1diff --git a/DOCS.md b/DOCS.md
  2index 1e04354..e0ac0a2 100644
  3--- a/DOCS.md
  4+++ b/DOCS.md
  5@@ -1225,6 +1225,15 @@
  6 @returns {number} 
  7 @constructor
  8 ```
  9+
 10+### ACTDOUBLE 
 11+
 12+```
 13+  Calculates the double-factorial of a number. 
 14+@param value - value or reference to calculate. 
 15+@returns {number} 
 16+@constructor
 17+```
 18 ## Statistical
 19 
 20 
 21diff --git a/TODO.md b/TODO.md
 22index e8ea9a8..b2bbc48 100644
 23--- a/TODO.md
 24+++ b/TODO.md
 25@@ -51,7 +51,6 @@ For example 64 tbs to a qt.
 26 * ROWS
 27 * VLOOKUP
 28 * COUNTBLANK
 29-* FACTDOUBLE
 30 * MULTINOMIAL
 31 * SERIESSUM
 32 * SUBTOTAL
 33diff --git a/dist/Formulas/AllFormulas.js b/dist/Formulas/AllFormulas.js
 34index f3cf143..41b3da6 100644
 35--- a/dist/Formulas/AllFormulas.js
 36+++ b/dist/Formulas/AllFormulas.js
 37@@ -72,6 +72,7 @@ exports.QUOTIENT = Math_1.QUOTIENT;
 38 exports.UPLUS = Math_1.UPLUS;
 39 exports.UMINUS = Math_1.UMINUS;
 40 exports.MROUND = Math_1.MROUND;
 41+exports.FACTDOUBLE = Math_1.FACTDOUBLE;
 42 var Info_1 = require("./Info");
 43 exports.NA = Info_1.NA;
 44 exports.ISTEXT = Info_1.ISTEXT;
 45diff --git a/dist/Formulas/Math.js b/dist/Formulas/Math.js
 46index 5788e20..9fddc50 100644
 47--- a/dist/Formulas/Math.js
 48+++ b/dist/Formulas/Math.js
 49@@ -1302,3 +1302,32 @@ var MROUND = function (value, factor) {
 50     return Math.round(v / f) * f;
 51 };
 52 exports.MROUND = MROUND;
 53+/**
 54+ * Calculates the double-factorial of a number.
 55+ * @param value - value or reference to calculate.
 56+ * @returns {number}
 57+ * @constructor
 58+ */
 59+var FACTDOUBLE = function (value) {
 60+    ArgsChecker_1.ArgsChecker.checkLength(arguments, 1, "FACTDOUBLE");
 61+    var n = Math.floor(TypeConverter_1.TypeConverter.firstValueAsNumber(value));
 62+    function factDoublePrivate(n) {
 63+        if (n <= 0) {
 64+            return 1;
 65+        }
 66+        else {
 67+            return n * factDoublePrivate(n - 2);
 68+        }
 69+    }
 70+    if (n === 0) {
 71+        return 0;
 72+    }
 73+    else if (n < 0) {
 74+        throw new Errors_1.NumError("Function FACTDOUBLE parameter 1 value is '" + n
 75+            + "'. It should be greater than or equal to 0.");
 76+    }
 77+    else {
 78+        return factDoublePrivate(n);
 79+    }
 80+};
 81+exports.FACTDOUBLE = FACTDOUBLE;
 82diff --git a/src/Formulas/AllFormulas.ts b/src/Formulas/AllFormulas.ts
 83index 5ad0f58..8402b80 100644
 84--- a/src/Formulas/AllFormulas.ts
 85+++ b/src/Formulas/AllFormulas.ts
 86@@ -69,7 +69,8 @@ import {
 87   QUOTIENT,
 88   UPLUS,
 89   UMINUS,
 90-  MROUND
 91+  MROUND,
 92+  FACTDOUBLE
 93 } from "./Math";
 94 import {
 95   NA,
 96@@ -337,5 +338,6 @@ export {
 97   ISLOGICAL,
 98   ISNUMBER,
 99   ISNONTEXT,
100-  MROUND
101+  MROUND,
102+  FACTDOUBLE
103 }
104\ No newline at end of file
105diff --git a/src/Formulas/Math.ts b/src/Formulas/Math.ts
106index 9adad14..c00ed74 100644
107--- a/src/Formulas/Math.ts
108+++ b/src/Formulas/Math.ts
109@@ -1314,6 +1314,33 @@ var MROUND = function (value, factor) {
110   return Math.round(v / f) * f;
111 };
112 
113+
114+/**
115+ * Calculates the double-factorial of a number.
116+ * @param value - value or reference to calculate.
117+ * @returns {number}
118+ * @constructor
119+ */
120+var FACTDOUBLE = function (value) {
121+  ArgsChecker.checkLength(arguments, 1, "FACTDOUBLE");
122+  var n = Math.floor(TypeConverter.firstValueAsNumber(value));
123+  function factDoublePrivate(n) {
124+    if (n <= 0) {
125+      return 1;
126+    } else {
127+      return n * factDoublePrivate(n - 2);
128+    }
129+  }
130+  if (n === 0) {
131+    return 0;
132+  } else if (n < 0) {
133+    throw new NumError("Function FACTDOUBLE parameter 1 value is '" + n
134+        + "'. It should be greater than or equal to 0.");
135+  } else {
136+    return factDoublePrivate(n);
137+  }
138+};
139+
140 export {
141   ABS,
142   ACOS,
143@@ -1385,5 +1412,6 @@ export {
144   QUOTIENT,
145   UPLUS,
146   UMINUS,
147-  MROUND
148+  MROUND,
149+  FACTDOUBLE
150 }
151\ No newline at end of file
152diff --git a/tests/Formulas/MathTest.ts b/tests/Formulas/MathTest.ts
153index 2d3dd5d..ab1a153 100644
154--- a/tests/Formulas/MathTest.ts
155+++ b/tests/Formulas/MathTest.ts
156@@ -69,7 +69,8 @@ import {
157   QUOTIENT,
158   UPLUS,
159   UMINUS,
160-  MROUND
161+  MROUND,
162+  FACTDOUBLE
163 } from "../../src/Formulas/Math";
164 import * as ERRORS from "../../src/Errors";
165 import {
166@@ -92,6 +93,23 @@ test("MROUND", function(){
167   }, ERRORS.NA_ERROR);
168 });
169 
170+
171+test("FACTDOUBLE", function(){
172+  assertEquals(FACTDOUBLE(7), 105);
173+  assertEquals(FACTDOUBLE(6), 48);
174+  assertEquals(FACTDOUBLE(3), 3);
175+  catchAndAssertEquals(function() {
176+    FACTDOUBLE(-1);
177+  }, ERRORS.NUM_ERROR);
178+  catchAndAssertEquals(function() {
179+    FACTDOUBLE.apply(this, []);
180+  }, ERRORS.NA_ERROR);
181+  catchAndAssertEquals(function() {
182+    FACTDOUBLE.apply(this, [10, 10]);
183+  }, ERRORS.NA_ERROR);
184+});
185+
186+
187 test("GAMMALN", function(){
188   assertEquals(GAMMALN(4.5), 2.453736570842444);
189   assertEquals(GAMMALN(3), 0.6931471805599443);
190diff --git a/tests/SheetFormulaTest.ts b/tests/SheetFormulaTest.ts
191index b7422de..fa889a5 100644
192--- a/tests/SheetFormulaTest.ts
193+++ b/tests/SheetFormulaTest.ts
194@@ -675,6 +675,10 @@ test("Sheet MROUND", function(){
195   assertFormulaEquals('=MROUND(21, 14)', 28);
196 });
197 
198+test("Sheet FACTDOUBLE", function(){
199+  assertFormulaEquals('=FACTDOUBLE(7)', 105);
200+});
201+
202 test("Sheet *", function(){
203   assertFormulaEquals('= 10 * 10', 100);
204   assertFormulaEquals('= 10 * 0', 0);