1diff --git a/DOCS.md b/DOCS.md
  2index a7ae0bb..1449b3d 100644
  3--- a/DOCS.md
  4+++ b/DOCS.md
  5@@ -1601,6 +1601,16 @@
  6 @returns {number} 
  7 @constructor
  8 ```
  9+
 10+### SUBTOTAL 
 11+
 12+```
 13+  Calculates subtotals. If a range already contains subtotals, these are not used for further calculations. 
 14+@param functionCode - A value that stands for another function: 1=AVERAGE, 2=COUNT, 3=COUNTA, 4=MAX, 5=MIN, 6=PRODUCT, 7=STDEV, 8=STDEVP, 9=SUM, 10=VAR, 11=VARP. 
 15+@param values - The ranges whose cells are included. 
 16+@returns {Array} 
 17+@constructor
 18+```
 19 ## Range
 20 
 21 
 22diff --git a/TODO.md b/TODO.md
 23index d31123a..40c27a9 100644
 24--- a/TODO.md
 25+++ b/TODO.md
 26@@ -38,20 +38,12 @@ Many of these formulas can be written by allowing the Sheet and Parser to return
 27 * COUNTBLANK - Requires changes to to Parser/Sheet so when we iterate through a range to return an array, we call a special function that accumulates all values, blank/null/undefined or otherwise.
 28 
 29 
 30-### Formulas to write
 31-* SUBTOTAL
 32-* CRITBINOM
 33-* F.DIST.RT
 34+### Easy formulas to write
 35 * HYPGEOMDIST
 36-* LOGINV
 37-* T.INV
 38-* T.INV.2T
 39-* TINV
 40-* TTEST
 41+* CRITBINOM
 42 * ZTEST
 43 * CLEAN
 44 * FIND
 45-* FINDB
 46 * JOIN
 47 * LEFT
 48 * LEN
 49@@ -63,11 +55,19 @@ Many of these formulas can be written by allowing the Sheet and Parser to return
 50 * REPLACE
 51 * REPT
 52 * RIGHT
 53-* ROMAN
 54 * SEARCH
 55-* SEARCHB
 56 * SUBSTITUTE
 57 * VALUE
 58+
 59+### Other formulas to write
 60+* F.DIST.RT
 61+* LOGINV
 62+* T.INV
 63+* T.INV.2T
 64+* TINV
 65+* TTEST
 66+* FINDB
 67+* SEARCHB
 68 * LOGEST
 69 * MDETERM
 70 * MINVERSE
 71diff --git a/dist/Formulas/AllFormulas.js b/dist/Formulas/AllFormulas.js
 72index 56af867..4122490 100644
 73--- a/dist/Formulas/AllFormulas.js
 74+++ b/dist/Formulas/AllFormulas.js
 75@@ -76,6 +76,7 @@ exports.FACTDOUBLE = Math_1.FACTDOUBLE;
 76 exports.UNARY_PERCENT = Math_1.UNARY_PERCENT;
 77 exports.MULTINOMIAL = Math_1.MULTINOMIAL;
 78 exports.SERIESSUM = Math_1.SERIESSUM;
 79+exports.SUBTOTAL = Math_1.SUBTOTAL;
 80 var Range_1 = require("./Range");
 81 exports.FREQUENCY = Range_1.FREQUENCY;
 82 exports.GROWTH = Range_1.GROWTH;
 83diff --git a/dist/Formulas/Math.js b/dist/Formulas/Math.js
 84index 4846561..5a348a2 100644
 85--- a/dist/Formulas/Math.js
 86+++ b/dist/Formulas/Math.js
 87@@ -7,6 +7,7 @@ var Serializer_1 = require("../Utilities/Serializer");
 88 var CriteriaFunctionFactory_1 = require("../Utilities/CriteriaFunctionFactory");
 89 var Errors_1 = require("../Errors");
 90 var MathHelpers_1 = require("../Utilities/MathHelpers");
 91+var Statistical_1 = require("./Statistical");
 92 /**
 93  * Returns the greatest common divisor of one or more integers.
 94  * @param values - The values or ranges whose factors to consider in a calculation to find the greatest common divisor.
 95@@ -1401,3 +1402,48 @@ var SERIESSUM = function (x, n, m, coefficients) {
 96     return result;
 97 };
 98 exports.SERIESSUM = SERIESSUM;
 99+/**
100+ * Calculates subtotals. If a range already contains subtotals, these are not used for further calculations.
101+ * @param functionCode - A value that stands for another function: 1=AVERAGE, 2=COUNT, 3=COUNTA, 4=MAX, 5=MIN,
102+ * 6=PRODUCT, 7=STDEV, 8=STDEVP, 9=SUM, 10=VAR, 11=VARP.
103+ * @param values - The ranges whose cells are included.
104+ * @returns {Array}
105+ * @constructor
106+ */
107+var SUBTOTAL = function (functionCode) {
108+    var values = [];
109+    for (var _i = 1; _i < arguments.length; _i++) {
110+        values[_i - 1] = arguments[_i];
111+    }
112+    ArgsChecker_1.ArgsChecker.checkAtLeastLength(arguments, 2, "SUBTOTAL");
113+    functionCode = TypeConverter_1.TypeConverter.firstValueAsNumber(functionCode);
114+    values = Filter_1.Filter.flattenAndThrow(values);
115+    switch (functionCode) {
116+        case 1:
117+            return Statistical_1.AVERAGE(values);
118+        case 2:
119+            return Statistical_1.COUNT(values);
120+        case 3:
121+            return Statistical_1.COUNTA(values);
122+        case 4:
123+            return Statistical_1.MAX(values);
124+        case 5:
125+            return Statistical_1.MIN(values);
126+        case 6:
127+            return PRODUCT.apply(this, values);
128+        case 7:
129+            return Statistical_1.STDEV(values);
130+        case 8:
131+            return Statistical_1.STDEVP(values);
132+        case 9:
133+            return SUM(values);
134+        case 10:
135+            return Statistical_1.VAR(values);
136+        case 11:
137+            return Statistical_1.VARP(values);
138+        default:
139+            throw new Errors_1.ValueError("Value '" + functionCode +
140+                "' does not correspond to a function for use in SUBTOTAL. Value should be between 1 to 11.");
141+    }
142+};
143+exports.SUBTOTAL = SUBTOTAL;
144diff --git a/src/Formulas/AllFormulas.ts b/src/Formulas/AllFormulas.ts
145index 0e7718f..12aff04 100644
146--- a/src/Formulas/AllFormulas.ts
147+++ b/src/Formulas/AllFormulas.ts
148@@ -73,7 +73,8 @@ import {
149   FACTDOUBLE,
150   UNARY_PERCENT,
151   MULTINOMIAL,
152-  SERIESSUM
153+  SERIESSUM,
154+  SUBTOTAL
155 } from "./Math";
156 import {
157   FREQUENCY,
158@@ -523,5 +524,6 @@ export {
159   TEXT,
160   FVSCHEDULE,
161   PV,
162-  RATE
163+  RATE,
164+  SUBTOTAL
165 }
166\ No newline at end of file
167diff --git a/src/Formulas/Math.ts b/src/Formulas/Math.ts
168index bc535b6..615a76c 100644
169--- a/src/Formulas/Math.ts
170+++ b/src/Formulas/Math.ts
171@@ -23,6 +23,17 @@ import {
172 import {
173   erf, gammaln
174 } from "../Utilities/MathHelpers";
175+import {
176+  AVERAGE,
177+  COUNT,
178+  COUNTA,
179+  MAX,
180+  MIN,
181+  STDEV,
182+  STDEVP,
183+  VAR,
184+  VARP
185+} from "./Statistical";
186 
187 
188 /**
189@@ -1408,6 +1419,48 @@ let SERIESSUM = function (x, n, m, coefficients) {
190 };
191 
192 
193+/**
194+ * Calculates subtotals. If a range already contains subtotals, these are not used for further calculations.
195+ * @param functionCode - A value that stands for another function: 1=AVERAGE, 2=COUNT, 3=COUNTA, 4=MAX, 5=MIN,
196+ * 6=PRODUCT, 7=STDEV, 8=STDEVP, 9=SUM, 10=VAR, 11=VARP.
197+ * @param values - The ranges whose cells are included.
198+ * @returns {Array}
199+ * @constructor
200+ */
201+let SUBTOTAL =  function (functionCode, ...values: Array<Array<any>>) {
202+  ArgsChecker.checkAtLeastLength(arguments, 2, "SUBTOTAL");
203+  functionCode = TypeConverter.firstValueAsNumber(functionCode);
204+  values = Filter.flattenAndThrow(values);
205+  switch (functionCode) {
206+    case 1:
207+      return AVERAGE(values);
208+    case 2:
209+      return COUNT(values);
210+    case 3:
211+      return COUNTA(values);
212+    case 4:
213+      return MAX(values);
214+    case 5:
215+      return MIN(values);
216+    case 6:
217+      return PRODUCT.apply(this, values);
218+    case 7:
219+      return STDEV(values);
220+    case 8:
221+      return STDEVP(values);
222+    case 9:
223+      return SUM(values);
224+    case 10:
225+      return VAR(values);
226+    case 11:
227+      return VARP(values);
228+    default:
229+      throw new ValueError("Value '" + functionCode +
230+          "' does not correspond to a function for use in SUBTOTAL. Value should be between 1 to 11.");
231+  }
232+};
233+
234+
235 export {
236   ABS,
237   ACOS,
238@@ -1483,5 +1536,6 @@ export {
239   FACTDOUBLE,
240   UNARY_PERCENT,
241   MULTINOMIAL,
242-  SERIESSUM
243+  SERIESSUM,
244+  SUBTOTAL
245 }
246\ No newline at end of file
247diff --git a/tests/Formulas/MathTest.ts b/tests/Formulas/MathTest.ts
248index 2102b4b..a76f85b 100644
249--- a/tests/Formulas/MathTest.ts
250+++ b/tests/Formulas/MathTest.ts
251@@ -73,7 +73,8 @@ import {
252   FACTDOUBLE,
253   UNARY_PERCENT,
254   MULTINOMIAL,
255-  SERIESSUM
256+  SERIESSUM,
257+  SUBTOTAL
258 } from "../../src/Formulas/Math";
259 import * as ERRORS from "../../src/Errors";
260 import {
261@@ -1529,4 +1530,29 @@ test("SERIESSUM", function(){
262   catchAndAssertEquals(function() {
263     SERIESSUM.apply(this, [1, 0, 1])
264   }, ERRORS.NA_ERROR);
265+});
266+
267+
268+test("SUBTOTAL", function(){
269+  assertEquals(SUBTOTAL(1, [1, 2, 3, 4, 5, 6, 7]), 4);
270+  assertEquals(SUBTOTAL(1, [1, 2, 3, 4, 5, 6, 7], [4, 5, 6, 7, 8, 9, 10]), 5.5);
271+  assertEquals(SUBTOTAL(2, [1, 2, 3, 4, 5, 6, 7], [4, 5, 6, 7, 8, 9, 10]), 14);
272+  assertEquals(SUBTOTAL(3, [1, 2, 3, 4, 5, 6, 7], [4, 5, 6, 7, 8, 9, 10]), 14);
273+  assertEquals(SUBTOTAL(4, [1, 2, 3, 4, 5, 6, 7], [4, 5, 6, 7, 8, 9, 10]), 10);
274+  assertEquals(SUBTOTAL(5, [1, 2, 3, 4, 5, 6, 7], [4, 5, 6, 7, 8, 9, 10]), 1);
275+  assertEquals(SUBTOTAL(6, [1, 2, 3, 4, 5, 6, 7], [4, 5, 6, 7, 8, 9, 10]), 3048192000);
276+  assertEquals(SUBTOTAL(7, [1, 2, 3, 4, 5, 6, 7], [4, 5, 6, 7, 8, 9, 10]), 2.5943726083138543);
277+  assertEquals(SUBTOTAL(8, [1, 2, 3, 4, 5, 6, 7], [4, 5, 6, 7, 8, 9, 10]), 2.5);
278+  assertEquals(SUBTOTAL(9, [1, 2, 3, 4, 5, 6, 7], [4, 5, 6, 7, 8, 9, 10]), 77);
279+  assertEquals(SUBTOTAL(10, [1, 2, 3, 4, 5, 6, 7], [4, 5, 6, 7, 8, 9, 10]), 6.730769230769231);
280+  assertEquals(SUBTOTAL(11, [1, 2, 3, 4, 5, 6, 7], [4, 5, 6, 7, 8, 9, 10]), 6.25);
281+  catchAndAssertEquals(function() {
282+    SUBTOTAL(0, [1, 2, 3, 4, 5, 6, 7]);
283+  }, ERRORS.VALUE_ERROR);
284+  catchAndAssertEquals(function() {
285+    SUBTOTAL(12, [1, 2, 3, 4, 5, 6, 7]);
286+  }, ERRORS.VALUE_ERROR);
287+  catchAndAssertEquals(function() {
288+    SUBTOTAL.apply(this, [1])
289+  }, ERRORS.NA_ERROR);
290 });
291\ No newline at end of file
292diff --git a/tests/SheetFormulaTest.ts b/tests/SheetFormulaTest.ts
293index 5cb1477..601e4f3 100644
294--- a/tests/SheetFormulaTest.ts
295+++ b/tests/SheetFormulaTest.ts
296@@ -1033,6 +1033,10 @@ test("Sheet RATE", function(){
297   assertFormulaEquals('=RATE(12, -100, 400, 100)', 0.2225948800332845);
298 });
299 
300+test("Sheet SUBTOTAL", function(){
301+  assertFormulaEquals('=SUBTOTAL([1], [1, 2, 3, 4, 5, 6, 7])', 4);
302+});
303+
304 test("Sheet parsing error", function(){
305   assertFormulaEqualsError('= 10e', PARSE_ERROR);
306   assertFormulaEqualsError('= SUM(', PARSE_ERROR);
307diff --git a/tests/Utils/Asserts.ts b/tests/Utils/Asserts.ts
308index e49fdd7..d113ba4 100644
309--- a/tests/Utils/Asserts.ts
310+++ b/tests/Utils/Asserts.ts
311@@ -82,12 +82,22 @@ function catchAndAssertEquals(toExecute : Function, expected) {
312   }
313 }
314 
315+/**
316+ * Print description of test, and run test.
317+ * @param {string} description - To print.
318+ * @param {Function} toRun - Test function to run.
319+ */
320 function test(description: string, toRun: Function) {
321   console.log("Test:", description);
322   toRun();
323 }
324 
325 
326+/**
327+ * Assert formula will result in a particular error.
328+ * @param {string} formula
329+ * @param {string} errorString
330+ */
331 function assertFormulaEqualsError(formula: string, errorString: string) {
332   let sheet  = new Sheet();
333   sheet.setCell("A1", formula);
334@@ -96,6 +106,11 @@ function assertFormulaEqualsError(formula: string, errorString: string) {
335   assertEquals(cell.getValue(), null);
336 }
337 
338+/**
339+ * Assert formula will result in a particular value.
340+ * @param {string} formula
341+ * @param expectation
342+ */
343 function assertFormulaEquals(formula: string, expectation: any) {
344   let sheet  = new Sheet();
345   sheet.setCell("A1", formula);
346@@ -104,6 +119,13 @@ function assertFormulaEquals(formula: string, expectation: any) {
347   assertEquals(cell.getValue(), expectation);
348 }
349 
350+/**
351+ * Assert formula will equal a result, depends on a specific cell reference.
352+ * @param {string} refId - Cell ID, eg: A1
353+ * @param value - Value for refId.
354+ * @param {string} formula - Formula to evaluate.
355+ * @param expectation - Expected result.
356+ */
357 function assertFormulaEqualsDependsOnReference(refId: string, value: any, formula: string, expectation: any) {
358   let sheet  = new Sheet();
359   sheet.setCell(refId, value);
360@@ -113,6 +135,11 @@ function assertFormulaEqualsDependsOnReference(refId: string, value: any, formul
361   assertEquals(cell.getValue(), expectation);
362 }
363 
364+/**
365+ * Assert that the evaluation of a formula results in a specific type.
366+ * @param {string} formula
367+ * @param {string} type
368+ */
369 function assertFormulaResultsInType(formula: string, type: string) {
370   let sheet  = new Sheet();
371   sheet.setCell("A1", formula);
372@@ -121,6 +148,11 @@ function assertFormulaResultsInType(formula: string, type: string) {
373   assertEquals(typeof cell.getValue(), type);
374 }
375 
376+/**
377+ * Assert formula will result in a particular array.
378+ * @param {string} formula
379+ * @param expectation
380+ */
381 function assertFormulaEqualsArray(formula: string, expectation: any) {
382   let sheet  = new Sheet();
383   sheet.setCell("A1", formula);
384@@ -132,7 +164,15 @@ function assertFormulaEqualsArray(formula: string, expectation: any) {
385   }
386 }
387 
388-// WARNING: Locking in Date by overriding prototypes.
389+/**
390+ * Lock in Date by overriding prototypes. WARNING: Should be used sparingly and cautiously.
391+ * @param year
392+ * @param month
393+ * @param day
394+ * @param hour
395+ * @param minute
396+ * @param second
397+ */
398 function lockDate(year, month, day, hour, minute, second)  {
399   let d = new Date(year, month, day, hour, minute, second);
400   Date.prototype.constructor = function () {