngine/Functions/MathTrig.cs
@@ -17,6 +17,7 @@
             ce.RegisterFunction("ABS", 1, Abs);
             ce.RegisterFunction("ACOS", 1, Acos);
             ce.RegisterFunction("ACOSH", 1, Acosh);
+            ce.RegisterFunction("ACOT", 1, Acot);
             ce.RegisterFunction("ASIN", 1, Asin);
             ce.RegisterFunction("ASINH", 1, Asinh);
             ce.RegisterFunction("ATAN", 1, Atan);
@@ -483,6 +484,18 @@
             return XLMath.ACosh(p[0]);
         }
 
+        private static object Acot(List<Expression> p)
+        {
+            double x = Math.Atan(1.0 / p[0]);
+
+            // Acot in Excel calculates the modulus of the function above.
+            // as the % operator is not the modulus, but the remainder, we have to calculate the modulus by hand:
+            while (x < 0)
+                x = x + Math.PI;
+
+            return x;
+        }
+
         private static object Asinh(List<Expression> p)
         {
             return XLMath.ASinh(p[0]);
diff --git a/ClosedXML_Tests/Excel/CalcEngine/MathTrigTests.cs b/ClosedXML_Tests/Excel/CalcEngine/MathTrigTests.cs
index dcb6719..ec28464 100644
--- a/ClosedXML_Tests/Excel/CalcEngine/MathTrigTests.cs
+++ b/ClosedXML_Tests/Excel/CalcEngine/MathTrigTests.cs
@@ -12,6 +12,33 @@
     {
         private readonly double tolerance = 1e-10;
 
+        [TestCase(-10, 3.041924001)]
+        [TestCase(-9, 3.030935432)]
+        [TestCase(-8, 3.017237659)]
+        [TestCase(-7, 2.999695599)]
+        [TestCase(-6, 2.976443976)]
+        [TestCase(-5, 2.944197094)]
+        [TestCase(-4, 2.89661399)]
+        [TestCase(-3, 2.819842099)]
+        [TestCase(-2, 2.677945045)]
+        [TestCase(-1, 2.35619449)]
+        [TestCase(0, 1.570796327)]
+        [TestCase(1, 0.785398163)]
+        [TestCase(2, 0.463647609)]
+        [TestCase(3, 0.321750554)]
+        [TestCase(4, 0.244978663)]
+        [TestCase(5, 0.19739556)]
+        [TestCase(6, 0.165148677)]
+        [TestCase(7, 0.141897055)]
+        [TestCase(8, 0.124354995)]
+        [TestCase(9, 0.110657221)]
+        [TestCase(10, 0.099668652)]
+        public void Acot_ReturnsCorrectValue(double input, double expectedResult)
+        {
+            var actual = (double)XLWorkbook.EvaluateExpr(string.Format(@"ACOT({0})", input.ToString(CultureInfo.InvariantCulture)));
+            Assert.AreEqual(expectedResult, actual, tolerance * 10);
+        }
+
         [TestCase(4, 3, 20)]
         [TestCase(10, 3, 220)]
         [TestCase(0, 0, 1)]