1diff --git a/src/RawFormulas/RawFormulas.ts b/src/RawFormulas/RawFormulas.ts
  2index b1f5aba..865dfaa 100644
  3--- a/src/RawFormulas/RawFormulas.ts
  4+++ b/src/RawFormulas/RawFormulas.ts
  5@@ -107,7 +107,6 @@ var DATEVALUE = function (dateString: string) : Date {
  6 var DAY = Formula["DAY"];
  7 var DAYS = Formula["DAYS"];
  8 var DAYS360 = Formula["DAYS360"];
  9-var DDB = Formula["DDB"];
 10 var DEVSQ = Formula["DEVSQ"];
 11 var DOLLAR = Formula["DOLLAR"];
 12 var DOLLARDE = Formula["DOLLARDE"];
 13@@ -129,6 +128,59 @@ var __COMPLEX = {
 14 };
 15 var YEARFRAC = Formula["YEARFRAC"];
 16 
 17+
 18+/**
 19+ * Calculates the depreciation of an asset for a specified period using the double-declining balance method.
 20+ * @param values[0] cost - The initial cost of the asset.
 21+ * @param values[1] salvage - The value of the asset at the end of depreciation.
 22+ * @param values[2] life - The number of periods over which the asset is depreciated.
 23+ * @param values[3] period - The single period within life for which to calculate depreciation.
 24+ * @param values[4] factor - [ OPTIONAL - 2 by default ] - The factor by which depreciation decreases.
 25+ * @returns {number} depreciation of an asset for a specified period
 26+ * @constructor
 27+ */
 28+var DDB = function (...values) : number {
 29+  ArgsChecker.checkLengthWithin(values, 4, 5);
 30+  var cost = TypeCaster.firstValueAsNumber(values[0]);
 31+  var salvage = TypeCaster.firstValueAsNumber(values[1]);
 32+  var life = TypeCaster.firstValueAsNumber(values[2]);
 33+  var period = TypeCaster.firstValueAsNumber(values[3]);
 34+  var factor = values.length === 5 ? TypeCaster.firstValueAsNumber(values[4]) : 2;
 35+
 36+  if (cost < 0) {
 37+    throw new CellError(ERRORS.NUM_ERROR, "Function DDB parameter 1 value is "
 38+      + cost + ". It should be greater than or equal to 0.");
 39+  }
 40+  if (salvage < 0) {
 41+    throw new CellError(ERRORS.NUM_ERROR, "Function DDB parameter 2 value is "
 42+      + salvage + ". It should be greater than or equal to 0.");
 43+  }
 44+  if (life < 0) {
 45+    throw new CellError(ERRORS.NUM_ERROR, "Function DDB parameter 3 value is "
 46+      + life + ". It should be greater than or equal to 0.");
 47+  }
 48+  if (period < 0) {
 49+    throw new CellError(ERRORS.NUM_ERROR, "Function DDB parameter 4 value is "
 50+      + period + ". It should be greater than or equal to 0.");
 51+  }
 52+  if (period > life) {
 53+    throw new CellError(ERRORS.NUM_ERROR, "Function DDB parameter 4 value is "
 54+      + life + ". It should be less than or equal to value of Function DB parameter 3 with "+ period +".");
 55+  }
 56+  if (salvage >= cost) {
 57+    return 0;
 58+  }
 59+
 60+  var total = 0;
 61+  var current = 0;
 62+  for (var i = 1; i <= period; i++) {
 63+    current = Math.min((cost - total) * (factor / life), (cost - salvage - total));
 64+    total += current;
 65+  }
 66+  return current;
 67+};
 68+
 69+
 70 /**
 71  * Calculates the depreciation of an asset for a specified period using the arithmetic declining balance method.
 72  * @param values[0] cost - The initial cost of the asset.
 73@@ -145,10 +197,7 @@ var DB = function (...values) : number {
 74   var salvage = TypeCaster.firstValueAsNumber(values[1]);
 75   var life = TypeCaster.firstValueAsNumber(values[2]);
 76   var period = TypeCaster.firstValueAsNumber(values[3]);
 77-  var month = 12;
 78-  if (values.length === 5) {
 79-    month = Math.floor(TypeCaster.firstValueAsNumber(values[4]));
 80-  }
 81+  var month = values.length === 5 ? Math.floor(TypeCaster.firstValueAsNumber(values[4])) : 12;
 82   if (cost < 0) {
 83     throw new CellError(ERRORS.NUM_ERROR, "Function DB parameter 1 value is "
 84       + cost + ". It should be greater than or equal to 0.");
 85diff --git a/tests/FormulasTest.ts b/tests/FormulasTest.ts
 86index 50e3d92..76faf86 100644
 87--- a/tests/FormulasTest.ts
 88+++ b/tests/FormulasTest.ts
 89@@ -682,13 +682,23 @@ catchAndAssertEquals(function() {
 90   DB(100, 50, 10, 2, 13);
 91 }, ERRORS.NUM_ERROR);
 92 catchAndAssertEquals(function() {
 93-  DB(100, 50, 10, 12, 13);
 94+  DB(100, 50, 10, 12, 2);
 95 }, ERRORS.NUM_ERROR);
 96 catchAndAssertEquals(function() {
 97   DB(100, -50, 10, 2, 12);
 98 }, ERRORS.NUM_ERROR);
 99 
100+
101+// Test DDB
102 assertEquals(DDB(100, 50, 10, 2, 2.25), 17.4375);
103+assertEquals(DDB(100, [50], 10, 2, "2.25"), 17.4375);
104+catchAndAssertEquals(function() {
105+  DDB(100, 50, 10, 12, 2.25);
106+}, ERRORS.NUM_ERROR);
107+catchAndAssertEquals(function() {
108+  DDB(100, -50, 10, 2, 12);
109+}, ERRORS.NUM_ERROR);
110+
111 
112 // Test DEC2BIN
113 assertEquals(DEC2BIN([100]), "1100100");