diff --git a/CHANGELOG.md b/CHANGELOG.md
index d6d252d5..d7b123ef 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,7 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 
 ### Added
 
-- Implemented the CHITEST() Statistical function.
+- Implemented the CHITEST() and CHISQ.DIST() Statistical function.
 - Support for ActiveSheet and SelectedCells in the ODS Reader and Writer. [PR #1908](https://github.com/PHPOffice/PhpSpreadsheet/pull/1908)
 
 ### Changed
diff --git a/src/PhpSpreadsheet/Calculation/Calculation.php b/src/PhpSpreadsheet/Calculation/Calculation.php
index 62524c37..c4a88bc3 100644
--- a/src/PhpSpreadsheet/Calculation/Calculation.php
+++ b/src/PhpSpreadsheet/Calculation/Calculation.php
@@ -488,22 +488,22 @@ class Calculation
         ],
         'CHIDIST' => [
             'category' => Category::CATEGORY_STATISTICAL,
-            'functionCall' => [Statistical\Distributions\ChiSquared::class, 'distribution'],
+            'functionCall' => [Statistical\Distributions\ChiSquared::class, 'distributionRightTail'],
             'argumentCount' => '2',
         ],
         'CHISQ.DIST' => [
             'category' => Category::CATEGORY_STATISTICAL,
-            'functionCall' => [Functions::class, 'DUMMY'],
+            'functionCall' => [Statistical\Distributions\ChiSquared::class, 'distributionLeftTail'],
             'argumentCount' => '3',
         ],
         'CHISQ.DIST.RT' => [
             'category' => Category::CATEGORY_STATISTICAL,
-            'functionCall' => [Statistical\Distributions\ChiSquared::class, 'distribution'],
+            'functionCall' => [Statistical\Distributions\ChiSquared::class, 'distributionRightTail'],
             'argumentCount' => '2',
         ],
         'CHIINV' => [
             'category' => Category::CATEGORY_STATISTICAL,
-            'functionCall' => [Statistical\Distributions\ChiSquared::class, 'inverse'],
+            'functionCall' => [Statistical\Distributions\ChiSquared::class, 'inverseRightTail'],
             'argumentCount' => '2',
         ],
         'CHISQ.INV' => [
@@ -513,7 +513,7 @@ class Calculation
         ],
         'CHISQ.INV.RT' => [
             'category' => Category::CATEGORY_STATISTICAL,
-            'functionCall' => [Statistical\Distributions\ChiSquared::class, 'inverse'],
+            'functionCall' => [Statistical\Distributions\ChiSquared::class, 'inverseRightTail'],
             'argumentCount' => '2',
         ],
         'CHITEST' => [
diff --git a/src/PhpSpreadsheet/Calculation/Statistical.php b/src/PhpSpreadsheet/Calculation/Statistical.php
index 5f557431..01caba14 100644
--- a/src/PhpSpreadsheet/Calculation/Statistical.php
+++ b/src/PhpSpreadsheet/Calculation/Statistical.php
@@ -297,8 +297,8 @@ class Statistical
      *
      * @Deprecated 1.18.0
      *
-     * @see Statistical\Distributions\ChiSquared::distribution()
-     *      Use the distribution() method in the Statistical\Distributions\ChiSquared class instead
+     * @see Statistical\Distributions\ChiSquared::distributionRightTail()
+     *      Use the distributionRightTail() method in the Statistical\Distributions\ChiSquared class instead
      *
      * @param float $value Value for the function
      * @param float $degrees degrees of freedom
@@ -307,7 +307,7 @@ class Statistical
      */
     public static function CHIDIST($value, $degrees)
     {
-        return Statistical\Distributions\ChiSquared::distribution($value, $degrees);
+        return Statistical\Distributions\ChiSquared::distributionRightTail($value, $degrees);
     }
 
     /**
@@ -317,8 +317,8 @@ class Statistical
      *
      * @Deprecated 1.18.0
      *
-     * @see Statistical\Distributions\ChiSquared::inverse()
-     *      Use the inverse() method in the Statistical\Distributions\ChiSquared class instead
+     * @see Statistical\Distributions\ChiSquared::inverseRightTail()
+     *      Use the inverseRightTail() method in the Statistical\Distributions\ChiSquared class instead
      *
      * @param float $probability Probability for the function
      * @param float $degrees degrees of freedom
@@ -327,7 +327,7 @@ class Statistical
      */
     public static function CHIINV($probability, $degrees)
     {
-        return Statistical\Distributions\ChiSquared::inverse($probability, $degrees);
+        return Statistical\Distributions\ChiSquared::inverseRightTail($probability, $degrees);
     }
 
     /**
@@ -2159,7 +2159,7 @@ class Statistical
     /**
      * TINV.
      *
-     * Returns the one-tailed probability of the chi-squared distribution.
+     * Returns the one-tailed probability of the Student-T distribution.
      *
      * @Deprecated 1.18.0
      *
diff --git a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/ChiSquared.php b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/ChiSquared.php
index dfd090de..409e5883 100644
--- a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/ChiSquared.php
+++ b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/ChiSquared.php
@@ -21,7 +21,7 @@ class ChiSquared
      *
      * @return float|string
      */
-    public static function distribution($value, $degrees)
+    public static function distributionRightTail($value, $degrees)
     {
         $value = Functions::flattenSingleValue($value);
         $degrees = Functions::flattenSingleValue($degrees);
@@ -48,16 +48,60 @@ class ChiSquared
     }
 
     /**
-     * CHIINV.
+     * CHIDIST.
      *
      * Returns the one-tailed probability of the chi-squared distribution.
      *
+     * @param mixed (float) $value Value for the function
+     * @param mixed (int) $degrees degrees of freedom
+     * @param mixed $cumulative
+     *
+     * @return float|string
+     */
+    public static function distributionLeftTail($value, $degrees, $cumulative)
+    {
+        $value = Functions::flattenSingleValue($value);
+        $degrees = Functions::flattenSingleValue($degrees);
+        $cumulative = Functions::flattenSingleValue($cumulative);
+
+        try {
+            $value = self::validateFloat($value);
+            $degrees = self::validateInt($degrees);
+            $cumulative = self::validateBool($cumulative);
+        } catch (Exception $e) {
+            return $e->getMessage();
+        }
+
+        if ($degrees < 1) {
+            return Functions::NAN();
+        }
+        if ($value < 0) {
+            if (Functions::getCompatibilityMode() == Functions::COMPATIBILITY_GNUMERIC) {
+                return 1;
+            }
+
+            return Functions::NAN();
+        }
+
+        if ($cumulative === true) {
+            return 1 - self::distributionRightTail($value, $degrees);
+        }
+
+        return (($value ** (($degrees / 2) - 1) * exp(-$value / 2))) /
+            ((2 ** ($degrees / 2)) * Gamma::gammaValue($degrees / 2));
+    }
+
+    /**
+     * CHIINV.
+     *
+     * Returns the inverse of the right-tailed probability of the chi-squared distribution.
+     *
      * @param mixed (float) $probability Probability for the function
      * @param mixed (int) $degrees degrees of freedom
      *
      * @return float|string
      */
-    public static function inverse($probability, $degrees)
+    public static function inverseRightTail($probability, $degrees)
     {
         $probability = Functions::flattenSingleValue($probability);
         $degrees = Functions::flattenSingleValue($degrees);
@@ -108,7 +152,7 @@ class ChiSquared
 
         $degrees = self::degrees($rows, $columns);
 
-        $result = self::distribution($result, $degrees);
+        $result = self::distributionRightTail($result, $degrees);
 
         return $result;
     }
diff --git a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/GammaBase.php b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/GammaBase.php
index 3f76787d..89170f7c 100644
--- a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/GammaBase.php
+++ b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/GammaBase.php
@@ -30,14 +30,15 @@ abstract class GammaBase
         $xLo = 0;
         $xHi = $alpha * $beta * 5;
 
-        $x = $xNew = 1;
         $dx = 1024;
+        $x = $xNew = 1;
         $i = 0;
 
         while ((abs($dx) > Functions::PRECISION) && (++$i <= self::MAX_ITERATIONS)) {
             // Apply Newton-Raphson step
             $result = self::calculateDistribution($x, $alpha, $beta, true);
             $error = $result - $probability;
+
             if ($error == 0.0) {
                 $dx = 0;
             } elseif ($error < 0.0) {
diff --git a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/NewtonRaphson.php b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/NewtonRaphson.php
index 298cdfaf..d4025f6f 100644
--- a/src/PhpSpreadsheet/Calculation/Statistical/Distributions/NewtonRaphson.php
+++ b/src/PhpSpreadsheet/Calculation/Statistical/Distributions/NewtonRaphson.php
@@ -20,8 +20,8 @@ class NewtonRaphson
         $xLo = 100;
         $xHi = 0;
 
-        $x = $xNew = 1;
         $dx = 1;
+        $x = $xNew = 1;
         $i = 0;
 
         while ((abs($dx) > Functions::PRECISION) && ($i++ < self::MAX_ITERATIONS)) {
diff --git a/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiInvTest.php b/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDistLeftTailTest.php
similarity index 58%
rename from tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiInvTest.php
rename to tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDistLeftTailTest.php
index 72680914..3c7a8d4e 100644
--- a/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiInvTest.php
+++ b/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDistLeftTailTest.php
@@ -6,7 +6,7 @@ use PhpOffice\PhpSpreadsheet\Calculation\Functions;
 use PhpOffice\PhpSpreadsheet\Calculation\Statistical;
 use PHPUnit\Framework\TestCase;
 
-class ChiInvTest extends TestCase
+class ChiDistLeftTailTest extends TestCase
 {
     protected function setUp(): void
     {
@@ -14,18 +14,18 @@ class ChiInvTest extends TestCase
     }
 
     /**
-     * @dataProvider providerCHIINV
+     * @dataProvider providerCHIDIST
      *
      * @param mixed $expectedResult
      */
-    public function testCHIINV($expectedResult, ...$args): void
+    public function testCHIDIST($expectedResult, ...$args): void
     {
-        $result = Statistical::CHIINV(...$args);
+        $result = Statistical\Distributions\ChiSquared::distributionLeftTail(...$args);
         self::assertEqualsWithDelta($expectedResult, $result, 1E-12);
     }
 
-    public function providerCHIINV()
+    public function providerCHIDIST()
     {
-        return require 'tests/data/Calculation/Statistical/CHIINV.php';
+        return require 'tests/data/Calculation/Statistical/CHIDISTLeftTail.php';
     }
 }
diff --git a/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDistTest.php b/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDistRightTailTest.php
similarity index 92%
rename from tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDistTest.php
rename to tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDistRightTailTest.php
index 9dc7326c..26bf5ab7 100644
--- a/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDistTest.php
+++ b/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiDistRightTailTest.php
@@ -6,7 +6,7 @@ use PhpOffice\PhpSpreadsheet\Calculation\Functions;
 use PhpOffice\PhpSpreadsheet\Calculation\Statistical;
 use PHPUnit\Framework\TestCase;
 
-class ChiDistTest extends TestCase
+class ChiDistRightTailTest extends TestCase
 {
     protected function setUp(): void
     {
@@ -26,6 +26,6 @@ class ChiDistTest extends TestCase
 
     public function providerCHIDIST()
     {
-        return require 'tests/data/Calculation/Statistical/CHIDIST.php';
+        return require 'tests/data/Calculation/Statistical/CHIDISTRightTail.php';
     }
 }
diff --git a/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiInvRightTailTest.php b/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiInvRightTailTest.php
new file mode 100644
index 00000000..75949f39
--- /dev/null
+++ b/tests/PhpSpreadsheetTests/Calculation/Functions/Statistical/ChiInvRightTailTest.php
@@ -0,0 +1,37 @@
+<?php
+
+namespace PhpOffice\PhpSpreadsheetTests\Calculation\Functions\Statistical;
+
+use PhpOffice\PhpSpreadsheet\Calculation\Functions;
+use PhpOffice\PhpSpreadsheet\Calculation\Statistical;
+use PHPUnit\Framework\TestCase;
+
+class ChiInvRightTailTest extends TestCase
+{
+    protected function setUp(): void
+    {
+        Functions::setCompatibilityMode(Functions::COMPATIBILITY_EXCEL);
+    }
+
+    /**
+     * @dataProvider providerCHIINV
+     *
+     * @param mixed $expectedResult
+     * @param mixed $probability
+     * @param mixed $degrees
+     */
+    public function testCHIINV($expectedResult, $probability, $degrees): void
+    {
+        $result = Statistical::CHIINV($probability, $degrees);
+        if (!is_string($expectedResult)) {
+            $reverse = Statistical\Distributions\ChiSquared::distributionRightTail($result, $degrees);
+            self::assertEqualsWithDelta($probability, $reverse, 1E-12);
+        }
+        self::assertEqualsWithDelta($expectedResult, $result, 1E-12);
+    }
+
+    public function providerCHIINV()
+    {
+        return require 'tests/data/Calculation/Statistical/CHIINVRightTail.php';
+    }
+}
diff --git a/tests/data/Calculation/Statistical/CHIDISTLeftTail.php b/tests/data/Calculation/Statistical/CHIDISTLeftTail.php
new file mode 100644
index 00000000..313061ac
--- /dev/null
+++ b/tests/data/Calculation/Statistical/CHIDISTLeftTail.php
@@ -0,0 +1,64 @@
+<?php
+
+return [
+    [
+        0.520499877813,
+        0.5, 1, true,
+    ],
+    [
+        0.207553748710,
+        2, 3, false,
+    ],
+    [
+        0.111565080074,
+        3, 2, false,
+    ],
+    [
+        0.776869839852,
+        3, 2, true,
+    ],
+    [
+        0.039646370521,
+        3, 9, false,
+    ],
+    [
+        0.035705027315,
+        3, 9, true,
+    ],
+    [
+        0.103349469094,
+        7.5, 8, false,
+    ],
+    [
+        0.516232618446,
+        7.5, 8, true,
+    ],
+    [
+        0.020666985354,
+        8, 3, false,
+    ],
+    [
+        0.953988294311,
+        8, 3, true,
+    ],
+    [
+        '#VALUE!',
+        'NaN', 3, true,
+    ],
+    [
+        '#VALUE!',
+        2, 'NaN', true,
+    ],
+    [
+        '#VALUE!',
+        2, 3, 'NaN',
+    ],
+    'Value < 0' => [
+        '#NUM!',
+        -8, 3, true,
+    ],
+    'Degrees < 1' => [
+        '#NUM!',
+        8, 0, true,
+    ],
+];
diff --git a/tests/data/Calculation/Statistical/CHIDIST.php b/tests/data/Calculation/Statistical/CHIDISTRightTail.php
similarity index 100%
rename from tests/data/Calculation/Statistical/CHIDIST.php
rename to tests/data/Calculation/Statistical/CHIDISTRightTail.php
diff --git a/tests/data/Calculation/Statistical/CHIINV.php b/tests/data/Calculation/Statistical/CHIINVRightTail.php
similarity index 82%
rename from tests/data/Calculation/Statistical/CHIINV.php
rename to tests/data/Calculation/Statistical/CHIINVRightTail.php
index f931a780..58b317e5 100644
--- a/tests/data/Calculation/Statistical/CHIINV.php
+++ b/tests/data/Calculation/Statistical/CHIINVRightTail.php
@@ -10,13 +10,21 @@ return [
         0.75, 10,
     ],
     [
-        18.30697345702,
-        0.050001, 10,
+        0.007716715545,
+        0.93, 1,
+    ],
+    [
+        1.021651247532,
+        0.6, 2,
     ],
     [
         0.45493642312,
         0.5, 1,
     ],
+    [
+        4.351460191096,
+        0.5, 5,
+    ],
     [
         0.101531044268,
         0.75, 1,