diff --git a/core/includes/common.inc b/core/includes/common.inc
index 46a7b8fc82..f32e581b00 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -251,9 +251,9 @@ function check_url($uri) {
  *   A translated string representation of the size.
  */
 function format_size($size, $langcode = NULL) {
-  if ($size < Bytes::KILOBYTE) {
+  if (abs($size) < Bytes::KILOBYTE) {
     if ($size == -1) {
-      return \Drupal::translation()->t('-1 byte', [], ['langcode' => $langcode]);
+      return \Drupal::translation()->translate('-1 byte', [], ['langcode' => $langcode]);
     }
     else {
       return \Drupal::translation()->formatPlural($size, '1 byte', '@count bytes', [], ['langcode' => $langcode]);
@@ -261,13 +261,15 @@ function format_size($size, $langcode = NULL) {
   }
   else {
     // Convert bytes to kilobytes.
+    $positive = $size >= 0;
+    $temp_size = abs($size);
     foreach (['B', 'KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'] as $unit) {
-      if (round($size, 2) < Bytes::KILOBYTE) {
+      if (round($temp_size, 2) < Bytes::KILOBYTE) {
         break;
       }
-      $size = $size / Bytes::KILOBYTE;
+      $temp_size = $temp_size / Bytes::KILOBYTE;
     }
-    $args = ['@size' => round($size, 2)];
+    $args = ['@size' => $positive ? round($temp_size, 2) : -round($temp_size, 2)];
     $options = ['langcode' => $langcode];
     switch ($unit) {
       case 'KB':
diff --git a/core/lib/Drupal/Component/Utility/Bytes.php b/core/lib/Drupal/Component/Utility/Bytes.php
index d625b12364..2a7102a4b0 100644
--- a/core/lib/Drupal/Component/Utility/Bytes.php
+++ b/core/lib/Drupal/Component/Utility/Bytes.php
@@ -28,7 +28,7 @@ public static function toInt($size) {
     // Remove the non-unit characters from the size.
     $unit = preg_replace('/[^bkmgtpezy]/i', '', $size);
     // Remove the non-numeric characters from the size.
-    $size = preg_replace('/[^0-9\.]/', '', $size);
+    $size = preg_replace('/[^0-9\.\-]/', '', $size);
     if ($unit) {
       // Find the position of the unit in the ordered string which is the power
       // of magnitude to multiply a kilobyte by.
diff --git a/core/tests/Drupal/KernelTests/Core/Common/SizeTest.php b/core/tests/Drupal/KernelTests/Core/Common/SizeTest.php
index e821cb01fa..333cf62552 100644
--- a/core/tests/Drupal/KernelTests/Core/Common/SizeTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Common/SizeTest.php
@@ -36,6 +36,7 @@ protected function setUp() {
       '-2 bytes' => -2,
       // Rounded to 1 MB (not 1000 or 1024 kilobyte!).
       '1 MB' => ($kb * $kb) - 1,
+      '-1 MB' => -(($kb * $kb) - 1),
       '3.77 GB' => 4053371676,
       // Megabytes.
       round(3623651 / ($this->exactTestCases['1 MB']), 2) . ' MB' => 3623651,
@@ -52,7 +53,7 @@ protected function setUp() {
   public function testCommonFormatSize() {
     foreach ([$this->exactTestCases, $this->roundedTestCases] as $test_cases) {
       foreach ($test_cases as $expected => $input) {
-        $result = format_size($input, NULL);
+        $result = (string) format_size($input, NULL);
         $this->assertSame($expected, $result, $expected . ' == ' . $result . ' (' . $input . ' bytes)');
       }
     }
@@ -63,7 +64,7 @@ public function testCommonFormatSize() {
    */
   public function testCommonParseSizeFormatSize() {
     foreach ($this->exactTestCases as $size) {
-      $parsed_size = Bytes::toInt($string = format_size($size, NULL));
+      $parsed_size = (int) Bytes::toInt($string = format_size($size, NULL));
       $this->assertSame($size, $parsed_size, $size . ' == ' . $parsed_size . ' (' . $string . ')');
     }
   }