diff --git a/src/SmsLength.php b/src/SmsLength.php
index 93da5a1..89902d9 100644
--- a/src/SmsLength.php
+++ b/src/SmsLength.php
@@ -162,12 +162,17 @@ private function inspect(string $messageContent): void
         // Any character outside the 7-bit alphabet switches the entire encoding to UCS-2
         $this->encoding = '7-bit';
         $this->size = 0;
+
         $mbLength = mb_strlen($messageContent, 'UTF-8');
         for ($i = 0; $i < $mbLength; $i++) {
             $char = mb_substr($messageContent, $i, 1, 'UTF-8');
             if (in_array($char, self::GSM0338_BASIC, true)) {
                 $this->size++;
             } elseif (in_array($char, self::GSM0338_EXTENDED, true)) {
+                // In cases where a double counted char straddles two messages, add padding to push it to the next part
+                if (($this->size + 2) % self::MAXIMUM_CHARACTERS_7BIT_CONCATENATED === 1) {
+                    $this->size++;
+                }
                 $this->size += 2;
             } else {
                 $this->encoding = 'ucs-2';
@@ -182,7 +187,14 @@ private function inspect(string $messageContent): void
             for ($i = 0; $i < $mbLength; $i++) {
                 $char = mb_substr($messageContent, $i, 1, 'UTF-8');
                 $utf16Hex = bin2hex(mb_convert_encoding($char, 'UTF-16', 'UTF-8'));
-                $this->size += strlen($utf16Hex) / 4;
+                $charSize = strlen($utf16Hex) / 4;
+
+                // In cases where a double counted char straddles two messages, add padding to push it to the next part
+                if ($charSize > 1 && ($this->size + $charSize) % self::MAXIMUM_CHARACTERS_UCS2_CONCATENATED === 1) {
+                    $this->size++;
+                }
+
+                $this->size += $charSize;
             }
         }
 
@@ -196,7 +208,7 @@ private function inspect(string $messageContent): void
 
         $this->messageCount = 1;
         if ($this->size > $singleSize) {
-            $this->messageCount = (int)ceil($this->size / $concatSize);
+            $this->messageCount = (int) ceil($this->size / $concatSize);
         }
     }
 }
diff --git a/tests/SmsLengthTest.php b/tests/SmsLengthTest.php
index 011bf83..0766bfe 100644
--- a/tests/SmsLengthTest.php
+++ b/tests/SmsLengthTest.php
@@ -78,8 +78,8 @@ public function providerSize(): array
             'long-gsm-exact' => [str_repeat('exact max', 153), '7-bit', 1377, 9, 1377],
 
             // long 7-bit extended
-            'long-gsm-ex-1' => [str_repeat(self::GSM0338_EXTENDED, 40), '7-bit', 720, 5, 765],
-            'long-gsm-ex-2' => [str_repeat(self::GSM0338_EXTENDED, 76), '7-bit', 1368, 9, 1377],
+            'long-gsm-ex-1' => [str_repeat(self::GSM0338_EXTENDED, 40), '7-bit', 724, 5, 765],
+            'long-gsm-ex-2' => [str_repeat(self::GSM0338_EXTENDED, 76), '7-bit', 1376, 9, 1377],
 
             // long UCS-2
             'long-ucs-1' => [str_repeat('simple msg plus •', 20), 'ucs-2', 340, 6, 402],
@@ -87,7 +87,22 @@ public function providerSize(): array
             'long-ucs-exact' => [str_repeat('exact•max', 67), 'ucs-2', 603, 9, 603],
 
             // empty
-            'empty' => ['', '7-bit', 0, 1, 160],
+            'empty messages' => ['', '7-bit', 0, 1, 160],
+
+            'test length calculates correctly when a GSM extended char straddles two messages' => [
+                'The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown f[x jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the.',
+                '7-bit',
+                307,
+                3,
+                459,
+            ],
+            'test length calculates correctly when a unicode char straddles two messages' => [
+                str_repeat('🌐', 67),
+                'ucs-2',
+                136,
+                3,
+                201,
+            ],
         ];
     }