diff --git a/core/lib/Drupal/Core/EventSubscriber/ExceptionTestSiteSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/ExceptionTestSiteSubscriber.php
index c6fbdd0..a091b4e 100644
--- a/core/lib/Drupal/Core/EventSubscriber/ExceptionTestSiteSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/ExceptionTestSiteSubscriber.php
@@ -74,10 +74,8 @@ public function on500(GetResponseForExceptionEvent $event) {
    */
   public function onResponse(FilterResponseEvent $event) {
     if ($this->canAddHeaders()) {
-      $response = $event->getResponse();
-      $memory_usage = memory_get_peak_usage(TRUE);
-      $response->headers->add([
-        'X-Drupal-Memory-Peak' => round($memory_usage / 1024 / 1024, 2),
+      $event->getResponse()->headers->add([
+        'X-Drupal-Memory-Peak' => memory_get_peak_usage(TRUE),
       ]);
     }
   }
diff --git a/core/modules/simpletest/src/Tests/SimpleTestTest.php b/core/modules/simpletest/src/Tests/SimpleTestTest.php
index 6b4d076..8d08883 100644
--- a/core/modules/simpletest/src/Tests/SimpleTestTest.php
+++ b/core/modules/simpletest/src/Tests/SimpleTestTest.php
@@ -221,8 +221,9 @@ function stubTest() {
     // This causes \Drupal\simpletest\WebTestBase::curlHeaderCallback() to fail.
     // Set an absurdly low limit and get a page.
     $old_limit = $this->memoryUsageTrigger;
-    $this->memoryUsageTrigger = 1;
-    $this->drupalGet($user->url());
+    $this->memoryUsageTrigger = '1M';
+    // Getting the front page will result in a redirect to /user/2.
+    $this->drupalGet('/');
     $this->memoryUsageTrigger = $old_limit;
   }
 
@@ -262,9 +263,13 @@ function confirmStubTestResults() {
 
     $this->assertAssertion("Debug: 'Foo'", 'Debug', 'Fail', 'SimpleTestTest.php', 'Drupal\simpletest\Tests\SimpleTestTest->stubTest()');
 
-    $this->assertAssertion("exceeded acceptable maximum (1MB)", 'Other', 'Fail', 'WebTestBase.php', 'Drupal\simpletest\WebTestBase->curlExec()');
+    // Check that we have two exceeded memory errors. One for the frontpage and
+    // one for the user page which the request is redirected to.
+    $base_url = rtrim(\Drupal::request()->getSchemeAndHttpHost() . '/' . \Drupal::request()->getBaseUrl(), '/');
+    $this->assertAssertion("exceeded acceptable maximum (1M) on url $base_url/", 'Other', 'Fail', 'WebTestBase.php', 'Drupal\simpletest\WebTestBase->curlExec()');
+    $this->assertAssertion("exceeded acceptable maximum (1M) on url $base_url/user/2", 'Other', 'Fail', 'WebTestBase.php', 'Drupal\simpletest\WebTestBase->curlExec()');
 
-    $this->assertEqual('17 passes, 4 fails, 2 exceptions, 4 debug messages', $this->childTestResults['summary']);
+    $this->assertEqual('17 passes, 5 fails, 2 exceptions, 4 debug messages', $this->childTestResults['summary']);
 
     $this->testIds[] = $test_id = $this->getTestIdFromResults();
     $this->assertTrue($test_id, 'Found test ID in results.');
diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index 44ce470..84a9411 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -11,6 +11,7 @@
 use Drupal\Component\FileCache\FileCacheFactory;
 use Drupal\Component\Serialization\Json;
 use Drupal\Component\Serialization\Yaml;
+use Drupal\Component\Utility\Bytes;
 use Drupal\Component\Utility\Html;
 use Drupal\Component\Utility\NestedArray;
 use Drupal\Component\Utility\UrlHelper;
@@ -71,9 +72,9 @@
    * in it's tests. The check can be disabled for a specific tests by setting it
    * to FALSE.
    *
-   * @var int|FALSE
+   * @var string|FALSE
    */
-  protected $memoryUsageTrigger = 64;
+  protected $memoryUsageTrigger = '64M';
 
   /**
    * The handle of the current cURL connection.
@@ -1413,10 +1414,10 @@ protected function curlHeaderCallback($curlHandler, $header) {
       // the header.
       call_user_func_array(array(&$this, 'error'), unserialize(urldecode($matches[1])));
     }
-    if ($this->memoryUsageTrigger && preg_match('/^X-Drupal-Memory-Peak: (.*)$/', $header, $matches) && (float) $matches[1] >= $this->memoryUsageTrigger) {
-      $this->fail(SafeMarkup::format('Child site memory usage (@siteMB) exceeded acceptable maximum (@maxMB) on url @url', [
-        '@url' => $this->getUrl(),
-        '@site' => $matches[1],
+    if ($this->memoryUsageTrigger && preg_match('/^X-Drupal-Memory-Peak: (.*)$/', $header, $matches) && (float) $matches[1] >= Bytes::toInt($this->memoryUsageTrigger)) {
+      $this->fail(SafeMarkup::format('Child site memory usage (@site) exceeded acceptable maximum (@max) on url @url', [
+        '@url' => curl_getinfo($curlHandler, CURLINFO_EFFECTIVE_URL),
+        '@site' => format_size($matches[1]),
         '@max' => $this->memoryUsageTrigger,
       ]));
     }